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
