Re: [PATCH 2/7] mm/gup: check ref_count instead of lru before migration

[David Hildenbrand <david@redhat.com>]

On 31.08.25 11:05, Hugh Dickins wrote:
> Will Deacon reports:-
> 
> When taking a longterm GUP pin via pin_user_pages(),
> __gup_longterm_locked() tries to migrate target folios that should not
> be longterm pinned, for example because they reside in a CMA region or
> movable zone. This is done by first pinning all of the target folios
> anyway, collecting all of the longterm-unpinnable target folios into a
> list, dropping the pins that were just taken and finally handing the
> list off to migrate_pages() for the actual migration.
> 
> It is critically important that no unexpected references are held on the
> folios being migrated, otherwise the migration will fail and
> pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is
> relatively easy to observe migration failures when running pKVM (which
> uses pin_user_pages() on crosvm's virtual address space to resolve
> stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and
> this results in the VM terminating prematurely.
> 
> In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its
> mapping of guest memory prior to the pinning. Subsequently, when
> pin_user_pages() walks the page-table, the relevant 'pte' is not
> present and so the faulting logic allocates a new folio, mlocks it
> with mlock_folio() and maps it in the page-table.
> 
> Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page()
> batch by pagevec"), mlock/munlock operations on a folio (formerly page),
> are deferred. For example, mlock_folio() takes an additional reference
> on the target folio before placing it into a per-cpu 'folio_batch' for
> later processing by mlock_folio_batch(), which drops the refcount once
> the operation is complete. Processing of the batches is coupled with
> the LRU batch logic and can be forcefully drained with
> lru_add_drain_all() but as long as a folio remains unprocessed on the
> batch, its refcount will be elevated.
> 
> This deferred batching therefore interacts poorly with the pKVM pinning
> scenario as we can find ourselves in a situation where the migration
> code fails to migrate a folio due to the elevated refcount from the
> pending mlock operation.
> 
> Hugh Dickins adds:-
> 
> !folio_test_lru() has never been a very reliable way to tell if an
> lru_add_drain_all() is worth calling, to remove LRU cache references
> to make the folio migratable: the LRU flag may be set even while the
> folio is held with an extra reference in a per-CPU LRU cache.
> 
> 5.18 commit 2fbb0c10d1e8 may have made it more unreliable.  Then 6.11
> commit 33dfe9204f29 ("mm/gup: clear the LRU flag of a page before adding
> to LRU batch") tried to make it reliable, by moving LRU flag clearing;
> but missed the mlock/munlock batches, so still unreliable as reported.
> 
> And it turns out to be difficult to extend 33dfe9204f29's LRU flag
> clearing to the mlock/munlock batches: if they do benefit from batching,
> mlock/munlock cannot be so effective when easily suppressed while !LRU.
> 
> Instead, switch to an expected ref_count check, which was more reliable
> all along: some more false positives (unhelpful drains) than before, and
> never a guarantee that the folio will prove migratable, but better.
> 
> Note for stable backports: requires 6.16 commit 86ebd50224c0 ("mm:
> add folio_expected_ref_count() for reference count calculation") and
> 6.17 commit ("mm: fix folio_expected_ref_count() when PG_private_2").
> 
> Reported-by: Will Deacon <will@kernel.org>
> Link: https://lore.kernel.org/linux-mm/20250815101858.24352-1-will@kernel.org/
> Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: <stable@vger.kernel.org>
> ---
>   mm/gup.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index adffe663594d..82aec6443c0a 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_unpinnable_folios(
>   			continue;
>   		}
>   
> -		if (!folio_test_lru(folio) && drain_allow) {
> +		if (drain_allow && folio_ref_count(folio) !=
> +				   folio_expected_ref_count(folio) + 1) {
>   			lru_add_drain_all();
>   			drain_allow = false;
>   		}

In general, to the fix idea

	Acked-by: David Hildenbrand <david@redhat.com>

But as raised in reply to patch #1, we have to be a bit careful about 
including private_2 in folio_expected_ref_count() at this point.

If we cannot include it in folio_expected_ref_count(), it's all going to 
be a mess until PG_private_2 is removed for good.

So that part still needs to be figured out.

-- 
Cheers

David / dhildenb