On 2/5/24 15:36, zhangpeng (AS) wrote: > 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@xxxxxxxxxx> writes: >>>>> On 2024/2/5 10:56, Huang, Ying wrote: >>>>>> Peng Zhang <zhangpeng362@xxxxxxxxxx> writes: >>>>>>> From: ZhangPeng <zhangpeng362@xxxxxxxxxx> >>>>>>> >>>>>>> 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@xxxxxxxxxx/ >>>>>>> [2] https://github.com/antonblanchard/will-it-scale/ >>>>>>> >>>>>>> Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx> >>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@xxxxxxxxx> >>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx> >>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> >>>>>>> --- >>>>>>> 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. > */ > I believe David was asking to add: "...but the race window seems small enough. If we lose the race during the check, the page_fault will be triggered. Butthe page table entry lock still make sure the correctness: - If the page cache is not reclaimed, the page_fault will work like the page fault was served already and bail out. - If the page cache is reclaimed, the major fault will be triggered, page cache is filled, page_fault also work like the page fault was served already and bail out. "
