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Date:   Wed, 23 Nov 2022 15:28:27 +0100
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Subject: Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE
 usage
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From:   Hans Verkuil <hverkuil@xs4all.nl>
To:     David Hildenbrand <david@redhat.com>,
        Tomasz Figa <tfiga@chromium.org>
Cc:     x86@kernel.org, linux-alpha@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        sparclinux@vger.kernel.org, linux-um@lists.infradead.org,
        etnaviv@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-samsung-soc@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-media@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-perf-users@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        John Hubbard <jhubbard@nvidia.com>,
        Peter Xu <peterx@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>, Nadav Amit <namit@vmware.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Shuah Khan <shuah@kernel.org>,
        Lucas Stach <l.stach@pengutronix.de>,
        David Airlie <airlied@gmail.com>,
        Oded Gabbay <ogabbay@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Christoph Hellwig <hch@infradead.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-kernel@vger.kernel.org
References: <20221116102659.70287-1-david@redhat.com>
 <20221116102659.70287-17-david@redhat.com>
 <36dd800b-d96b-af39-d0de-a5a8ca1034dd@xs4all.nl>
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On 23/11/2022 14:26, Hans Verkuil wrote:
> 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.

In any case, for this patch:

Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Regards,

	Hans

> 
> 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@xs4all.nl>
>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>> Cc: Tomasz Figa <tfiga@chromium.org>
>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.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;
>