summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorZhangPeng <zhangpeng362@huawei.com>2024-03-06 16:38:09 +0800
committerAndrew Morton <akpm@linux-foundation.org>2024-03-06 13:04:20 -0800
commit58f327f2ce80f9c7b4a70e9cf017ae8810d44a20 (patch)
tree7a306d55d42575fbfc4103c57e2d702666329d25
parent4839e79c7eae747810c81848729e050eb5440547 (diff)
filemap: avoid unnecessary major faults in filemap_fault()
A major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) in application, which leading to an unexpected issue[1]. This is caused by temporarily cleared PTE during a read+clear/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. This issue affects our traffic analysis service. The inbound traffic is heavy. If a major fault occurs, the I/O schedule is triggered and the original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If other applications are operating disks, the system needs to wait for more than 10 ms. However, the inbound traffic is heavy and the NIC buffer is small. As a result, packet loss occurs. But the traffic analysis service can't tolerate packet loss. Fix this by holding PTL and rechecking the PTE in filemap_fault() before triggering a major fault. We do this check only if vma is VM_LOCKED to reduce the performance impact in common scenarios. In our product environment, there were 7 major faults every 12 hours. After the patch is applied, no major fault have been triggered. Testing file 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. We do this check only if vma is VM_LOCKED, therefore, no performance regressions is caused for most common cases. The test results are as follows: processes processes_idle threads threads_idle ext4 private file write: 0.22% 0.26% 1.21% -0.15% ext4 private file read: 0.03% 1.00% 1.39% 0.34% ext4 shared file write: -0.50% -0.02% -0.14% -0.02% ramdisk private file write: 0.07% 0.02% 0.53% 0.04% ramdisk private file read: 0.01% 1.60% -0.32% -0.02% [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ [2] https://github.com/antonblanchard/will-it-scale/ Link: https://lkml.kernel.org/r/20240306083809.1236634-1-zhangpeng362@huawei.com Signed-off-by: ZhangPeng <zhangpeng362@huawei.com> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com> Suggested-by: "Huang, Ying" <ying.huang@intel.com> Suggested-by: David Hildenbrand <david@redhat.com> Reviewed-by: "Huang, Ying" <ying.huang@intel.com> Reviewed-by: David Hildenbrand <david@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
-rw-r--r--mm/filemap.c46
1 files changed, 46 insertions, 0 deletions
diff --git a/mm/filemap.c b/mm/filemap.c
index b4858d89f1b1..31ab455c4537 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3181,6 +3181,48 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
return fpin;
}
+static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ vm_fault_t ret = 0;
+ pte_t *ptep;
+
+ /*
+ * We might have COW'ed a pagecache folio and might now have an mlocked
+ * anon folio mapped. The original pagecache folio is not mlocked and
+ * might have been evicted. During a read+clear/modify/write update of
+ * the PTE, such as done in do_numa_page()/change_pte_range(), we
+ * temporarily clear the PTE under PT lock and might detect it here as
+ * "none" when not holding the PT lock.
+ *
+ * Not rechecking the PTE under PT lock could result in an unexpected
+ * major fault in an mlock'ed region. Recheck only for this special
+ * scenario while holding the PT lock, to not degrade non-mlocked
+ * scenarios. Recheck the PTE without PT lock firstly, thereby reducing
+ * the number of times we hold PT lock.
+ */
+ if (!(vma->vm_flags & VM_LOCKED))
+ return 0;
+
+ if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID))
+ return 0;
+
+ ptep = pte_offset_map(vmf->pmd, vmf->address);
+ if (unlikely(!ptep))
+ return VM_FAULT_NOPAGE;
+
+ if (unlikely(!pte_none(ptep_get_lockless(ptep)))) {
+ ret = VM_FAULT_NOPAGE;
+ } else {
+ spin_lock(vmf->ptl);
+ if (unlikely(!pte_none(ptep_get(ptep))))
+ ret = VM_FAULT_NOPAGE;
+ spin_unlock(vmf->ptl);
+ }
+ pte_unmap(ptep);
+ return ret;
+}
+
/**
* filemap_fault - read in file data for page fault handling
* @vmf: struct vm_fault containing details of the fault
@@ -3236,6 +3278,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
mapping_locked = true;
}
} else {
+ ret = filemap_fault_recheck_pte_none(vmf);
+ 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);