Re: [PATCH v3] mm: migrate: requeue destination folio on deferred split queue

[Andrew Morton]

On Fri, 13 Mar 2026 13:40:29 +0300 Usama Arif <usama.arif@xxxxxxxxx> wrote:

> 
> 
> On 13/03/2026 03:52, Andrew Morton wrote:
> > On Thu, 12 Mar 2026 17:16:30 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:
> > 
> >>> By the time migrate_folio_move() runs, partially mapped folios without a
> >>> pin have already been split by migrate_pages_batch().  So only two cases
> >>> remain on the deferred list at this point:
> >>>   1. Partially mapped folios with a pin (split failed).
> >>>   2. Fully mapped but potentially underused folios.
> >>> The recorded partially_mapped state is forwarded to deferred_split_folio()
> >>> so that the destination folio is correctly re-queued in both cases.
> >>>
> >>> Reported-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> >>> Fixes: dafff3f4c850 ("mm: split underused THPs")
> >>
> >> Seems the commit is merged in 6.12.  And I assume the user impact on
> >> THP-shrinker enabled systems is visible.  If so, should we Cc stable@ ?
> > 
> > I think the user impact should be visible to backport, but the
> > changelog is elusive on details?
> > 
> 
> 
> The original patches added THPs to deferred_list at fault/collapse, they
> got removed but not added back to the list after migration.
> This patch adds them to the deferred_list on migration. The user would
> not expect the THPs to get removed from deferred_list on migration, so
> this fixes user expectations.

Maybe users just won't notice?

If we can't identify any benefit to userspace then I don't think this
patch meets the criteria for backporting.

> I have CC-ed stable@xxxxxxxxxxxxxxx to this email. Should I resend the patch
> with CC stable in commit message?

That's OK, I update changelogs.  A lot.