Re: [PATCH 1/6] mm: Allow per-VMA locks on file-backed VMAs

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Apr 7, 2023 at 2:36 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Apr 07, 2023 at 01:26:08PM -0700, Suren Baghdasaryan wrote:
> > True. do_swap_page() has the same issue. Can we move these
> > count_vm_event() calls to the end of handle_mm_fault():
>
> I was going to suggest something similar, so count that as an
> Acked-by.  This will change the accounting for the current retry
> situation, where we drop the mmap_lock in filemap_fault(), initiate I/O
> and return RETRY.  I think that's probably a good change though; I don't
> see why applications should have their fault counters incremented twice
> for that situation.
>
> >        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);
> > +       }
>
> Nit: this is a bitmask, so we should probably have:
>
>         if (!(ret & VM_FAULT_RETRY)) {
>
> in case somebody's ORed it with VM_FAULT_MAJOR or something.

The fix is posted at
https://lore.kernel.org/all/20230414175444.1837474-1-surenb@google.com/

>
> Hm, I wonder if we're handling that correctly; if we need to start I/O,
> do we preserve VM_FAULT_MAJOR returned from the first attempt?  Not
> in a position where I can look at the code right now.