Re: [PATCH v2] mm/hugetlb: fix subpool accounting after cgroup charge failure

[Zhao Li <enderaoelyther@gmail.com>]

On Tue, Apr 28, 2026 at 11:08:04AM +0200, Oscar Salvador wrote:
> I found that last sentence misleading, because we do not really
> care about hugetlb cgroup charge/uncharge (besides that being of
> the reasons we end up on error path) but rather the fact that we
> fiddle with subpool->used_hpages and we need to undo that when we
> rollback.

Agreed - reframed in v3.  The commit body now states the bug as
the unwind missing the used_hpages rollback, without pinning it to
the cgroup-charge case, and the subject is narrowed to "fix
max-only subpool accounting on alloc_hugetlb_folio failure".

> Well, that does not quite explain the problem I think, at least
> not clear enough?  [...]

Fair - that explanation got tangled because v2's design itself was
trying to compensate for racing min crossings.  v3 sidesteps it
entirely: the gbl_chg > 0 cleanup is now restricted to
(max_hpages != -1, min_hpages == -1).  In that configuration
hugepage_subpool_put_pages()'s min-restoration branch is dead, so a
direct used_hpages-- under spool->lock is the exact inverse of the
speculative bump - no h->resv_huge_pages++ needed, no rsv_hpages
publication, no racing-put reasoning.

Mounts with min_hpages != -1 are left at v1 behaviour for now.
That quadrant has an inherited race that also exists at
hugetlb_reserve_pages()'s out_put_pages cleanup, so a coordinated
fix belongs in a separate RFC rather than this stable backport.

> I would split the comment in two parts and place them within the
> block they belong, otherwise it sounds confusing.
>
> Subpools, reservations and hugetlb make a very head-spinning
> situation, so let us make our life easier.

Done - one short comment per branch placed inside the relevant
code block in v3.  Hopefully easier to follow now.

v3:
  https://lore.kernel.org/linux-mm/20260428113037.88766-2-enderaoelyther@gmail.com/

Thanks for the review.

--
Zhao Li