linux-kernel - Re: [PATCH] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages

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Message-ID: <CAFgQCTtFmK1=7a4ewb+Dy3JZk=rxthi6ZAJBkkMaTgW2DxtubA@mail.gmail.com>
Date:   Fri, 31 May 2019 18:40:57 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

On Fri, May 31, 2019 at 7:52 AM Ira Weiny <ira.weiny@...el.com> wrote:
>
> On Thu, May 30, 2019 at 04:21:19PM -0700, John Hubbard wrote:
> > On 5/30/19 2:47 PM, Ira Weiny wrote:
> > > On Thu, May 30, 2019 at 06:54:04AM +0800, Pingfan Liu wrote:
> > [...]
> > >> +                          for (j = i; j < nr; j++)
> > >> +                                  put_page(pages[j]);
> > >
> > > Should be put_user_page() now.  For now that just calls put_page() but it is
> > > slated to change soon.
> > >
> > > I also wonder if this would be more efficient as a check as we are walking the
> > > page tables and bail early.
> > >
> > > Perhaps the code complexity is not worth it?
> >
> > Good point, it might be worth it. Because now we've got two loops that
> > we run, after the interrupts-off page walk, and it's starting to look like
> > a potential performance concern.
>
> FWIW I don't see this being a huge issue at the moment.  Perhaps those more
> familiar with CMA can weigh in here.  How was this issue found?  If it was
> found by running some test perhaps that indicates a performance preference?
>
I found the bug by reading code. And I do not see any performance
concern. Bailing out early contritute little to performance, as we
fall on the slow path immediately.

Regards,
  Pingfan
[....]