diff mbox series

[11/13] ext4: correct gdblock calculation in add_new_gdb_meta_bg to support non first group

Message ID 20230629120044.1261968-12-shikemeng@huaweicloud.com
State New
Headers
Series fixes and cleanups to ext4 resize |

Commit Message

Kemeng Shi June 29, 2023, noon UTC 
  In add_new_gdb_meta_bg, we assume that group could be non first
group in meta block group as we call ext4_meta_bg_first_block_no
to get first block of meta block group rather than call
ext4_group_first_block_no for passed group directly. Then ext4_bg_has_super
should be called with first group in meta group rather than new added
group. Or we can call ext4_group_first_block_no instead of
ext4_meta_bg_first_block_no to assume only first group of
meta group will be passed.
Either way, ext4_meta_bg_first_block_no will be useless and
could be removed.

This patch do it in first way to make add_new_gdb_meta_bg support non
first group in meta block group.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
---
 fs/ext4/resize.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

Comments

Kemeng Shi Aug. 17, 2023, 3:38 a.m. UTC | #1 
on 8/16/2023 11:45 AM, Theodore Ts'o wrote:
> On Thu, Jun 29, 2023 at 08:00:42PM +0800, Kemeng Shi wrote:
>> In add_new_gdb_meta_bg, we assume that group could be non first
>> group in meta block group as we call ext4_meta_bg_first_block_no
>> to get first block of meta block group rather than call
>> ext4_group_first_block_no for passed group directly. Then ext4_bg_has_super
>> should be called with first group in meta group rather than new added
>> group. Or we can call ext4_group_first_block_no instead of
>> ext4_meta_bg_first_block_no to assume only first group of
>> meta group will be passed.
>> Either way, ext4_meta_bg_first_block_no will be useless and
>> could be removed.
> 
> Unfortunately, I spent more time trying to understand the commit
> description than the C code.  Perhaps this might be a better way of
> describing the situation?
> 
Sorry for my poor language again, :( I will try to improve this.
> The ext4_new descs() function calls ext4_meta_bg_first_block_no() with
> the group paramter when the group is the first group of a meta_bg
> (e.g., when (group % EXT4_DESC_PER_BLOCK) is zero.  So we can simplify
> things a bit by removing ext4_meta_bg_first_block_no() and an open
> coding its logic.
> 
> Does this make more sense to tou?
> 
This patch tries to correct gdbblock calculation in add_new_gdb_meta_bg
in case group from caller is not the first group of meta_bg which is
supposed to be handled by add_new_gdb_meta_bg.
We should call ext4_bg_has_super with first group in meta_bg instead
of group which could be non first group in meta_bg to calculate gdb
of meta_bg.
Fortunately, the only caller ext4_add_new_descs always call
add_new_gdb_meta_bg with first group of meta_bg and no real issue
will happen.


> 					- Ted
>
Kemeng Shi Aug. 22, 2023, 2:48 a.m. UTC | #2 
on 8/19/2023 12:54 AM, Theodore Ts'o wrote:
> On Fri, Aug 18, 2023 at 03:09:35PM +0800, Kemeng Shi wrote:
>>> Is following comment looks good to you:
>>
>> When all reserved primary blocks are consumed, we create meta_bg group and
>> allocate new primary block at first block or block after backup superblock
>> (if exsiting) in first group of meta_bg group.
>> This function is only called when first group of meta_bg is added.
> 
> Well, it's possible to create a file system where all of the block
> group descriptors use meta_bg, and there are no "traditional" block
> group descriptors.  And so what happens is if there is no available
> space in the existing block group descriptors for the new block group,
> and there are no reserved block group descriptors (I'd remove
> "primary" as that's not something that we've used traditionally), then
> what happens is that the meta_bg feature will get enabled, and 
> es->s_first_meta_bg will get set to the first block group that is
> managed using meta_bg.  s_first_meta_bg must be a multiple of
> EXT4_DESC_PER_BLOCK(sb).
> 
> Some of this is documented in Documentation/filesystems/ext4/blockgroup.rst
> already.
>
As these information into comment of add_new_gdb_meta_bg could help to some
dgree. I summary the information to:

If there is no available space in the existing block group descriptors for
the new block group and there are no reserved block group descriptors, then
the meta_bg feature will get enabled, and es->s_first_meta_bg will get set
to the first block group that is managed using meta_bg and s_first_meta_bg
must be a multiple of EXT4_DESC_PER_BLOCK(sb).
This function will be called when first group of meta_bg is added to bring
new group descriptors block of new added meta_bg.

Or I will leave comments unchange in next version if it's redundant to you.
Thanks!
> Cheers,
> 
> 						- Ted
>
diff mbox series

Patch

diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
index b9507e432496..da832466ce74 100644
--- a/fs/ext4/resize.c
+++ b/fs/ext4/resize.c
@@ -110,12 +110,6 @@  static ext4_group_t ext4_meta_bg_first_group(struct super_block *sb,
 	       EXT4_DESC_PER_BLOCK_BITS(sb);
 }
 
-static ext4_fsblk_t ext4_meta_bg_first_block_no(struct super_block *sb,
-					     ext4_group_t group) {
-	group = ext4_meta_bg_first_group(sb, group);
-	return ext4_group_first_block_no(sb, group);
-}
-
 static ext4_grpblk_t ext4_group_overhead_blocks(struct super_block *sb,
 						ext4_group_t group) {
 	ext4_grpblk_t overhead;
@@ -954,8 +948,9 @@  static int add_new_gdb_meta_bg(struct super_block *sb,
 	unsigned long gdb_num = group / EXT4_DESC_PER_BLOCK(sb);
 	int err;
 
-	gdblock = ext4_meta_bg_first_block_no(sb, group) +
-		   ext4_bg_has_super(sb, group);
+	group = ext4_meta_bg_first_group(sb, group);
+	gdblock = ext4_group_first_block_no(sb, group) +
+		  ext4_bg_has_super(sb, group);
 	gdb_bh = ext4_sb_bread(sb, gdblock, 0);
 	if (IS_ERR(gdb_bh))
 		return PTR_ERR(gdb_bh);