Messages in this thread | | | Date | Thu, 23 Nov 2023 17:09:04 +0800 | Subject | Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault() | From | "zhangpeng (AS)" <> |
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On 2023/11/23 16:36, Huang, Ying wrote:
> Peng Zhang <zhangpeng362@huawei.com> writes: > >> 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> > Suggested-by: "Huang, Ying" <ying.huang@intel.com> > > :-)
Yes! :-)
>> --- >> 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.
If ptep == NULL, we may just need to return VM_FAULT_SIGBUS. I'll add it in the next version.
Thanks!
> -- > 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);
-- Best Regards, Peng
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