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

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On 22.11.22 13:25, Hans Verkuil wrote:
Hi Tomasz, David,

On 11/8/22 05:45, Tomasz Figa wrote:
Hi David,

On Tue, Nov 8, 2022 at 1:19 AM David Hildenbrand <david@xxxxxxxxxx> wrote:

FOLL_FORCE is really only for debugger access. According to commit
707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
writable"), the pinned pages are always writable.

Actually that patch is only a workaround to temporarily disable
support for read-only pages as they seemed to suffer from some
corruption issues in the retrieved user pages. We expect to support
read-only pages as hardware input after. That said, FOLL_FORCE doesn't
sound like the right thing even in that case, but I don't know the
background behind it being added here in the first place. +Hans
Verkuil +Marek Szyprowski do you happen to remember anything about it?

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.

I'm not sure if the 'force' argument of get_user_pages() at that time
even meant the same as FOLL_FORCE today. From what I can tell it has just
been faithfully used ever since, but I have my doubt that anyone understands
the reason behind it since it was never explained.

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
(non-zero write argument) where you also want to be able to read from the
buffer. That is certainly something that some drivers need to do post-capture
fixups.

But 'force' was also always set for read buffers, and I don't know if that
was something that was actually needed, or just laziness.

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

Yes. The only problematic corner case I can imagine is if someone has a VMA without write permissions (no PROT_WRITE/VM_WRITE) and wants to pin user space pages as a read buffer. We'd specify now FOLL_WRITE without FOLL_FORCE and GUP would reject that: write access without write permissions is invalid.

There would be no way around "fixing" this implementation to not specify FOLL_WRITE when only reading from user-space pages. Not sure what the implications are regarding that corruption that was mentioned in 707947247e95.

Having said that, I assume such a scenario is unlikely -- but you might know better how user space usually uses this interface. There would be three options:

1) Leave the FOLL_FORCE hack in for now, which I *really* want to avoid.
2) Remove FOLL_FORCE and see if anybody even notices (this patch) and
   leave the implementation as is for now.
3) Remove FOLL_FORCE and fixup the implementation to only specify
   FOLL_WRITE if the pages will actually get written to.

3) would most probably ideal, however, I am no expert on that code and can't do it (707947247e95 confuses me). So naive me would go with 2) first.

--
Thanks,

David / dhildenb





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