linux-kernel - Re: [PATCH 4/6] mm: add a gup_fixup_start

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Date:   Wed, 29 May 2019 09:19:56 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-mips@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] mm: add a gup_fixup_start_addr hook

Hi Christoph,

On Sat, 25 May 2019 at 14:33, Christoph Hellwig <hch@....de> wrote:
> diff --git a/mm/gup.c b/mm/gup.c
> index f173fcbaf1b2..1c21ecfbf38b 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2117,6 +2117,10 @@ static void gup_pgd_range(unsigned long addr, unsigned long end,
>         } while (pgdp++, addr = next, addr != end);
>  }
>
> +#ifndef gup_fixup_start_addr
> +#define gup_fixup_start_addr(start)    (start)
> +#endif

As you pointed out in a subsequent reply, we could use the
untagged_addr() macro from Andrey (or a shorter "untag_addr" if you
want it to look like a verb).

>  #ifndef gup_fast_permitted
>  /*
>   * Check if it's allowed to use __get_user_pages_fast() for the range, or
> @@ -2145,7 +2149,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>         unsigned long flags;
>         int nr = 0;
>
> -       start &= PAGE_MASK;
> +       start = gup_fixup_start_addr(start) & PAGE_MASK;
>         len = (unsigned long) nr_pages << PAGE_SHIFT;
>         end = start + len;
>
> @@ -2218,7 +2222,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>         unsigned long addr, len, end;
>         int nr = 0, ret = 0;
>
> -       start &= PAGE_MASK;
> +       start = gup_fixup_start_addr(start) & PAGE_MASK;
>         addr = start;
>         len = (unsigned long) nr_pages << PAGE_SHIFT;
>         end = start + len;

In Andrey's patch [1] we don't fix __get_user_pages_fast(), only
__get_user_pages() as it needs to do a find_vma() search. I wonder
whether this is actually necessary for the *_fast() versions. If the
top byte is non-zero (i.e. tagged address), 'end' would also have the
same tag. The page table macros like pgd_index() and pgd_addr_end()
already take care of masking out the top bits (at least for arm64)
since they need to work on kernel address with the top bits all 1. So
gup_pgd_range() should cope with tagged addresses already.

[1] https://lore.kernel.org/lkml/d234cd71774f35229bdfc0a793c34d6712b73093.1557160186.git.andreyknvl@google.com/

-- 
Catalin