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

[David Hildenbrand]

On 20.12.24 03:55, Donet Tom wrote:
> On 12/19/24 18:28, David Hildenbrand wrote:
> > On 19.12.24 13:47, 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.
> >
> > But what about if the PTE is not in the TLB yet, and you get an access
> > from another CPU just after clearing the PTE (but not flushing the
> > TLB)? The other CPU will still observe PTE=none, trigger a fault etc.
> Yes, in this scenario, the migration will fail. Do you think the
> migration test
> failure, even after a retry, should be considered a major issue that
> must be fixed?

I think it is something we should definitely improve, but I think our 
page migration should handle this in a better way, not the unmap logic. 
I recall we discussed with Dev some ideas on how to improve that?


I'm pretty sure one can trigger similar case using a tmpfs file and 
using read/write in a loop instead of memory access -> page faults. So 
where racing with page faults is completely out of the picture.

--
Cheers,

David / dhildenb