From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id BEFEE30594F; Fri, 30 Jan 2026 08:11:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769760666; cv=none; b=if57Tx9D8awCkqmuh0C7oL9P1OtK30S6OOvNRIjFbL2xpcEhY+CvOjdZfCUd4IkC3pLWGgjXLgYYFOFBWyKf1LGzTIhJ2Ve+rb+hKJetYtWG1TdXNoL7edCr5oXKALLg0ptVMC4ZK9ncJq0/j25xnYBf9kOCA6JgtrnmtZ1hUJQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769760666; c=relaxed/simple; bh=/s65cuwG2s7quRiocr+iGjbTPoSdj2e6uUzazyRUKBI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nffS46tY2rYB3lpWd3toY7uN+tcN3tCjV3TuOJCklErnKMCRyYbs/bWAIamovRtN6NUyr9T6XYkCcL0rMn1o/7B172hxutU4xU/2BvyZ4ek5JC7gdLUxkDgYTbbHH5sGw/FlWc5IV7fvXHA+/A/cRAo2byhbKt6+Aa7mY+1wh5A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 343F1153B; Fri, 30 Jan 2026 00:10:56 -0800 (PST) Received: from [10.164.18.94] (unknown [10.164.18.94]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 20D8D3F632; Fri, 30 Jan 2026 00:10:56 -0800 (PST) Message-ID: <1a33fe3e-b0dd-4553-95b4-89619b9229d2@arm.com> Date: Fri, 30 Jan 2026 13:40:54 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: khugepaged: fix NR_FILE_PAGES and NR_SHMEM in collapse_file() To: Shakeel Butt , Andrew Morton Cc: Johannes Weiner , Rik van Riel , Song Liu , Kiryl Shutsemau , Usama Arif , David Hildenbrand , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Barry Song , Lance Yang , Matthew Wilcox , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org References: <20260130042925.2797946-1-shakeel.butt@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <20260130042925.2797946-1-shakeel.butt@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 30/01/26 9:59 am, Shakeel Butt wrote: > In META's fleet, we observed high-level cgroups showing zero file memcg > stats while their descendants had non-zero values. Investigation using > drgn revealed that these parent cgroups actually had negative file stats, > aggregated from their children. > > This issue became more frequent after deploying thp-always more widely, > pointing to a correlation with THP file collapsing. The root cause is > that collapse_file() assumes old folios and the new THP belong to the > same node and memcg. When this assumption breaks, stats become skewed. > The bug affects not just memcg stats but also per-numa stats, and not > just NR_FILE_PAGES but also NR_SHMEM. > > The assumption breaks in scenarios such as: > > 1. Small folios allocated on one node while the THP gets allocated on a > different node. > > 2. A package downloader running in one cgroup populates the page cache, > while a job in a different cgroup executes the downloaded binary. > > 3. A file shared between processes in different cgroups, where one > process faults in the pages and khugepaged (or madvise(COLLAPSE)) > collapses them on behalf of the other. > > Fix the accounting by explicitly incrementing stats for the new THP and > decrementing stats for the old folios being replaced. > > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages") > Signed-off-by: Shakeel Butt > --- Thanks. Reviewed-by: Dev Jain > mm/khugepaged.c | 16 +++++++++------- > 1 file changed, 9 insertions(+), 7 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 1d994b6c58c6..fa1e57fd2c46 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2195,16 +2195,13 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > xas_lock_irq(&xas); > } > > - if (is_shmem) > + if (is_shmem) { > + lruvec_stat_mod_folio(new_folio, NR_SHMEM, HPAGE_PMD_NR); > lruvec_stat_mod_folio(new_folio, NR_SHMEM_THPS, HPAGE_PMD_NR); > - else > + } else { > lruvec_stat_mod_folio(new_folio, NR_FILE_THPS, HPAGE_PMD_NR); > - > - if (nr_none) { > - lruvec_stat_mod_folio(new_folio, NR_FILE_PAGES, nr_none); > - /* nr_none is always 0 for non-shmem. */ > - lruvec_stat_mod_folio(new_folio, NR_SHMEM, nr_none); > } > + lruvec_stat_mod_folio(new_folio, NR_FILE_PAGES, HPAGE_PMD_NR); > > /* > * Mark new_folio as uptodate before inserting it into the > @@ -2238,6 +2235,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr, > */ > list_for_each_entry_safe(folio, tmp, &pagelist, lru) { > list_del(&folio->lru); > + lruvec_stat_mod_folio(folio, NR_FILE_PAGES, > + -folio_nr_pages(folio)); > + if (is_shmem) > + lruvec_stat_mod_folio(folio, NR_SHMEM, > + -folio_nr_pages(folio)); I notice here that we don't need to do accounting for NR_SHMEM_THPS or NR_FILE_THPS - but the following bit: if (folio_order(folio) == HPAGE_PMD_ORDER && folio->index == start) in the khugepaged code, seems to suggest that we can reach this stat accounting path with a PMD order old folio, if folio->index != start. But this condition should not be possible; a folio is always order-aligned within the file, which means the folio->index here is PMD-aligned. The entry of collapse_file() asserts that start is also PMD-aligned (guaranteed by thp_vma_allowable_order in khugepaged_scan_mm_slot). Therefore start must equal folio->index. If I am not missing something here, I'll send a patch to convert this to a VM_WARN_ON. > folio->mapping = NULL; > folio_clear_active(folio); > folio_clear_unevictable(folio);