From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.13]) (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 A078CBE4A for ; Mon, 5 Feb 2024 06:54:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.175.65.13 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707116082; cv=none; b=Dwp4kjmxQmAweT+GpMRFdcquAWZndiApl8WO8ysDmW+6ODAW3bBHAKPB7i20t7Gd9KocRKqGUH8DA3E0UOAmdZYdyLyCO06Uwvxx4K2Ig+LmjNaEbgdwHXHEpCpToutnOQ1ftargRPY50seBFg4SXBxx8bVxBejj9HXXr+nh6mw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707116082; c=relaxed/simple; bh=9RL5KR1hpJcQae+rJ4QHCvOl5EYuXsE3JTpP7Zi4wtY=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=Pnl1TmgTVNdITdjrymQQBouOaT059WMvQD7QB3x8DkDv7/UB4+dVcQcElTKh6N5HTHwzxClA49jzMrLs5wwIBiKmPcCy8uGosqNcFnInOZ01G/9y/PKjK3/RM2n0TmuANgnO/NSloMoOkOEvgV/X8ySverc4KkfsxAwrNXeKhAs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=MYrp53hL; arc=none smtp.client-ip=198.175.65.13 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="MYrp53hL" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1707116080; x=1738652080; h=from:to:cc:subject:in-reply-to:references:date: message-id:mime-version; bh=9RL5KR1hpJcQae+rJ4QHCvOl5EYuXsE3JTpP7Zi4wtY=; b=MYrp53hLcklRtfNo+/hVj2EG58UNDRAu7KJXPUVppVM8HQPmc/MaVxBP T82JO+Z5fOhg2MrqWfJcyesEYE18g3rpZmSZvoLLst04yN+PglP2+5J44 /k576YX4bl/p8gwmlW7oDtOmKekrPkuhUmX7ncAaWHAgfricBvPAatGSK VhMLHmgL40MA6QZ+tLN9dshXSP+vbK3QRbl+ojP2JUyFLNFfvnRBMHzwb XXaxi1X6gAkobS3DY1e7Gao4Z/k0qhI+NFP4hQJc8+rXnZS6TsUoY32PU lcTg8pkBizSXPBfFFMzdas/z5q3+LkmhuQsTXrr8PYlD/Q+pAafR7s/PS Q==; X-IronPort-AV: E=McAfee;i="6600,9927,10974"; a="11553999" X-IronPort-AV: E=Sophos;i="6.05,242,1701158400"; d="scan'208";a="11553999" Received: from orviesa006.jf.intel.com ([10.64.159.146]) by orvoesa105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Feb 2024 22:54:39 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.05,242,1701158400"; d="scan'208";a="930460" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by orviesa006-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Feb 2024 22:54:37 -0800 From: "Huang, Ying" To: "zhangpeng (AS)" Cc: , , , , , , , , , Subject: Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault() In-Reply-To: <85e03dd9-8bd7-d516-ebe4-84dd449a9fb2@huawei.com> (zhangpeng's message of "Mon, 5 Feb 2024 14:43:07 +0800") References: <20240204093526.212636-1-zhangpeng362@huawei.com> <87zfwf39ha.fsf@yhuang6-desk2.ccr.corp.intel.com> <85e03dd9-8bd7-d516-ebe4-84dd449a9fb2@huawei.com> Date: Mon, 05 Feb 2024 14:52:40 +0800 Message-ID: <87mssf2yiv.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=ascii "zhangpeng (AS)" writes: > On 2024/2/5 10:56, Huang, Ying wrote: > >> Peng Zhang writes: >>> From: ZhangPeng >>> >>> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) >>> in application, which leading to an unexpected performance issue[1]. >>> >>> This caused by temporarily cleared PTE during a read/modify/write update >>> of the PTE, eg, do_numa_page()/change_pte_range(). >>> >>> For the data segment of the user-mode program, the global variable area >>> is a private mapping. After the pagecache is loaded, the private anonymous >>> page is generated after the COW is triggered. Mlockall can lock COW pages >>> (anonymous pages), but the original file pages cannot be locked and may >>> be reclaimed. If the global variable (private anon page) is accessed when >>> vmf->pte is zeroed in numa fault, a file page fault will be triggered. >>> >>> At this time, the original private file page may have been reclaimed. >>> If the page cache is not available at this time, a major fault will be >>> triggered and the file will be read, causing additional overhead. >>> >>> Fix this by rechecking the PTE without acquiring PTL in filemap_fault() >>> before triggering a major fault. >>> >>> Testing file anonymous page read and write page fault performance in ext4 >>> and ramdisk using will-it-scale[2] on a x86 physical machine. The data >>> is the average change compared with the mainline after the patch is >>> applied. The test results are within the range of fluctuation, and there >>> is no obvious difference. The test results are as follows: >>> processes processes_idle threads threads_idle >>> ext4 file write: -1.14% -0.08% -1.87% 0.13% >>> ext4 file read: 0.03% -0.65% -0.51% -0.08% >>> ramdisk file write: -1.21% -0.21% -1.12% 0.11% >>> ramdisk file read: 0.00% -0.68% -0.33% -0.02% >>> >>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ >>> [2] https://github.com/antonblanchard/will-it-scale/ >>> >>> Suggested-by: "Huang, Ying" >>> Suggested-by: Yin Fengwei >>> Signed-off-by: ZhangPeng >>> Signed-off-by: Kefeng Wang >>> --- >>> RFC->v1: >>> - Add error handling when ptep == NULL per Huang, Ying and Matthew Wilcox >>> - Check the PTE without acquiring PTL in filemap_fault(), suggested by >>> Huang, Ying and Yin Fengwei >>> - Add pmd_none() check before PTE map >>> - Update commit message and add performance test information >>> >>> mm/filemap.c | 18 ++++++++++++++++++ >>> 1 file changed, 18 insertions(+) >>> >>> diff --git a/mm/filemap.c b/mm/filemap.c >>> index 142864338ca4..b29cdeb6a03b 100644 >>> --- a/mm/filemap.c >>> +++ b/mm/filemap.c >>> @@ -3238,6 +3238,24 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >>> mapping_locked = true; >>> } >>> } else { >>> + if (!pmd_none(*vmf->pmd)) { >>> + pte_t *ptep; >>> + >>> + ptep = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd, >>> + vmf->address, &vmf->ptl); >>> + if (unlikely(!ptep)) >>> + return VM_FAULT_NOPAGE; >>> + /* >>> + * Recheck pte as the pte can be cleared temporarily >>> + * during a read/modify/write update. >>> + */ >> I think that we should add some comments here about the racy checking. > > I'll add comments in a v2 as follows: > /* > * Recheck PTE as the PTE can be cleared temporarily > * during a read/modify/write update of the PTE, eg, > * do_numa_page()/change_pte_range(). This will trigger > * a major fault, even if we use mlockall, which may > * affect performance. > */ Sorry, my previous words aren't clear enough. I mean some comments as follows, We don't hold PTL here, so the check is still racy. But acquiring PTL hurts performance and the race window seems small enough. -- Best Regards, Huang, Ying >>> + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) >>> + ret = VM_FAULT_NOPAGE; >>> + pte_unmap(ptep); >>> + if (unlikely(ret)) >>> + return ret; >>> + } >>> + >>> /* No page in the page cache at all */ >>> count_vm_event(PGMAJFAULT); >>> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);