Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Hans Verkuil <hverkuil@xs4all.nl>
Cc: David Hildenbrand <david@redhat.com>,
	dri-devel@lists.freedesktop.org, linux-mm@kvack.org,
	Nadav Amit <namit@vmware.com>,
	linux-kselftest@vger.kernel.org,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org,
	Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	John Hubbard <jhubbard@nvidia.com>,
	etnaviv@lists.freedesktop.org, Peter Xu <peterx@redhat.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Oded Gabbay <ogabbay@kernel.org>,
	linux-kernel@vger.kernel.org, Tomasz Figa <tfiga@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH RFC 16/19] mm/frame-vector: remove FOLL_FORCE usage
Date: Tue, 22 Nov 2022 09:33:16 -0800	[thread overview]
Message-ID: <CAHk-=wh0nuSn=zYB1z9bHXJRPi33mmbpv-Z6z7ARkHQupbQ3fQ@mail.gmail.com> (raw)
In-Reply-To: <6175d780-3307-854c-448a-8e6c7ad0772c@xs4all.nl>

On Tue, Nov 22, 2022 at 4:25 AM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> I tracked the use of 'force' all the way back to the first git commit
> (2.6.12-rc1) in the very old video-buf.c. So it is very, very old and the
> reason is lost in the mists of time.

Well, not entirely.

For archaeology reasons, I went back to the old BK history, which
exists as a git conversion in

    https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/

and there you can actually find it.

Not with a lot of explanations, though - it's commit b7649ef789
("[PATCH] videobuf update"):

    This updates the video-buf.c module (helper module for video buffer
    management).  Some memory management fixes, also some adaptions to the
    final v4l2 api.

but it went from

         err = get_user_pages(current,current->mm,
-                            data, dma->nr_pages,
-                            rw == READ, 0, /* don't force */
+                            data & PAGE_MASK, dma->nr_pages,
+                            rw == READ, 1, /* force */
                             dma->pages, NULL);

in that commit.

So it goes back to October 2002.

> Looking at this old LWN article https://lwn.net/Articles/28548/ suggests
> that it might be related to calling get_user_pages for write buffers

The timing roughly matches.

> I assume that removing FOLL_FORCE from 'FOLL_FORCE|FOLL_WRITE' will still
> allow drivers to read from the buffer?

The issue with some of the driver hacks has been that

 - they only want to read, and the buffer may be read-only

 - they then used FOLL_WRITE despite that, because they want to break
COW (due to the issue that David is now fixing with his series)

 - but that means that the VM layer says "nope, you can't write to
this read-only user mapping"

 - ... and then they use FOLL_FORCE to say "yes, I can".

iOW, the FOLL_FORCE may be entirely due to an (incorrect, but
historically needed) FOLL_WRITE.

             Linus