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

[Jinchao Wang <wangjinchao600@gmail.com>]

On Fri, Jan 09, 2026 at 03:18:37PM +0100, David Hildenbrand (Red Hat) wrote:
> On 1/9/26 15:16, Jinchao Wang wrote:
> > On Fri, Jan 09, 2026 at 02:39:08PM +0100, David Hildenbrand (Red Hat) wrote:
> > > On 1/9/26 04:47, Jinchao Wang wrote:
> > > > 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
> > > 
> > > 
> > > I'm confused. Isn't it unmap_and_move_huge_page() that grabs the
> > > i_mmap_rwsem during hugetlb_page_mapping_lock_write() (where we do a
> > > try-lock)?
> > Yes, but the lock is released before remove_migration_ptes().
> > 
> > Task A can enter the race window between
> > 	i_mmap_unlock_write(mapping)
> > and
> > 	remove_migration_ptes() -> i_mmap_lock_read(mapping).
> > 
> > This window was introduced by the change below:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/diff/mm/migrate.c?id=336bf30eb765
> 
> try_to_migrate() is not the problem, but remove_migration_ptes() ?
> 
> Anyhow, I saw that Willy sent out a version.
Thank you for letting me know.

> 
> -- 
> Cheers
> 
> David