From nobody Mon Dec 8 00:16:14 2025 Received: from out-189.mta1.migadu.com (out-189.mta1.migadu.com [95.215.58.189]) (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 7E15D199237 for ; Mon, 6 Oct 2025 17:51:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.189 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759773086; cv=none; b=GW5IdaSJNHa63ZslF9fkRe0M09lHIM8KjGYky4bZlO2Zt9e5Ek918ddnGv9AY1H+gReHHKMRoHsXJbWC+9pHERaIfUWvJLlEgNmgktJfGElGKWICl0UAv2KWO2VKBZqqubFAsoEnzxr9NwW9mB+cKIyOQVTdx9yImS8zoJchQhI= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759773086; c=relaxed/simple; bh=eDUXs7ScxA5xbX00G5qcGbvHjCgTcMBubaO0lWT/R5w=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=IKmSj5PESv5/rIqJHbW/ZU45s8B1wUVU2Q6UpQIYGAVxHz5lHN5hBoOI2+93BFPA3gZGG8oH/YBd58CBAWz3DZaAu19s1A9C2nJ3fonwILDW73AphwfUvGIsDyPF85IAcCPt0Ft4PtuN5ZpVu6v0Mjf14VzXlTqGbffx6ZTNVe8= 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=NAg+ao8U; arc=none smtp.client-ip=95.215.58.189 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="NAg+ao8U" 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=1759773081; 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=qleRauGEj+bb9sgaaA3graHFC3gh/W99qbG2F0J96jE=; b=NAg+ao8U/6KBCPsnfzZcwwdqcMtsZXjQyFTJxdE789iThOvflZWEQU3w522WKFskq38XKO J0Hi/whc6xsJ9GIbFJUnywtlB+zqXnGhEQU5665RzuZZSAw6U85NOhf/OGZwPmc0uO4Rb2 pjXTZj8TdAkUNpQgPF4cotBJHjDl41A= From: Roman Gushchin To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Roman Gushchin , "Matthew Wilcox (Oracle)" , Jan Kara , Dev Jain , linux-mm@kvack.org Subject: [PATCH v3] mm: readahead: make thp readahead conditional to mmap_miss logic Date: Mon, 6 Oct 2025 10:51:06 -0700 Message-ID: <20251006175106.377411-1-roman.gushchin@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" Commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings") introduced a special handling for VM_HUGEPAGE mappings: even if the readahead is disabled, 1 or 2 HPAGE_PMD_ORDER pages are allocated. This change causes a significant regression for containers with a tight memory.max limit, if VM_HUGEPAGE is widely used. Prior to this commit, mmap_miss logic would eventually lead to the readahead disablement, effectively reducing the memory pressure in the cgroup. With this change the kernel is trying to allocate 1-2 huge pages for each fault, no matter if these pages are used or not before being evicted, increasing the memory pressure multi-fold. To fix the regression, let's make the new VM_HUGEPAGE conditional to the mmap_miss check, but keep independent from the ra->ra_pages. This way the main intention of commit 4687fdbb805a ("mm/filemap: Support VM_HUGEPAGE for file mappings") stays intact, but the regression is resolved. The logic behind this changes is simple: even if a user explicitly requests using huge pages to back the file mapping (using VM_HUGEPAGE flag), under a very strong memory pressure it's better to fall back to ordinary pages. Signed-off-by: Roman Gushchin Cc: Matthew Wilcox (Oracle) Cc: Jan Kara Cc: Dev Jain Cc: linux-mm@kvack.org -- v3: fixed VM_SEQ_READ handling for the THP case (by Jan Kara) v2: fixed VM_SEQ_READ handling (by Dev Jain) Reviewed-by: Dev Jain Reviewed-by: Jan Kara --- mm/filemap.c | 68 +++++++++++++++++++++++++++++----------------------- 1 file changed, 38 insertions(+), 30 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index a52dd38d2b4a..ec731ac05551 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3235,11 +3235,47 @@ static struct file *do_sync_mmap_readahead(struct v= m_fault *vmf) DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff); struct file *fpin =3D NULL; vm_flags_t vm_flags =3D vmf->vma->vm_flags; + bool force_thp_readahead =3D false; unsigned short mmap_miss; =20 -#ifdef CONFIG_TRANSPARENT_HUGEPAGE /* Use the readahead code, even if readahead is disabled */ - if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <=3D MAX_PAGECACHE_ORDER)= { + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && + (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <=3D MAX_PAGECACHE_ORDER) + force_thp_readahead =3D true; + + if (!force_thp_readahead) { + /* + * If we don't want any read-ahead, don't bother. + * VM_EXEC case below is already intended for random access. + */ + if ((vm_flags & (VM_RAND_READ | VM_EXEC)) =3D=3D VM_RAND_READ) + return fpin; + + if (!ra->ra_pages) + return fpin; + + if (vm_flags & VM_SEQ_READ) { + fpin =3D maybe_unlock_mmap_for_io(vmf, fpin); + page_cache_sync_ra(&ractl, ra->ra_pages); + return fpin; + } + } + + if (!(vm_flags & VM_SEQ_READ)) { + /* Avoid banging the cache line if not needed */ + mmap_miss =3D READ_ONCE(ra->mmap_miss); + if (mmap_miss < MMAP_LOTSAMISS * 10) + WRITE_ONCE(ra->mmap_miss, ++mmap_miss); + + /* + * Do we miss much more than hit in this file? If so, + * stop bothering with read-ahead. It will only hurt. + */ + if (mmap_miss > MMAP_LOTSAMISS) + return fpin; + } + + if (force_thp_readahead) { fpin =3D maybe_unlock_mmap_for_io(vmf, fpin); ractl._index &=3D ~((unsigned long)HPAGE_PMD_NR - 1); ra->size =3D HPAGE_PMD_NR; @@ -3254,34 +3290,6 @@ static struct file *do_sync_mmap_readahead(struct vm= _fault *vmf) page_cache_ra_order(&ractl, ra); return fpin; } -#endif - - /* - * If we don't want any read-ahead, don't bother. VM_EXEC case below is - * already intended for random access. - */ - if ((vm_flags & (VM_RAND_READ | VM_EXEC)) =3D=3D VM_RAND_READ) - return fpin; - if (!ra->ra_pages) - return fpin; - - if (vm_flags & VM_SEQ_READ) { - fpin =3D maybe_unlock_mmap_for_io(vmf, fpin); - page_cache_sync_ra(&ractl, ra->ra_pages); - return fpin; - } - - /* Avoid banging the cache line if not needed */ - mmap_miss =3D READ_ONCE(ra->mmap_miss); - if (mmap_miss < MMAP_LOTSAMISS * 10) - WRITE_ONCE(ra->mmap_miss, ++mmap_miss); - - /* - * Do we miss much more than hit in this file? If so, - * stop bothering with read-ahead. It will only hurt. - */ - if (mmap_miss > MMAP_LOTSAMISS) - return fpin; =20 if (vm_flags & VM_EXEC) { /* --=20 2.51.0