Re: [PATCH 1/1] mm: handle swap page faults if the faulting page can be locked

[Matthew Wilcox]

On Fri, Apr 14, 2023 at 11:00:43AM -0700, Suren Baghdasaryan wrote:
> When page fault is handled under VMA lock protection, all swap page
> faults are retried with mmap_lock because folio_lock_or_retry
> implementation has to drop and reacquire mmap_lock if folio could
> not be immediately locked.
> Instead of retrying all swapped page faults, retry only when folio
> locking fails.

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

Let's just review what can now be handled under the VMA lock instead of
the mmap_lock, in case somebody knows better than me that it's not safe.

 - We can call migration_entry_wait().  This will wait for PG_locked to
   become clear (in migration_entry_wait_on_locked()).  As previously
   discussed offline, I think this is safe to do while holding the VMA
   locked.
 - We can call remove_device_exclusive_entry().  That calls
   folio_lock_or_retry(), which will fail if it can't get the VMA lock.
 - We can call pgmap->ops->migrate_to_ram().  Perhaps somebody familiar
   with Nouveau and amdkfd could comment on how safe this is?
 - I believe we can't call handle_pte_marker() because we exclude UFFD
   VMAs earlier.
 - We can call swap_readpage() if we allocate a new folio.  I haven't
   traced through all this code to tell if it's OK.

So ... I believe this is all OK, but we're definitely now willing to
wait for I/O from the swap device while holding the VMA lock when we
weren't before.  And maybe we should make a bigger deal of it in the
changelog.

And maybe we shouldn't just be failing the folio_lock_or_retry(),
maybe we should be waiting for the folio lock with the VMA locked.