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Message-ID: <0da1813a-7e37-bc35-cf8d-8c41590f3b1a@xs4all.nl>
Date: Wed, 23 Nov 2022 15:28:27 +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,
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Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Jason Gunthorpe <jgg@...pe.ca>,
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Peter Xu <peterx@...hat.com>,
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Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
Vlastimil Babka <vbabka@...e.cz>,
Matthew Wilcox <willy@...radead.org>,
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Oded Gabbay <ogabbay@...nel.org>,
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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
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@...all.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@...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;
>
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