Re: [PATCH v2] khugepaged: Reduce race probability between migration and khugepaged

[Dev Jain]

On 03/07/25 2:55 pm, David Hildenbrand wrote:
> On 03.07.25 07:48, Dev Jain wrote:
> > Suppose a folio is under migration, and khugepaged is also trying to
> > collapse it. collapse_pte_mapped_thp() will retrieve the folio from the
> > page cache via filemap_lock_folio(), thus taking a reference on the 
> > folio
> > and sleeping on the folio lock, since the lock is held by the migration
> > path. Migration will then fail in
> > __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of
> > such a race happening (leading to migration failure) by bailing out
> > if we detect a PMD is marked with a migration entry.
> >
> > This fixes the migration-shared-anon-thp testcase failure on Apple M3.
> >
> > Note that, this is not a "fix" since it only reduces the chance of
> > interference of khugepaged with migration, wherein both the kernel
> > functionalities are deemed "best-effort".
> >
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >
> > v1->v2:
> >   - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, 
> > Anshuman)
> >   - Add a comment (Lorenzo)
> >
> > v1:
> >   - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@xxxxxxx/
> >
> >   mm/khugepaged.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 1aa7ca67c756..3fdefc4f4984 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -941,6 +941,15 @@ static inline int check_pmd_state(pmd_t *pmd)
> >         if (pmd_none(pmde))
> >           return SCAN_PMD_NONE;
> > +
> > +    /*
> > +     * The folio may be under migration when khugepaged is trying to
> > +     * collapse it. Migration success or failure will eventually end
> > +     * up with the PMD still pointing to a PMD-order folio, so return
> > +     * SCAN_PMD_MAPPED.
>
> Nit: the last part (, so return ..) is obvious from the code.
>
> I would have written
>
> /*
>  * The folio may be under migration when khugepaged is trying to
>  * collapse it. Migration success or failure will eventually end
>  * up with a present PMD entry again.
>  */
Thanks for the suggestion, but

PMD pointing to PMD-order folio necessarily implies present PMD entry,

but the converse is not true? For example it may point to a PTE table.

So I wanted to make it explicitly clear that the transient entry will be

converted to a PMD-THP in the end.


> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Thanks!