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Date:   Fri, 7 Apr 2023 22:36:50 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        Punit Agrawal <punit.agrawal@bytedance.com>
Subject: Re: [PATCH 1/6] mm: Allow per-VMA locks on file-backed VMAs
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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.

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.