Re: [PATCH] mm/huge_memory: fix dereferencing invalid pmd migration entry

[Zi Yan]

On 17 Apr 2025, at 1:29, Hugh Dickins wrote:

> On Tue, 15 Apr 2025, Zi Yan wrote:
>>
>> Anyway, we need to figure out why both THP migration and deferred_split_scan()
>> hold the THP lock first, which sounds impossible to me. Or some other execution
>> interleaving is happening.
>
> I think perhaps you're missing that an anon_vma lookup points to a
> location which may contain the folio of interest, but might instead
> contain another folio: and weeding out those other folios is precisely
> what the "folio != pmd_folio((*pmd)" check (and the "risk of replacing
> the wrong folio" comment a few lines above it) is for.

Yes, from Gavin’s commit log, I thought both migration and deferred split
are working on the same folio. But after reread it along with your explanation,
now I understand that both are working on the same pmd migration entry.

Thank you for the explanation.

>
> The "BUG: unable to handle page fault" comes about because that other
> folio might actually be being migrated at this time, so we encounter
> a PMD migration entry instead of a valid PMD entry.  But if it's the
> folio we're looking for, our folio lock excludes a racing migration,
> so it would never be a PMD migration entry for our folio.
>
> Hugh


Best Regards,
Yan, Zi