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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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: ch4hp7i4o8ykyyjjbr8cfnmw57nbtqd9 X-Rspamd-Queue-Id: EDE19C0007 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1779721081-763470 X-HE-Meta: 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 9RinU2aI 0+jv+rIkw/a3YEZ9SpE/daYA1uauuW2TBGsvLq1dC0XFIcAl1kzFgoiF3eNNyvnW4kIc/OXIS0wTspHQ78Qedg8UkmRznuo5R9CetwZwOoCf+nH5iySiOkADoAQgJjqaXgpBh1y1muPlWrSzh8Bvx8f3Dn4CGI2oT4H58CwpTOkR5qnz0rRKPN7HLHOf1WIyNVK89ffiYOIa1denbl46E+eAOO+gtfK4U3lrC2IPa/q8N+laAJ9PQz38h8w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 --- 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 vm_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 = 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; mmap_miss = READ_ONCE(file->f_ra.mmap_miss); -- 2.52.0