linux-kernel - Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

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Date:   Wed, 25 Jan 2023 11:15:34 -0800
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

On Wed, Jan 25, 2023 at 10:06 AM Yang Shi <shy828301@...il.com> wrote:
>
> On Tue, Jan 24, 2023 at 5:58 PM Zach O'Keefe <zokeefe@...gle.com> wrote:
> >
> > During collapse, in a few places we check to see if a given small page
> > has any unaccounted references.  If the refcount on the page doesn't
> > match our expectations, it must be there is an unknown user concurrently
> > interested in the page, and so it's not safe to move the contents
> > elsewhere. However, the unaccounted pins are likely an ephemeral state.
> >
> > In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
> > collapse may succeed on retry.
>
> The page may be DMA pinned (for example, pin_user_pages()), it is not
> worth retrying for such pages. But it may also not be worth optimizing
> for this case at this point.
>
> So the patch looks good to me. Reviewed-by: Yang Shi <shy828301@...il.com>

Thanks as always, Yang, and good point about DMA pinning. As you
mentioned, I don't know if it's worth considering that too much right
now, as it's unlikely these two uses (MADV_COLLAPSE and DMA pining)
would be used together. We can revisit if necessary later if it's an
issue, but for now, I think it's a win that MADV_COLLAPSE (+ a bounded
userspace retry loop based off erno) is more likely to succeed.