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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 by holding ptl in filemap_fault() before > triggering a major fault. > > [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ > > Signed-off-by: ZhangPeng > Signed-off-by: Kefeng Wang > --- > mm/filemap.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 71f00539ac00..bb5e6a2790dc 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) > mapping_locked = true; > } > } else { > + pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, > + vmf->address, &vmf->ptl); > + if (ptep) { > + /* > + * Recheck pte with ptl locked as the pte can be cleared > + * temporarily during a read/modify/write update. > + */ > + if (unlikely(!pte_none(ptep_get(ptep)))) > + ret = VM_FAULT_NOPAGE; > + pte_unmap_unlock(ptep, vmf->ptl); > + if (unlikely(ret)) > + return ret; > + } I am curious. Did you try not to take PTL here and just check whether PTE is not NONE? Regards Yin, Fengwei > + > /* No page in the page cache at all */ > count_vm_event(PGMAJFAULT); > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);