Re: [PATCH v3] mm/hugetlb: fix max-only subpool accounting on alloc_hugetlb_folio failure

[Ackerley Tng]

Zhao Li <enderaoelyther@xxxxxxxxx> writes:

> alloc_hugetlb_folio() calls hugepage_subpool_get_pages() when map_chg
> is set.  For a subpool with max_hpages != -1, that bumps used_hpages
> regardless of whether it returns gbl_chg = 0 (rsv slot consumed) or
> gbl_chg > 0 (used_hpages slot only).  If the allocation later fails
> before a folio is returned, the unwind must undo the used_hpages
> bump.  The old cleanup only ran for !gbl_chg, leaking used_hpages on
> the gbl_chg > 0 path.
>
> For gbl_chg > 0 on max-only subpools (max_hpages != -1, min_hpages
> == -1), hugepage_subpool_get_pages() took only a speculative
> used_hpages slot.  Drop that slot directly under spool->lock.  In
> that configuration hugepage_subpool_put_pages() cannot restore
> rsv_hpages, so the direct decrement is the exact inverse and is
> race-free against concurrent puts.  This matches the used_hpages-only
> part of hugetlb_reserve_pages()'s out_put_pages cleanup, but
> restricts it to the max-only case where no rsv_hpages restoration is
> possible.
>
> Mounts with min_hpages != -1 are left unchanged for now.  v2's
> approach (hugepage_subpool_put_pages() + h->resv_huge_pages++ to
> back a restored rsv_hpages slot) double-counts global backing under
> concurrent free_huge_folio() and creates phantom reservations under
> concurrent hugetlb_unreserve_pages().  Safe cleanup of that quadrant
> needs a coordinated fix across multiple call sites.
>
> Reproduced on size=20M hugetlbfs with the faulting task in a hugetlb
> cgroup whose limit is exceeded.  Vanilla leaks 6/8 hugepages of
> subpool quota; this patch leaks 0/8.  Verified under QEMU.
>
> Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
> Cc: stable@xxxxxxxxxxxxxxx # v6.15+
> Signed-off-by: Zhao Li <enderaoelyther@xxxxxxxxx>
> ---
> Changes in v3:
> - Replace v2's hugepage_subpool_put_pages() + h->resv_huge_pages++ on
>   the gbl_chg > 0 branch with a direct used_hpages-- under spool->lock.
> - Restrict the cleanup to (max_hpages != -1, min_hpages == -1) where
>   the direct decrement is the exact inverse of the speculative bump.
>
> Changes in v2:
> - Skip the gbl_chg > 0 cleanup when max_hpages is unset.
> - Add hugepage_subpool_put_pages() + h->resv_huge_pages++ on the
>   gbl_chg > 0 branch.
>
>  mm/hugetlb.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f24bf49be047e..cfdeaf6394c5b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3025,13 +3025,24 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
>  		hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h),
>  						    h_cg);
>  out_subpool_put:
> -	/*
> -	 * put page to subpool iff the quota of subpool's rsv_hpages is used
> -	 * during hugepage_subpool_get_pages.
> -	 */
> -	if (map_chg && !gbl_chg) {
> -		gbl_reserve = hugepage_subpool_put_pages(spool, 1);
> -		hugetlb_acct_memory(h, -gbl_reserve);
> +	if (map_chg) {
> +		if (!gbl_chg) {
> +			/* Full inverse when subpool_get_pages() consumed rsv_hpages. */
> +			gbl_reserve = hugepage_subpool_put_pages(spool, 1);
> +			hugetlb_acct_memory(h, -gbl_reserve);
> +		} else if (gbl_chg > 0 && spool && spool->min_hpages == -1 &&
> +			   spool->max_hpages != -1) {
> +			unsigned long flags;
> +
> +			/*
> +			 * For max-only subpools, subpool_get_pages() took only a
> +			 * speculative used_hpages slot. Drop that slot directly.
> +			 */
> +			spin_lock_irqsave(&spool->lock, flags);
> +			if (spool->used_hpages > 0)
> +				spool->used_hpages--;
> +			unlock_or_release_subpool(spool, flags);
> +		}
>  	}
>
>
> --
> 2.50.1 (Apple Git-155)

This fixes out_subpool_put in alloc_hugetlb_folio() on a mount that
specifies max_size only, and does not introduce any further bugs AFAICT.

Tested-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Further details at [1].

[1] https://lore.kernel.org/all/CAEvNRgHQLifmcD1NO6cd7EVcCiQb6gJgNUo4HkFhZAvzuwMq0w@xxxxxxxxxxxxxx/