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 by holding ptl in filemap_fault() before > triggering a major fault. > > [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@xxxxxxxxxx/ > > Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx> > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx> :-) > --- > 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; > + } > + Need to deal with ptep == NULL. Although that is high impossible. -- Best Regards, Huang, Ying > /* No page in the page cache at all */ > count_vm_event(PGMAJFAULT); > count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
