Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault()

["zhangpeng (AS)" <zhangpeng362@huawei.com>]

On 2023/11/23 13:26, Yin Fengwei wrote:

> On 11/23/23 12:12, zhangpeng (AS) wrote:
>> On 2023/11/23 9:09, Yin Fengwei wrote:
>>
>>> 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?
>> Thank you for your reply.
>>
>> If we don't take PTL, the current use case won't trigger this issue either.
> Is this verified by testing or just in theory?

If we add a delay between ptep_modify_prot_start() and ptep_modify_prot_commit(),
this issue will also trigger. Without delay, we haven't reproduced this problem
so far.

>> In most cases, if we don't take PTL, this issue won't be triggered. However,
>> there is still a possibility of triggering this issue. The corner case is that
>> task 2 triggers a page fault when task 1 is between ptep_modify_prot_start()
>> and ptep_modify_prot_commit() in do_numa_page(). Furthermore,task 2 passes the
>> check whether the PTE is not NONE before task 1 updates PTE in
>> ptep_modify_prot_commit() without taking PTL.
> There is very limited operations between ptep_modify_prot_start() and
> ptep_modify_prot_commit(). While the code path from page fault to this check is
> long. My understanding is it's very likely the PTE is not NONE when do PTE check
> here without hold PTL (This is my theory. :)).

Yes, there is a high probability that this issue won't occur without taking PTL.

> In the other side, acquiring/releasing PTL may bring performance impaction. It may
> not be big deal because the IO operations in this code path. But it's better to
> collect some performance data IMHO.

We tested the performance of file private mapping page fault (page_fault2.c of
will-it-scale [1]) and file shared mapping page fault (page_fault3.c of will-it-scale).
The difference in performance (in operations per second) before and after patch
applied is about 0.7% on a x86 physical machine.

[1] https://github.com/antonblanchard/will-it-scale/tree/master

>
> Regards
> Yin, Fengwei
>
>>> Regards
>>> Yin, Fengwei
>>>
>>>> +
>>>>            /* No page in the page cache at all */
>>>>            count_vm_event(PGMAJFAULT);
>>>>            count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);

-- 
Best Regards,
Peng