"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. -- 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);
