linux-kernel - Re: [PATCHv2 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages

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Message-ID: <CAFgQCTtXQtL0DQdqQCUEKMjDYB-AFA0SsJsvcbhqUtNbiCu1Eg@mail.gmail.com>
Date:   Tue, 4 Jun 2019 15:24:54 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ira Weiny <ira.weiny@...el.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>,
        John Hubbard <jhubbard@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Keith Busch <keith.busch@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

On Tue, Jun 4, 2019 at 3:08 PM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Jun 03, 2019 at 04:56:10PM -0700, Ira Weiny wrote:
> > On Mon, Jun 03, 2019 at 09:42:06AM -0700, Christoph Hellwig wrote:
> > > > +#if defined(CONFIG_CMA)
> > >
> > > You can just use #ifdef here.
> > >
> > > > +static inline int reject_cma_pages(int nr_pinned, unsigned int gup_flags,
> > > > + struct page **pages)
> > >
> > > Please use two instead of one tab to indent the continuing line of
> > > a function declaration.
> > >
> > > > +{
> > > > + if (unlikely(gup_flags & FOLL_LONGTERM)) {
> > >
> > > IMHO it would be a little nicer if we could move this into the caller.
> >
> > FWIW we already had this discussion and thought it better to put this here.
> >
> > https://lkml.org/lkml/2019/5/30/1565
>
> I don't see any discussion like this.  FYI, this is what I mean,
> code might be easier than words:
>
>
> diff --git a/mm/gup.c b/mm/gup.c
> index ddde097cf9e4..62d770b18e2c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2197,6 +2197,27 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
>         return ret;
>  }
>
> +#ifdef CONFIG_CMA
> +static int reject_cma_pages(struct page **pages, int nr_pinned)
> +{
> +       int i = 0;
> +
> +       for (i = 0; i < nr_pinned; i++)
> +               if (is_migrate_cma_page(pages[i])) {
> +                       put_user_pages(pages + i, nr_pinned - i);
> +                       return i;
> +               }
> +       }
> +
> +       return nr_pinned;
> +}
> +#else
> +static inline int reject_cma_pages(struct page **pages, int nr_pinned)
> +{
> +       return nr_pinned;
> +}
> +#endif /* CONFIG_CMA */
> +
>  /**
>   * get_user_pages_fast() - pin user pages in memory
>   * @start:     starting user address
> @@ -2237,6 +2258,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>                 ret = nr;
>         }
>
> +       if (nr && unlikely(gup_flags & FOLL_LONGTERM))
> +               nr = reject_cma_pages(pages, nr);
> +
Yeah. Looks better to keep reject_cma_pages() away from gup flags.

>         if (nr < nr_pages) {
>                 /* Try to get the remaining pages with get_user_pages */
>                 start += nr << PAGE_SHIFT;