From nobody Mon Jun 8 23:56:00 2026 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B3611387566 for ; Mon, 25 May 2026 14:58:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.185 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779721092; cv=none; b=hM1+r1dz9noDLme696uDgxC/BkFrZhH8wrPsMeMU/YkV1xW6xLDzM/q6SNemB134NeUTrgwxOc2EwQNQpwFroqotZH9+z//ejovWnUbQpzDxv187lzeJj1LrfIinBSBQ79Tl3bxfUv/Wx8tHRkvVrOWQagmMUf/W/F+JgdxBjdc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779721092; c=relaxed/simple; bh=MnKo4XIq4kXlUq+SnaCGu9YxR/BDe077y2SVa0ho314=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=lP2Heuc4jujacO83CtzyWwpYM/1YUeZHlIYf71pF4cSnUjDa9FDUtAgqEa7xor1BhCW6sYdvZnDlm96fb1kbFhw/Ic2vqVb3qD5k5EHk0hUxWcPKXBuRpbKZ1Bx0wtFtuoS24S2+2PCC1mtPATBcLoMWN/ip3kr+3RQh5SaZ714= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Dhw9rzwZ; arc=none smtp.client-ip=91.218.175.185 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Dhw9rzwZ" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779721078; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=fl57Ady6J/3/I/Lz0zKZ4/vMXEK3F3tyaeoZhQFZ73M=; b=Dhw9rzwZEUbDdhxSLI5HfhEQ/j1sgVyG1jehGc9eIYhUwabxAyasFX0LpXESkHtuEmJe65 NYtTfr+kbM8Lp9muiWBelV0+xWGNcp5kIc9TO3LLIRSo1KD2cKM4zd78k8Ce3QfLSaNtvs sDyFye7xTf/wJlMhJ1KHFIWnkEZLdaw= From: Usama Arif To: Andrew Morton , jack@suse.cz, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org, david@kernel.org Cc: hannes@cmpxchg.org, kas@kernel.org, shakeel.butt@linux.dev, kernel-team@meta.com, Usama Arif Subject: [PATCH] mm: make mmap_miss accounting symmetric for VM_SEQ_READ Date: Mon, 25 May 2026 07:57:51 -0700 Message-ID: <20260525145751.2671248-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" do_sync_mmap_readahead() skips both the mmap_miss increment and the MMAP_LOTSAMISS check for VM_SEQ_READ mappings, since sequential access is non-speculative and should always read ahead. The two decrement sites in do_async_mmap_readahead() and filemap_map_pages() do not mirror this skip, so concurrent faults on a VM_SEQ_READ mapping can still drive ra->mmap_miss down to zero through the decrement paths even though nothing in the sync path ever increments it. The counter itself is per-file (file->f_ra.mmap_miss), so it can be moved by any VMA mapping the file, not just the one currently faulting. Skip the decrement for VM_SEQ_READ in both decrement sites so the counter only moves for mappings that also participate in the increment side. No functional change for VM_SEQ_READ users, since the increment-side gate already prevents the counter from being consulted on their behalf, but it stops a VM_SEQ_READ mapping from biasing the counter for other mappings of the same file. Signed-off-by: Usama Arif Reviewed-by: Jan Kara Reviewed-by: William Kucharski --- mm/filemap.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 5aaba0d3e81d..cca20e350c95 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3433,8 +3433,13 @@ static struct file *do_async_mmap_readahead(struct v= m_fault *vmf, * Don't touch the mmap_miss counter to avoid decreasing it multiple * times for a single folio and break the balance with mmap_miss * increase in do_sync_mmap_readahead(). + * + * VM_SEQ_READ mappings skip the mmap_miss increment in + * do_sync_mmap_readahead(), so skip the decrement here as well to + * keep the counter symmetric. */ - if (likely(!folio_test_locked(folio))) { + if (likely(!folio_test_locked(folio)) && + !(vmf->vma->vm_flags & VM_SEQ_READ)) { mmap_miss =3D READ_ONCE(ra->mmap_miss); if (mmap_miss) WRITE_ONCE(ra->mmap_miss, --mmap_miss); @@ -3935,10 +3940,15 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, * In such situation, read-ahead is only a waste of IO. * Don't decrease mmap_miss in this scenario to make sure * we can stop read-ahead. + * + * VM_SEQ_READ mappings skip the mmap_miss increment in + * do_sync_mmap_readahead(), so skip the decrement here as + * well to keep the counter symmetric. */ if ((map_ret & VM_FAULT_NOPAGE) && !(vmf->flags & FAULT_FLAG_TRIED) && - !folio_test_workingset(folio)) { + !folio_test_workingset(folio) && + !(vma->vm_flags & VM_SEQ_READ)) { unsigned short mmap_miss; =20 mmap_miss =3D READ_ONCE(file->f_ra.mmap_miss); --=20 2.52.0