Re: [PATCH v5 6/7] mm/gup: Take mmap_lock in get_dump_page()

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jann Horn <jannh@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v5 6/7] mm/gup: Take mmap_lock in get_dump_page()
Date: Thu, 27 Aug 2020 10:13:49 -0700	[thread overview]
Message-ID: <CAHk-=wj=QXUkKXLzyWxJ49L80Heu2Z_RoHSahRt+zPq8W4du=g@mail.gmail.com> (raw)
In-Reply-To: <20200827114932.3572699-7-jannh@google.com>

On Thu, Aug 27, 2020 at 4:50 AM Jann Horn <jannh@google.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