Messages in this thread | | | Date | Mon, 5 Feb 2024 14:42:39 +0800 | Subject | Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault() | From | "zhangpeng (AS)" <> |
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On 2024/2/5 10:54, Huang, Ying wrote:
> Peng Zhang <zhangpeng362@huawei.com> writes: >> From: ZhangPeng <zhangpeng362@huawei.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% > IIUC, this is the regression test results. Right? Can you also show > improvement test results to justify the change?
Sure, I'll add the improvement test results as follows: processes processes_idle threads threads_idle ext4 private file write: -1.14% -0.08% -1.87% 0.13% ext4 shared file write: 0.14% -0.53% 2.88% -0.77% ext4 private file read: 0.03% -0.65% -0.51% -0.08% tmpfs private file write: -0.34% -0.11% 0.20% 0.15% tmpfs shared file write: 0.96% 0.10% 2.78% -0.34% ramdisk private file write: -1.21% -0.21% -1.12% 0.11% ramdisk private file read: 0.00% -0.68% -0.33% -0.02%
> > -- > Best Regards, > Huang, Ying > >> [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@intel.com> >> Suggested-by: Yin Fengwei <fengwei.yin@intel.com> >> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com> >> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.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);
-- Best Regards, Peng
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