linux-kernel - Re: [PATCH v5 6/7] mm/gup: Take mmap_lock in get_dump

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Message-ID: <CAHk-=wj=QXUkKXLzyWxJ49L80Heu2Z_RoHSahRt+zPq8W4du=g@mail.gmail.com>
Date:   Thu, 27 Aug 2020 10:13:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v5 6/7] mm/gup: Take mmap_lock in get_dump_page()

On Thu, Aug 27, 2020 at 4:50 AM Jann Horn <jannh@...gle.com> wrote:
>
> Properly take the mmap_lock before calling into the GUP code from
> get_dump_page(); and play nice, allowing the GUP code to drop the mmap_lock
> if it has to sleep.

Hmm. Of all the patches in the series, this simple one is now the only
one I feel makes for ugly code. Certainly not uglier than it used to
be,  but also not as pretty as it could be..

I think you're pretty much just re-implementing
get_user_pages_unlocked(), aren't you?

There are differences - you use mmap_read_lock_killable(), for
example. But I think get_user_pages_unlocked() should too.

The other difference is that you don't set FOLL_TOUCH. So it's not
*exactly* the same thing, but it's close enough that I get the feeling
that this should be cleaned up to use a common helper between the two.

That said, I suspect that falls under the heading of "future cleanup".
I don't think there's any need to re-spin this series for this, it's
just the only slightly negative reaction I had for the whole series
now.

                 Linus