From: Joel Fernandes <joel@joelfernandes.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org, mhocko@suse.com,
riel@redhat.com, minchan@google.com, hannes@cmpxchg.org,
kirill.shutemov@linux.intel.com
Subject: Re: rmap for shmem pages
Date: Thu, 12 Dec 2019 09:43:13 -0500 [thread overview]
Message-ID: <20191212144313.GC129023@google.com> (raw)
In-Reply-To: <20191212105521.kwqn6hiudt3tbr6g@box>
On Thu, Dec 12, 2019 at 01:55:21PM +0300, Kirill A. Shutemov wrote:
> On Wed, Dec 11, 2019 at 06:51:08PM -0500, Joel Fernandes wrote:
> > Hi,
> > I am looking into a weird behavior of the VmRSS counter in /proc/pid/status
> > which I think is a bug.
> >
> > The issue is if a memfd is mapped into the same process twice as MAP_SHARED, and
> > you write to both maps, then the RSS doubles even though the pages were
> > mapped elsewhere within the same address space before. We expect RSS to
> > increase only once in this case, not twice.
>
> VmRSS is a best effort. Meeting your expectation would require rmap walk
> (as your patch does). It's way too expensive for fault path (especially
> under page table lock) just to keep percise statistics. Sorry, but you
> will not get it.
No problem. I was under the same impression that this is difficult to do in a
scalable way. I am glad I reached out but if anyone has any other ideas, let
me know.
thanks,
- Joel
>
> --
> Kirill A. Shutemov
prev parent reply other threads:[~2019-12-12 14:43 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-11 23:51 rmap for shmem pages Joel Fernandes
2019-12-12 10:55 ` Kirill A. Shutemov
2019-12-12 14:43 ` Joel Fernandes [this message]
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