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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=KES7eykU; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 28A04606A31C; Wed, 28 Oct 2026 15:10:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=KES7eykU Received: by smtp.kernel.org (Postfix) with ESMTPSA id 093A41F01828; Wed, 28 Oct 2026 15:10:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793200203; bh=DKgJKWTBOJzl2PJZHSD17HbgcFOWBXCXBURY+ctwIP8=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=KES7eykUL+zeWVcVO1skU8I0d52Ib30WHpZ9YCxGj0MVaBt1znSoXAttcUK2mTMJV 8mXHsKjiSoQkWQ+brKDsuhLMVMz/36Zkj/UYd1BSbQxpM6h0TVaz+aEtfbDS7O9CK8 /MHalScRJSfZpJdXd6s1JRmuFxHhOZgjujKcJkMdhYd5PfOV54WPLaNkKx5K5vAvPu y1IKgg8CgsR68JTHAvyCSjn5Z4STBz0Dl/6LxMOXOdw7ErIj3BN977WmNZm9EL51tW YgPuLxmgdrqX79Y183LOW74kPEJgOKh2LMbgw7lFpcyjUvVHsrOWw8KVsDvv7uLPOQ EJHrU3wxZr3hw== Message-ID: <477298f9-59cb-439f-a4ea-b662bdb3154d@kernel.org> Date: Wed, 28 Oct 2026 16:09:59 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/hugetlb: fix max-only subpool accounting on alloc_hugetlb_folio failure From: "David Hildenbrand (Arm)" To: Zhao Li , Andrew Morton Cc: mawupeng1@huawei.com, Muchun Song , Oscar Salvador , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260427145247.84157-2-enderaoelyther@gmail.com> <20260428030712.66256-2-enderaoelyther@gmail.com> <20260428113037.88766-2-enderaoelyther@gmail.com> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/13/26 17:04, David Hildenbrand (Arm) wrote: > On 4/28/26 13:30, Zhao Li wrote: >> 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@vger.kernel.org # v6.15+ >> Signed-off-by: Zhao Li >> --- >> 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) { > > > I agree with Muchuns comment about removing the "gbl_chg > 0" check. > > You could consider the following structure, which reduces indentation and avoids > touching the existing handling. > > if (map_chg && !gbl_chg) { > ... > } else if (map_chg && spool && spool->min_hpages == -1 && > spool->max_hpages != -1) { > ... > } > > But the code would read a lot nicer as > > if (map_chg && !gbl_chg) { > ... > } else if (map_chg) { > hugepage_subpool_XXX_pages() > } > > Whereby the spool checks etc are handled in there. > > Muchun things that we might just be able to use hugepage_subpool_put_pages(). It > that's possible, great. Otherwise create a new helper that wraps all the logic > below. > Any update on this? -- Cheers, David