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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: C412C120015 X-Stat-Signature: xdr8a8m58b5wu7u4f4z9ybga6p53dm1w X-Rspam-User: X-HE-Tag: 1707118283-847351 X-HE-Meta: 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 wZ2wB6lB 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 05.02.24 08:24, zhangpeng (AS) wrote: > On 2024/2/5 14:52, Huang, Ying wrote: > >> "zhangpeng (AS)" writes: >>> On 2024/2/5 10:56, Huang, Ying wrote: >>>> Peng Zhang writes: >>>>> From: ZhangPeng >>>>> >>>>> 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@huawei.com/ >>>>> [2] https://github.com/antonblanchard/will-it-scale/ >>>>> >>>>> Suggested-by: "Huang, Ying" >>>>> Suggested-by: Yin Fengwei >>>>> Signed-off-by: ZhangPeng >>>>> Signed-off-by: Kefeng Wang >>>>> --- >>>>> 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. > > Got it. 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. > * We don't hold PTL here as acquiring PTL hurts > * performance. So the check is still racy, but > * the race window seems small enough. > */ It'd be worth spelling out what happens when we lose the race. -- Cheers, David / dhildenb