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. -- Best Regards, Huang, Ying > + 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);
