From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-112.freemail.mail.aliyun.com (out30-112.freemail.mail.aliyun.com [115.124.30.112]) (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 7D96B262FFC for ; Fri, 19 Sep 2025 02:52:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.112 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758250377; cv=none; b=QzfxhQ0S8STFdTgKY71HmQsBqVrimPM/ZjDs8Rt/b73F0q9Dqbzh7Ak70gs5ZVdbK/iiTXhc2rH4yYK5D4ojFireJBiyvFPBLFmalPs0pqb5Xsqaj7qBaQ+UT8V2qOuuWInACFLGR6lSUC5shp+6bFg+mpfO1JzqO1RjzeG+Ez0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758250377; c=relaxed/simple; bh=PiQCNPunC3FQaQFnasvZXtFdq9x2Gkfp9H0MO9LAFXQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iV9ZuivhP5NqbRaG2brx2CqxbHbBsVBsAfGk2z5CI+Hak9anuTs8V+KWy/baoY7VgScQxnb3GZg9ySMyyMA9VOTm0VS2QBxJXf+IjH8oincGLX2hLD2j51BJv0sfWzZKEU7NVieX9SyRZLCQVkLDvLnEyLevc/PXEpaBY0Q7gGE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=skDSLzZ+; arc=none smtp.client-ip=115.124.30.112 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="skDSLzZ+" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1758250372; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=gVFSgUKKAuFnEqSEgabfcIAa5Tbls2E1LiZ7DjuSkOM=; b=skDSLzZ+mPBXfnNIPL+k8jhyDEpW2AcATdjj3gzRqrupGROoVY3Utg8SNE7ZDlY1FsO+3/JD+oVyYoYX93Uk8eOkd/mjVhrSApitaRWJVX4H4jG+OmMExARhq+E4/LpdtYRCmQdF43unJW4/0m7TfEItJjmb3/8WDnOa+pyYOfg= Received: from 30.74.144.118(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WoIF0kP_1758250369 cluster:ay36) by smtp.aliyun-inc.com; Fri, 19 Sep 2025 10:52:50 +0800 Message-ID: Date: Fri, 19 Sep 2025 10:52:49 +0800 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 1/2] mm/fault: Try to map the entire file folio in finish_fault() To: Lorenzo Stoakes , David Hildenbrand Cc: kirill@shutemov.name, Andrew Morton , Hugh Dickins , Matthew Wilcox , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Rik van Riel , Harry Yoo , Johannes Weiner , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau , "hughd@google.com" References: <20250918112157.410172-1-kirill@shutemov.name> <20250918112157.410172-2-kirill@shutemov.name> <6e6f596a-1817-45d6-b674-04e8aefde6d4@redhat.com> <962c9c49-8603-4a57-ba07-36e395eb48a5@lucifer.local> From: Baolin Wang In-Reply-To: <962c9c49-8603-4a57-ba07-36e395eb48a5@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2025/9/18 21:13, Lorenzo Stoakes wrote: > On Thu, Sep 18, 2025 at 01:30:32PM +0200, David Hildenbrand wrote: >> On 18.09.25 13:21, kirill@shutemov.name wrote: >>> From: Kiryl Shutsemau >>> >>> The finish_fault() function uses per-page fault for file folios. This >>> only occurs for file folios smaller than PMD_SIZE. >>> >>> The comment suggests that this approach prevents RSS inflation. >>> However, it only prevents RSS accounting. The folio is still mapped to >>> the process, and the fact that it is mapped by a single PTE does not >>> affect memory pressure. Additionally, the kernel's ability to map >>> large folios as PMD if they are large enough does not support this >>> argument. >>> >>> When possible, map large folios in one shot. This reduces the number of >>> minor page faults and allows for TLB coalescing. >>> >>> Mapping large folios at once will allow the rmap code to mlock it on >>> add, as it will recognize that it is fully mapped and mlocking is safe. >>> >>> Signed-off-by: Kiryl Shutsemau >>> --- >>> mm/memory.c | 9 ++------- >>> 1 file changed, 2 insertions(+), 7 deletions(-) >>> >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 0ba4f6b71847..812a7d9f6531 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -5386,13 +5386,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf) >>> nr_pages = folio_nr_pages(folio); >>> - /* >>> - * Using per-page fault to maintain the uffd semantics, and same >>> - * approach also applies to non shmem/tmpfs faults to avoid >>> - * inflating the RSS of the process. >>> - */ >>> - if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) || >>> - unlikely(needs_fallback)) { >>> + /* Using per-page fault to maintain the uffd semantics */ >>> + if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) { >>> nr_pages = 1; >>> } else if (nr_pages > 1) { >>> pgoff_t idx = folio_page_idx(folio, page); >> >> I could have sworn that we recently discussed that. >> >> Ah yes, there it is >> >> https://lkml.kernel.org/r/a1c9ba0f-544d-4204-ad3b-60fe1be2ab32@linux.alibaba.com >> >> CCing Baolin as he wanted to look into this. >> >> -- >> Cheers >> >> David / dhildenb >> > > Yeah Baolin already did work here [0] so let's get his input first I think! :) > > [0]:https://lore.kernel.org/linux-mm/440940e78aeb7430c5cc8b6d2088ae98265b9809.1751599072.git.baolin.wang@linux.alibaba.com/ Thanks CCing me. Also CCing Hugh. Hugh previously suggested adding restrictions to the mapping of file folios (using fault_around_bytes). However, personally, I am not inclined to use fault_around_bytes to control, because: 1. This doesn't cause serious write amplification issues. 2. It will inflate the RSS of the process, but does it matter? It seems not very important. 3. The default configuration for 'fault_around_bytes' is 65536 (16 pages), which is too small for mapping large file folios. 4. We could try adjusting 'fault_around_bytes' to a larger value, but we've found in real customer environments that 'fault_around_bytes' can lead to more aggressive readahead, impacting performance. So if 'fault_around_bytes' controls more, it will bring more different intersecting factors into play. Therefore, I personally prefer Kiryl's patch (it's what I intended to do, but I haven't had the time:().