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

[zhangpeng (AS)]

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@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?

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