linux-kernel - [PATCH] filemap: avoid unnecessary major faults in filemap

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Message-ID: <20240204093526.212636-1-zhangpeng362@huawei.com>
Date: Sun, 4 Feb 2024 17:35:26 +0800
From: Peng Zhang <zhangpeng362@...wei.com>
To: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
CC: <akpm@...ux-foundation.org>, <willy@...radead.org>,
	<ying.huang@...el.com>, <fengwei.yin@...el.com>,
	<aneesh.kumar@...ux.ibm.com>, <shy828301@...il.com>, <hughd@...gle.com>,
	<david@...hat.com>, <wangkefeng.wang@...wei.com>, <zhangpeng362@...wei.com>
Subject: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

From: ZhangPeng <zhangpeng362@...wei.com>

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" <ying.huang@...el.com>
Suggested-by: Yin Fengwei <fengwei.yin@...el.com>
Signed-off-by: ZhangPeng <zhangpeng362@...wei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
---
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.
+			 */
+			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);
-- 
2.25.1