Messages in this thread | | | Date | Mon, 5 Feb 2024 15:36:01 +0800 | Subject | Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault() | From | "zhangpeng (AS)" <> |
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On 2024/2/5 15:31, David Hildenbrand wrote:
> On 05.02.24 08:24, zhangpeng (AS) wrote: >> On 2024/2/5 14:52, Huang, Ying wrote: >> >>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes: >>>> On 2024/2/5 10:56, 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% >>>>>> >>>>>> [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. >>>>>> + */ >>>>> 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. >> >> Got it. 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. >> * We don't hold PTL here as acquiring PTL hurts >> * performance. So the check is still racy, but >> * the race window seems small enough. >> */ > > It'd be worth spelling out what happens when we lose the race. > I'll add what happens when we lose the race 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. * We don't hold PTL here as acquiring PTL hurts * performance. So the check is still racy, but * the race window seems small enough. */
-- Best Regards, Peng
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