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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 28 Apr 2023 09:03:27 -0700
Message-ID: <CAJuCfpHZSEjG2nti5iWbPQjWgXcHZ95hFS2FKwaPC-sZPdSznw@mail.gmail.com>
Subject: Re: [GIT PULL] MM updates for 6.4-rc1
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        mm-commits@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Lorenzo Stoakes <lstoakes@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>
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On Thu, Apr 27, 2023 at 8:03=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Apr 27, 2023 at 3:33=E2=80=AFPM Andrew Morton <akpm@linux-foundat=
ion.org> wrote:
> >
> > - Suren Baghdasaryan has improved mmap_lock scalability by switching to
> >   per-VMA locking.
>
> .. related to this, we have that new PER_VMA_LOCK_STATS config entry,
> which is 'default y' and has basically no help test.
>
> Please don't do that.
>
> I don't think any of the VM debugging stuff should likely be 'default
> y' in the first place, but if they are, they should most definitely
> have a good *explanation* for why they should be on for a normal user.

Understood.
I wanted these stats enabled by default to be able to identify
possible pathological cases and to also let users disable them if they
can't tolerate even a small overhead in the pagefault path. Should I
document this reasoning for the config option?

>
>                       Linus