Re: [PATCH] mm: migration :shared anonymous migration test is failing

Baolin Wang
2024-12-20T02:31:49Z

On 2024/12/19 20:47, Donet Tom wrote:
> The migration selftest is currently failing for shared anonymous
> mappings due to a race condition.
>
> During migration, the source folio's PTE is unmapped by nuking the
> PTE, flushing the TLB,and then marking the page for migration
> (by creating the swap entries). The issue arises when, immediately
> after the PTE is nuked and the TLB is flushed, but before the page
> is marked for migration, another thread accesses the page. This
> triggers a page fault, and the page fault handler invokes
> do_pte_missing() instead of do_swap_page(), as the page is not yet
> marked for migration.
>
> In the fault handling path, do_pte_missing() calls __do_fault()
> ->shmem_fault() -> shmem_get_folio_gfp() -> filemap_get_entry().
> This eventually calls folio_try_get(), incrementing the reference
> count of the folio undergoing migration. The thread then blocks
> on folio_lock(), as the migration path holds the lock. This
> results in the migration failing in __migrate_folio(), which expects
> the folio's reference count to be 2. However, the reference count is
> incremented by the fault handler, leading to the failure.
>
> The issue arises because, after nuking the PTE and before marking the
> page for migration, the page is accessed. To address this, we have
> updated the logic to first nuke the PTE, then mark the page for
> migration, and only then flush the TLB. With this patch, If the page is
> accessed immediately after nuking the PTE, the TLB entry is still
> valid, so no fault occurs. After marking the page for migration,

IMO, I don't think this assumption is correct. At this point, the TLB
entry might also be evicted, so a page fault could still occur. It's
just a matter of probability.

Additionally, IIUC, if another thread is accessing the shmem folio
causing the migration to fail, I think this is expected, and migration
failure is not a vital issue?