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[2a03:2880:31ff:19::face:b00c]) by smtp.gmail.com with ESMTPSA id i13-20020a05640200cd00b00559c71e70e3sm1547951edu.79.2024.01.17.09.11.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Jan 2024 09:11:09 -0800 (PST) From: Breno Leitao To: Muchun Song , Andrew Morton Cc: lstoakes@gmail.com, willy@infradead.org, mike.kravetz@oracle.com, Rik van Riel , linux-mm@kvack.org (open list:HUGETLB SUBSYSTEM), linux-kernel@vger.kernel.org (open list) Subject: [PATCH 1/2] mm/hugetlb: Restore the reservation if needed Date: Wed, 17 Jan 2024 09:10:57 -0800 Message-Id: <20240117171058.2192286-1-leitao@debian.org> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1788358442355505237 X-GMAIL-MSGID: 1788358442355505237 Currently there is a bug that a huge page could be stolen, and when the original owner tries to fault in it, it causes a page fault. You can achieve that by: 1) Creating a single page echo 1 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages 2) mmap() the page above with MAP_HUGETLB into (void *ptr1). * This will mark the page as reserved 3) touch the page, which causes a page fault and allocates the page * This will move the page out of the free list. * It will also unreserved the page, since there is no more free page 4) madvise(MADV_DONTNEED) the page * This will free the page, but not mark it as reserved. 5) Allocate a secondary page with mmap(MAP_HUGETLB) into (void *ptr2). * it should fail, but, since there is no more available page. * But, since the page above is not reserved, this mmap() succeed. 6) Faulting at ptr1 will cause a SIGBUS * it will try to allocate a huge page, but there is none available A full reproducer is in selftest. See https://lore.kernel.org/all/20240105155419.1939484-1-leitao@debian.org/ Fix this by restoring the reserved page if necessary. If the page being unmapped has HPAGE_RESV_OWNER set, and needs a reservation, set the restore_reserve flag, which will move the page from free to reserved. Suggested-by: Rik van Riel Signed-off-by: Breno Leitao --- mm/hugetlb.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index ed1581b670d4..fa2c17767e44 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5677,6 +5677,16 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma, hugetlb_count_sub(pages_per_huge_page(h), mm); hugetlb_remove_rmap(page_folio(page)); + if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) && + vma_needs_reservation(h, vma, start)) { + /* + * Restore the reservation if needed, otherwise the + * backing page could be stolen by someone. + */ + folio_set_hugetlb_restore_reserve(page_folio(page)); + vma_add_reservation(h, vma, address); + } + spin_unlock(ptl); tlb_remove_page_size(tlb, page, huge_page_size(h)); /*