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    SubjectRe: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault()
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    Hi Peng,

    On 11/22/23 22:00, Peng Zhang wrote:
    > From: ZhangPeng <zhangpeng362@huawei.com>
    >
    > 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@huawei.com/
    >
    > Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
    > Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
    > ---
    > 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);

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