[12/13] writeback: add a writeback iterator

  Refactor the code left in write_cache_pages into an iterator that the
file system can call to get the next folio for a writeback operation:

	struct folio *folio = NULL;

	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
		error = <do per-foli writeback>;
	}

The twist here is that the error value is passed by reference, so that
the iterator can restore it when breaking out of the loop.

Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
iterator and needs is just kept in the write_cache_pages legacy wrapper.
in preparation for eventually killing it off.

Heavily based on a for_each* based iterator from Matthew Wilcox.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 include/linux/writeback.h |   4 +
 mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
 2 files changed, 118 insertions(+), 78 deletions(-)
  

On Sat 03-02-24 08:11:46, Christoph Hellwig wrote:
> Refactor the code left in write_cache_pages into an iterator that the
> file system can call to get the next folio for a writeback operation:
> 
> 	struct folio *folio = NULL;
> 
> 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> 		error = <do per-foli writeback>;
> 	}
> 
> The twist here is that the error value is passed by reference, so that
> the iterator can restore it when breaking out of the loop.
> 
> Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> iterator and needs is just kept in the write_cache_pages legacy wrapper.
> in preparation for eventually killing it off.
> 
> Heavily based on a for_each* based iterator from Matthew Wilcox.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  include/linux/writeback.h |   4 +
>  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
>  2 files changed, 118 insertions(+), 78 deletions(-)
> 
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index f67b3ea866a0fb..9845cb62e40b2d 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -82,6 +82,7 @@ struct writeback_control {
>  	/* internal fields used by the ->writepages implementation: */
>  	struct folio_batch fbatch;
>  	pgoff_t index;
> +	int saved_err;
>  
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct bdi_writeback *wb;	/* wb this writeback is issued under */
> @@ -366,6 +367,9 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
>  
>  bool wb_over_bg_thresh(struct bdi_writeback *wb);
>  
> +struct folio *writeback_iter(struct address_space *mapping,
> +		struct writeback_control *wbc, struct folio *folio, int *error);
> +
>  typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
>  				void *data);
>  
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3abb053e70580e..5fe4cdb7dbd61a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2325,18 +2325,18 @@ void __init page_writeback_init(void)
>  }
>  
>  /**
> - * tag_pages_for_writeback - tag pages to be written by write_cache_pages
> + * tag_pages_for_writeback - tag pages to be written by writeback
>   * @mapping: address space structure to write
>   * @start: starting page index
>   * @end: ending page index (inclusive)
>   *
>   * This function scans the page range from @start to @end (inclusive) and tags
> - * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
> - * that write_cache_pages (or whoever calls this function) will then use
> - * TOWRITE tag to identify pages eligible for writeback.  This mechanism is
> - * used to avoid livelocking of writeback by a process steadily creating new
> - * dirty pages in the file (thus it is important for this function to be quick
> - * so that it can tag pages faster than a dirtying process can create them).
> + * all pages that have DIRTY tag set with a special TOWRITE tag.  The caller
> + * can then use the TOWRITE tag to identify pages eligible for writeback.
> + * This mechanism is used to avoid livelocking of writeback by a process
> + * steadily creating new dirty pages in the file (thus it is important for this
> + * function to be quick so that it can tag pages faster than a dirtying process
> + * can create them).
>   */
>  void tag_pages_for_writeback(struct address_space *mapping,
>  			     pgoff_t start, pgoff_t end)
> @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
>  }
>  
>  /**
> - * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * writeback_iter - iterate folio of a mapping for writeback
>   * @mapping: address space structure to write
> - * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> - * @writepage: function called for each page
> - * @data: data passed to writepage function
> + * @wbc: writeback context
> + * @folio: previously iterated folio (%NULL to start)
> + * @error: in-out pointer for writeback errors (see below)
>   *
> - * If a page is already under I/O, write_cache_pages() skips it, even
> - * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
> - * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
> - * and msync() need to guarantee that all the data which was dirty at the time
> - * the call was made get new I/O started against them.  If wbc->sync_mode is
> - * WB_SYNC_ALL then we were called for data integrity and we must wait for
> - * existing IO to complete.
> - *
> - * To avoid livelocks (when other process dirties new pages), we first tag
> - * pages which should be written back with TOWRITE tag and only then start
> - * writing them. For data-integrity sync we have to be careful so that we do
> - * not miss some pages (e.g., because some other process has cleared TOWRITE
> - * tag we set). The rule we follow is that TOWRITE tag can be cleared only
> - * by the process clearing the DIRTY tag (and submitting the page for IO).
> - *
> - * To avoid deadlocks between range_cyclic writeback and callers that hold
> - * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
> - * we do not loop back to the start of the file. Doing so causes a page
> - * lock/page writeback access order inversion - we should only ever lock
> - * multiple pages in ascending page->index order, and looping back to the start
> - * of the file violates that rule and causes deadlocks.
> + * This function returns the next folio for the writeback operation described by
> + * @wbc on @mapping and  should be called in a while loop in the ->writepages
> + * implementation.
>   *
> - * Return: %0 on success, negative error code otherwise
> + * To start the writeback operation, %NULL is passed in the @folio argument, and
> + * for every subsequent iteration the folio returned previously should be passed
> + * back in.
> + *
> + * If there was an error in the per-folio writeback inside the writeback_iter()
> + * loop, @error should be set to the error value.
> + *
> + * Once the writeback described in @wbc has finished, this function will return
> + * %NULL and if there was an error in any iteration restore it to @error.
> + *
> + * Note: callers should not manually break out of the loop using break or goto
> + * but must keep calling writeback_iter() until it returns %NULL.
> + *
> + * Return: the folio to write or %NULL if the loop is done.
>   */
> -int write_cache_pages(struct address_space *mapping,
> -		      struct writeback_control *wbc, writepage_t writepage,
> -		      void *data)
> +struct folio *writeback_iter(struct address_space *mapping,
> +		struct writeback_control *wbc, struct folio *folio, int *error)
>  {
> -	int ret = 0;
> -	int error;
> -	struct folio *folio;
> -	pgoff_t end;		/* Inclusive */
> -
> -	if (wbc->range_cyclic) {
> -		wbc->index = mapping->writeback_index; /* prev offset */
> -		end = -1;
> -	} else {
> -		wbc->index = wbc->range_start >> PAGE_SHIFT;
> -		end = wbc->range_end >> PAGE_SHIFT;
> -	}
> -	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> -		tag_pages_for_writeback(mapping, wbc->index, end);
> -
> -	folio_batch_init(&wbc->fbatch);
> +	if (!folio) {
> +		folio_batch_init(&wbc->fbatch);
> +		wbc->saved_err = *error = 0;
>  
> -	for (;;) {
> -		folio = writeback_get_folio(mapping, wbc);
> -		if (!folio)
> -			break;
> +		/*
> +		 * For range cyclic writeback we remember where we stopped so
> +		 * that we can continue where we stopped.
> +		 *
> +		 * For non-cyclic writeback we always start at the beginning of
> +		 * the passed in range.
> +		 */
> +		if (wbc->range_cyclic)
> +			wbc->index = mapping->writeback_index;
> +		else
> +			wbc->index = wbc->range_start >> PAGE_SHIFT;
>  
> -		error = writepage(folio, wbc, data);
> +		/*
> +		 * To avoid livelocks when other processes dirty new pages, we
> +		 * first tag pages which should be written back and only then
> +		 * start writing them.
> +		 *
> +		 * For data-integrity writeback we have to be careful so that we
> +		 * do not miss some pages (e.g., because some other process has
> +		 * cleared the TOWRITE tag we set).  The rule we follow is that
> +		 * TOWRITE tag can be cleared only by the process clearing the
> +		 * DIRTY tag (and submitting the page for I/O).
> +		 */
> +		if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> +			tag_pages_for_writeback(mapping, wbc->index,
> +					wbc_end(wbc));
> +	} else {
>  		wbc->nr_to_write -= folio_nr_pages(folio);
>  
> -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> -			folio_unlock(folio);
> -			error = 0;
> -		}
> +		WARN_ON_ONCE(*error > 0);
>  
>  		/*
>  		 * For integrity writeback we have to keep going until we have
> @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
>  		 * wbc->nr_to_write or encounter the first error.
>  		 */
>  		if (wbc->sync_mode == WB_SYNC_ALL) {
> -			if (error && !ret)
> -				ret = error;
> +			if (*error && !wbc->saved_err)
> +				wbc->saved_err = *error;
>  		} else {
> -			if (error || wbc->nr_to_write <= 0)
> +			if (*error || wbc->nr_to_write <= 0)
>  				goto done;
>  		}
>  	}
>  
> -	/*
> -	 * For range cyclic writeback we need to remember where we stopped so
> -	 * that we can continue there next time we are called.  If  we hit the
> -	 * last page and there is more work to be done, wrap back to the start
> -	 * of the file.
> -	 *
> -	 * For non-cyclic writeback we always start looking up at the beginning
> -	 * of the file if we are called again, which can only happen due to
> -	 * -ENOMEM from the file system.
> -	 */
> -	folio_batch_release(&wbc->fbatch);
> -	if (wbc->range_cyclic)
> -		mapping->writeback_index = 0;
> -	return ret;
> +	folio = writeback_get_folio(mapping, wbc);
> +	if (!folio) {
> +		/*
> +		 * To avoid deadlocks between range_cyclic writeback and callers
> +		 * that hold pages in PageWriteback to aggregate I/O until
> +		 * the writeback iteration finishes, we do not loop back to the
> +		 * start of the file.  Doing so causes a page lock/page
> +		 * writeback access order inversion - we should only ever lock
> +		 * multiple pages in ascending page->index order, and looping
> +		 * back to the start of the file violates that rule and causes
> +		 * deadlocks.
> +		 */
> +		if (wbc->range_cyclic)
> +			mapping->writeback_index = 0;
> +
> +		/*
> +		 * Return the first error we encountered (if there was any) to
> +		 * the caller.
> +		 */
> +		*error = wbc->saved_err;
> +	}
> +	return folio;
>  
>  done:
>  	folio_batch_release(&wbc->fbatch);
>  	if (wbc->range_cyclic)
>  		mapping->writeback_index = folio->index + folio_nr_pages(folio);
> +	return NULL;
> +}
> +
> +/**
> + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * @mapping: address space structure to write
> + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> + * @writepage: function called for each page
> + * @data: data passed to writepage function
> + *
> + * Return: %0 on success, negative error code otherwise
> + *
> + * Note: please use writeback_iter() instead.
> + */
> +int write_cache_pages(struct address_space *mapping,
> +		      struct writeback_control *wbc, writepage_t writepage,
> +		      void *data)
> +{
> +	struct folio *folio = NULL;
> +	int error;
> +
> +	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> +		error = writepage(folio, wbc, data);
> +		if (error == AOP_WRITEPAGE_ACTIVATE) {
> +			folio_unlock(folio);
> +			error = 0;
> +		}
> +	}
> +
>  	return error;
>  }
>  EXPORT_SYMBOL(write_cache_pages);
> -- 
> 2.39.2
>
  

On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> Refactor the code left in write_cache_pages into an iterator that the
> file system can call to get the next folio for a writeback operation:
> 
> 	struct folio *folio = NULL;
> 
> 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> 		error = <do per-foli writeback>;
> 	}
> 
> The twist here is that the error value is passed by reference, so that
> the iterator can restore it when breaking out of the loop.
> 
> Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> iterator and needs is just kept in the write_cache_pages legacy wrapper.
> in preparation for eventually killing it off.
> 
> Heavily based on a for_each* based iterator from Matthew Wilcox.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  include/linux/writeback.h |   4 +
>  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
>  2 files changed, 118 insertions(+), 78 deletions(-)
> 
..
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3abb053e70580e..5fe4cdb7dbd61a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
..
> @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
>  }
>  
>  /**
..
>   */
> -int write_cache_pages(struct address_space *mapping,
> -		      struct writeback_control *wbc, writepage_t writepage,
> -		      void *data)
> +struct folio *writeback_iter(struct address_space *mapping,
> +		struct writeback_control *wbc, struct folio *folio, int *error)
>  {
..
> +	} else {
>  		wbc->nr_to_write -= folio_nr_pages(folio);
>  
> -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> -			folio_unlock(folio);
> -			error = 0;
> -		}
> +		WARN_ON_ONCE(*error > 0);

Why the warning on writeback error here? It looks like new behavior, but
maybe I missed something. Otherwise the factoring LGTM.

Brian

>  
>  		/*
>  		 * For integrity writeback we have to keep going until we have
> @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
>  		 * wbc->nr_to_write or encounter the first error.
>  		 */
>  		if (wbc->sync_mode == WB_SYNC_ALL) {
> -			if (error && !ret)
> -				ret = error;
> +			if (*error && !wbc->saved_err)
> +				wbc->saved_err = *error;
>  		} else {
> -			if (error || wbc->nr_to_write <= 0)
> +			if (*error || wbc->nr_to_write <= 0)
>  				goto done;
>  		}
>  	}
>  
> -	/*
> -	 * For range cyclic writeback we need to remember where we stopped so
> -	 * that we can continue there next time we are called.  If  we hit the
> -	 * last page and there is more work to be done, wrap back to the start
> -	 * of the file.
> -	 *
> -	 * For non-cyclic writeback we always start looking up at the beginning
> -	 * of the file if we are called again, which can only happen due to
> -	 * -ENOMEM from the file system.
> -	 */
> -	folio_batch_release(&wbc->fbatch);
> -	if (wbc->range_cyclic)
> -		mapping->writeback_index = 0;
> -	return ret;
> +	folio = writeback_get_folio(mapping, wbc);
> +	if (!folio) {
> +		/*
> +		 * To avoid deadlocks between range_cyclic writeback and callers
> +		 * that hold pages in PageWriteback to aggregate I/O until
> +		 * the writeback iteration finishes, we do not loop back to the
> +		 * start of the file.  Doing so causes a page lock/page
> +		 * writeback access order inversion - we should only ever lock
> +		 * multiple pages in ascending page->index order, and looping
> +		 * back to the start of the file violates that rule and causes
> +		 * deadlocks.
> +		 */
> +		if (wbc->range_cyclic)
> +			mapping->writeback_index = 0;
> +
> +		/*
> +		 * Return the first error we encountered (if there was any) to
> +		 * the caller.
> +		 */
> +		*error = wbc->saved_err;
> +	}
> +	return folio;
>  
>  done:
>  	folio_batch_release(&wbc->fbatch);
>  	if (wbc->range_cyclic)
>  		mapping->writeback_index = folio->index + folio_nr_pages(folio);
> +	return NULL;
> +}
> +
> +/**
> + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * @mapping: address space structure to write
> + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> + * @writepage: function called for each page
> + * @data: data passed to writepage function
> + *
> + * Return: %0 on success, negative error code otherwise
> + *
> + * Note: please use writeback_iter() instead.
> + */
> +int write_cache_pages(struct address_space *mapping,
> +		      struct writeback_control *wbc, writepage_t writepage,
> +		      void *data)
> +{
> +	struct folio *folio = NULL;
> +	int error;
> +
> +	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> +		error = writepage(folio, wbc, data);
> +		if (error == AOP_WRITEPAGE_ACTIVATE) {
> +			folio_unlock(folio);
> +			error = 0;
> +		}
> +	}
> +
>  	return error;
>  }
>  EXPORT_SYMBOL(write_cache_pages);
> -- 
> 2.39.2
> 
>
  

On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > Refactor the code left in write_cache_pages into an iterator that the
> > file system can call to get the next folio for a writeback operation:
> > 
> > 	struct folio *folio = NULL;
> > 
> > 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > 		error = <do per-foli writeback>;
> > 	}
> > 
> > The twist here is that the error value is passed by reference, so that
> > the iterator can restore it when breaking out of the loop.
> > 
> > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > in preparation for eventually killing it off.
> > 
> > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  include/linux/writeback.h |   4 +
> >  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
> >  2 files changed, 118 insertions(+), 78 deletions(-)
> > 
> ...
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> ...
> > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> >  }
> >  
> >  /**
> ...
> >   */
> > -int write_cache_pages(struct address_space *mapping,
> > -		      struct writeback_control *wbc, writepage_t writepage,
> > -		      void *data)
> > +struct folio *writeback_iter(struct address_space *mapping,
> > +		struct writeback_control *wbc, struct folio *folio, int *error)
> >  {
> ...
> > +	} else {
> >  		wbc->nr_to_write -= folio_nr_pages(folio);
> >  
> > -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > -			folio_unlock(folio);
> > -			error = 0;
> > -		}
> > +		WARN_ON_ONCE(*error > 0);
> 
> Why the warning on writeback error here? It looks like new behavior, but
> maybe I missed something. Otherwise the factoring LGTM.

Err, sorry.. I glossed over the > 0 check and read it as < 0.
Disregard, this seems reasonable to me as long as we no longer expect
those AOP returns (which I'm not really clear on either, but anyways..):

Reviewed-by: Brian Foster <bfoster@redhat.com>

> 
> Brian
> 
> >  
> >  		/*
> >  		 * For integrity writeback we have to keep going until we have
> > @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
> >  		 * wbc->nr_to_write or encounter the first error.
> >  		 */
> >  		if (wbc->sync_mode == WB_SYNC_ALL) {
> > -			if (error && !ret)
> > -				ret = error;
> > +			if (*error && !wbc->saved_err)
> > +				wbc->saved_err = *error;
> >  		} else {
> > -			if (error || wbc->nr_to_write <= 0)
> > +			if (*error || wbc->nr_to_write <= 0)
> >  				goto done;
> >  		}
> >  	}
> >  
> > -	/*
> > -	 * For range cyclic writeback we need to remember where we stopped so
> > -	 * that we can continue there next time we are called.  If  we hit the
> > -	 * last page and there is more work to be done, wrap back to the start
> > -	 * of the file.
> > -	 *
> > -	 * For non-cyclic writeback we always start looking up at the beginning
> > -	 * of the file if we are called again, which can only happen due to
> > -	 * -ENOMEM from the file system.
> > -	 */
> > -	folio_batch_release(&wbc->fbatch);
> > -	if (wbc->range_cyclic)
> > -		mapping->writeback_index = 0;
> > -	return ret;
> > +	folio = writeback_get_folio(mapping, wbc);
> > +	if (!folio) {
> > +		/*
> > +		 * To avoid deadlocks between range_cyclic writeback and callers
> > +		 * that hold pages in PageWriteback to aggregate I/O until
> > +		 * the writeback iteration finishes, we do not loop back to the
> > +		 * start of the file.  Doing so causes a page lock/page
> > +		 * writeback access order inversion - we should only ever lock
> > +		 * multiple pages in ascending page->index order, and looping
> > +		 * back to the start of the file violates that rule and causes
> > +		 * deadlocks.
> > +		 */
> > +		if (wbc->range_cyclic)
> > +			mapping->writeback_index = 0;
> > +
> > +		/*
> > +		 * Return the first error we encountered (if there was any) to
> > +		 * the caller.
> > +		 */
> > +		*error = wbc->saved_err;
> > +	}
> > +	return folio;
> >  
> >  done:
> >  	folio_batch_release(&wbc->fbatch);
> >  	if (wbc->range_cyclic)
> >  		mapping->writeback_index = folio->index + folio_nr_pages(folio);
> > +	return NULL;
> > +}
> > +
> > +/**
> > + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> > + * @mapping: address space structure to write
> > + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> > + * @writepage: function called for each page
> > + * @data: data passed to writepage function
> > + *
> > + * Return: %0 on success, negative error code otherwise
> > + *
> > + * Note: please use writeback_iter() instead.
> > + */
> > +int write_cache_pages(struct address_space *mapping,
> > +		      struct writeback_control *wbc, writepage_t writepage,
> > +		      void *data)
> > +{
> > +	struct folio *folio = NULL;
> > +	int error;
> > +
> > +	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > +		error = writepage(folio, wbc, data);
> > +		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > +			folio_unlock(folio);
> > +			error = 0;
> > +		}
> > +	}
> > +
> >  	return error;
> >  }
> >  EXPORT_SYMBOL(write_cache_pages);
> > -- 
> > 2.39.2
> > 
> > 
> 
>
  

On Tue 06-02-24 09:54:01, Brian Foster wrote:
> On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> > On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > > Refactor the code left in write_cache_pages into an iterator that the
> > > file system can call to get the next folio for a writeback operation:
> > > 
> > > 	struct folio *folio = NULL;
> > > 
> > > 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > > 		error = <do per-foli writeback>;
> > > 	}
> > > 
> > > The twist here is that the error value is passed by reference, so that
> > > the iterator can restore it when breaking out of the loop.
> > > 
> > > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > > in preparation for eventually killing it off.
> > > 
> > > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > > 
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > ---
> > >  include/linux/writeback.h |   4 +
> > >  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
> > >  2 files changed, 118 insertions(+), 78 deletions(-)
> > > 
> > ...
> > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > ...
> > > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > >  }
> > >  
> > >  /**
> > ...
> > >   */
> > > -int write_cache_pages(struct address_space *mapping,
> > > -		      struct writeback_control *wbc, writepage_t writepage,
> > > -		      void *data)
> > > +struct folio *writeback_iter(struct address_space *mapping,
> > > +		struct writeback_control *wbc, struct folio *folio, int *error)
> > >  {
> > ...
> > > +	} else {
> > >  		wbc->nr_to_write -= folio_nr_pages(folio);
> > >  
> > > -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > -			folio_unlock(folio);
> > > -			error = 0;
> > > -		}
> > > +		WARN_ON_ONCE(*error > 0);
> > 
> > Why the warning on writeback error here? It looks like new behavior, but
> > maybe I missed something. Otherwise the factoring LGTM.
> 
> Err, sorry.. I glossed over the > 0 check and read it as < 0.
> Disregard, this seems reasonable to me as long as we no longer expect
> those AOP returns (which I'm not really clear on either, but anyways..):
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>

So my understanding is that AOP_WRITEPAGE_ACTIVATE should be now handled
directly by the caller of ->writepage hook and not by writeback_iter()
which is the reason why the warning is here.

								Honza
  

On Wed, Feb 07, 2024 at 09:42:24AM +0100, Jan Kara wrote:
> On Tue 06-02-24 09:54:01, Brian Foster wrote:
> > On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> > > On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > > > Refactor the code left in write_cache_pages into an iterator that the
> > > > file system can call to get the next folio for a writeback operation:
> > > > 
> > > > 	struct folio *folio = NULL;
> > > > 
> > > > 	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > > > 		error = <do per-foli writeback>;
> > > > 	}
> > > > 
> > > > The twist here is that the error value is passed by reference, so that
> > > > the iterator can restore it when breaking out of the loop.
> > > > 
> > > > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > > > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > > > in preparation for eventually killing it off.
> > > > 
> > > > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > > > 
> > > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > > > ---
> > > >  include/linux/writeback.h |   4 +
> > > >  mm/page-writeback.c       | 192 ++++++++++++++++++++++----------------
> > > >  2 files changed, 118 insertions(+), 78 deletions(-)
> > > > 
> > > ...
> > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > > > --- a/mm/page-writeback.c
> > > > +++ b/mm/page-writeback.c
> > > ...
> > > > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > > >  }
> > > >  
> > > >  /**
> > > ...
> > > >   */
> > > > -int write_cache_pages(struct address_space *mapping,
> > > > -		      struct writeback_control *wbc, writepage_t writepage,
> > > > -		      void *data)
> > > > +struct folio *writeback_iter(struct address_space *mapping,
> > > > +		struct writeback_control *wbc, struct folio *folio, int *error)
> > > >  {
> > > ...
> > > > +	} else {
> > > >  		wbc->nr_to_write -= folio_nr_pages(folio);
> > > >  
> > > > -		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > > -			folio_unlock(folio);
> > > > -			error = 0;
> > > > -		}
> > > > +		WARN_ON_ONCE(*error > 0);
> > > 
> > > Why the warning on writeback error here? It looks like new behavior, but
> > > maybe I missed something. Otherwise the factoring LGTM.
> > 
> > Err, sorry.. I glossed over the > 0 check and read it as < 0.
> > Disregard, this seems reasonable to me as long as we no longer expect
> > those AOP returns (which I'm not really clear on either, but anyways..):
> > 
> > Reviewed-by: Brian Foster <bfoster@redhat.com>
> 
> So my understanding is that AOP_WRITEPAGE_ACTIVATE should be now handled
> directly by the caller of ->writepage hook and not by writeback_iter()
> which is the reason why the warning is here.
> 

Yeah, I wasn't really familiar with the AOP error codes, saw that
multiple existed and just assumed they might be arbitrarily relevant
across different aop callbacks (and so then filtered the check/clear for
WRITEPAGE_ACTIVATE out of my brain ;). On taking a closer look, it seems
like the only other one (AOP_TRUNCATED_PAGE) doesn't have any relevance
to ->writepage(), so this all makes more sense to me now. Thanks.

Brian

> 								Honza
> -- 
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
>