On Fri, Apr 7, 2023 at 1:12 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 07, 2023 at 10:54:00AM -0700, Suren Baghdasaryan wrote:
> > On Tue, Apr 4, 2023 at 6:59 AM Matthew Wilcox (Oracle)
> > <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > The fault path will immediately fail in handle_mm_fault(), so this
> > > is the minimal step which allows the per-VMA lock to be taken on
> > > file-backed VMAs. There may be a small performance reduction as a
> > > little unnecessary work will be done on each page fault. See later
> > > patches for the improvement.
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > ---
> > > mm/memory.c | 9 ++++-----
> > > 1 file changed, 4 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index fdaec7772fff..f726f85f0081 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -5223,6 +5223,9 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> > > flags & FAULT_FLAG_REMOTE))
> > > return VM_FAULT_SIGSEGV;
> > >
> > > + if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma))
> > > + return VM_FAULT_RETRY;
> > > +
> >
> > There are count_vm_event(PGFAULT) and count_memcg_event_mm(vma->vm_mm,
> > PGFAULT) earlier in this function. Returning here and retrying I think
> > will double-count this page fault. Returning before this accounting
> > should fix this issue.
>
> You're right, but this will be an issue with later patches in the series
> anyway as we move the check further and further down the call-chain.
> For that matter, it's an issue in do_swap_page() right now, isn't it?
> I suppose we don't care too much because it's the rare case where we go
> into do_swap_page() and so the stats are "correct enough".
True. do_swap_page() has the same issue. Can we move these
count_vm_event() calls to the end of handle_mm_fault():
vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
unsigned int flags, struct pt_regs *regs)
{
vm_fault_t ret;
__set_current_state(TASK_RUNNING);
- count_vm_event(PGFAULT);
- count_memcg_event_mm(vma->vm_mm, PGFAULT);
ret = sanitize_fault_flags(vma, &flags);
if (ret)
- return ret;
- goto out;
...
mm_account_fault(regs, address, flags, ret);
+out:
+ if (ret != VM_FAULT_RETRY) {
+ count_vm_event(PGFAULT);
+ count_memcg_event_mm(vma->vm_mm, PGFAULT);
+ }
return ret;
}
?
