On 09/01/22 18:41, Baolin Wang wrote: > On some architectures (like ARM64), it can support CONT-PTE/PMD size > hugetlb, which means it can support not only PMD/PUD size hugetlb > (2M and 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size > specified. > > So when looking up a CONT-PTE size hugetlb page by follow_page(), it > will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE > size hugetlb in follow_page_pte(). However this pte entry lock is incorrect > for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to > get the correct lock, which is mm->page_table_lock. > > That means the pte entry of the CONT-PTE size hugetlb under current > pte lock is unstable in follow_page_pte(), we can continue to migrate > or poison the pte entry of the CONT-PTE size hugetlb, which can cause > some potential race issues, even though they are under the 'pte lock'. > > For example, suppose thread A is trying to look up a CONT-PTE size > hugetlb page by move_pages() syscall under the lock, however antoher > thread B can migrate the CONT-PTE hugetlb page at the same time, which > will cause thread A to get an incorrect page, if thread A also wants to > do page migration, then data inconsistency error occurs. > > Moreover we have the same issue for CONT-PMD size hugetlb in > follow_huge_pmd(). > > To fix above issues, rename the follow_huge_pmd() as follow_huge_pmd_pte() > to handle PMD and PTE level size hugetlb, which uses huge_pte_lock() to > get the correct pte entry lock to make the pte entry stable. > > Cc: <stable@xxxxxxxxxxxxxxx> > Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> > --- > Changes from v2: > - Combine PMD and PTE level hugetlb handling into one function. > - Drop unnecessary patches. > - Update the commit message. Baolin, were you able to at least exercise the new code paths? Especially the path for CONT_PTE. Code looks fine to me. Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> It is a little hackish, but this is only for backports. So, I think it is OK. We may want to point out that code cleanup and simplification is going upstream that will address these issues in a more elegant manner. > > Mike, please fold this patch into your series. Thanks. If I understand Andrew, this can go in as a separate patch for backport to address potential bugs. I will provide a cleanup/simplification that will remove this going forward. Andrew also asked for a Fixes tag. Support for CONT_PMD/_PTE was added with bb9dd3df8ee9 "arm64: hugetlb: refactor find_num_contig()". Patch series "Support for contiguous pte hugepages", v4. However, I do not believe these code paths were executed until migration support was added with 5480280d3f2d "arm64/mm: enable HugeTLB migration for contiguous bit HugeTLB pages" I would go with 5480280d3f2d. -- Mike Kravetz
