Re: [PATCH] mm/migrate: fix hugetlbfs deadlock by respecting lock ordering

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Jinchao Wang <wangjinchao600@gmail.com> writes:

> Fix an AB-BA deadlock between hugetlbfs_punch_hole() and page migration.
>
> The deadlock occurs because migration violates the lock ordering defined
> in mm/rmap.c for hugetlbfs:
>
>   * hugetlbfs PageHuge() take locks in this order:
>   * hugetlb_fault_mutex
>   * vma_lock
>   * mapping->i_mmap_rwsem
>   * folio_lock
>
> The following trace illustrates the inversion:
>
> Task A (punch_hole):             Task B (migration):
> --------------------             -------------------
> 1. i_mmap_lock_write(mapping)    1. folio_lock(folio)
> 2. folio_lock(folio)             2. i_mmap_lock_read(mapping)
>    (blocks waiting for B)           (blocks waiting for A)
>
> Task A is blocked in the punch-hole path:
>   hugetlbfs_fallocate
>     hugetlbfs_punch_hole
>       hugetlbfs_zero_partial_page
>         folio_lock
>
> Task B is blocked in the migration path:
>   migrate_pages
>     unmap_and_move_huge_page
>       remove_migration_ptes
>         __rmap_walk_file
>           i_mmap_lock_read
>
> To fix this, adjust unmap_and_move_huge_page() to respect the established
> hierarchy. If i_mmap_rwsem is acquired during try_to_migrate(), hold it
> until remove_migration_ptes() completes.
>
> This utilizes the existing retry logic, which unlocks the folio and
> returns -EAGAIN if hugetlb_folio_mapping_lock_write() fails.
>
> Link: https://lore.kernel.org/all/68e9715a.050a0220.1186a4.000d.GAE@google.com/
> Link: https://lore.kernel.org/all/20260108123957.1123502-2-wangjinchao600@gmail.com
> Reported-by: syzbot+2d9c96466c978346b55f@syzkaller.appspotmail.com
> Suggested-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>

Can you provide a "Fixes:" tag?  That is helpful for backporting the bug
fix.

---
Best Regards,
Huang, Ying