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

[Yin Fengwei]

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@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>
>>> ---
>>>   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?

> 
> 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. :)).

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.


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);
>