linux-kernel - Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL

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Date:   Wed, 23 Nov 2022 14:26:10 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     David Hildenbrand <david@...hat.com>,
        Tomasz Figa <tfiga@...omium.org>
Cc:     x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        sparclinux@...r.kernel.org, linux-um@...ts.infradead.org,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Shuah Khan <shuah@...nel.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE
 usage

Hi David, Tomasz,

On 16/11/2022 11:26, David Hildenbrand wrote:
> FOLL_FORCE is really only for ptrace access. According to commit
> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> writable"), get_vaddr_frames() currently pins all pages writable as a
> workaround for issues with read-only buffers.

I've decided to revert 707947247e95: I have not been able to reproduce the problem
described in that commit, and Tomasz reported that it caused problems with a
specific use-case they encountered. I'll post that patch soon and I expect it
to land in 6.2. It will cause a conflict with this patch, though.

If the problem described in that patch occurs again, then I will revisit it
and hopefully do a better job than I did before. That commit was not my
finest moment.

Regards,

	Hans

> 
> FOLL_FORCE, however, seems to be a legacy leftover as it predates
> commit 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are
> always writable"). Let's just remove it.
> 
> Once the read-only buffer issue has been resolved, FOLL_WRITE could
> again be set depending on the DMA direction.
> 
> Cc: Hans Verkuil <hverkuil@...all.nl>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Tomasz Figa <tfiga@...omium.org>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  drivers/media/common/videobuf2/frame_vector.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
> index 542dde9d2609..062e98148c53 100644
> --- a/drivers/media/common/videobuf2/frame_vector.c
> +++ b/drivers/media/common/videobuf2/frame_vector.c
> @@ -50,7 +50,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>  	start = untagged_addr(start);
>  
>  	ret = pin_user_pages_fast(start, nr_frames,
> -				  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> +				  FOLL_WRITE | FOLL_LONGTERM,
>  				  (struct page **)(vec->ptrs));
>  	if (ret > 0) {
>  		vec->got_ref = true;