Re: [PATCH 3/3] [media] v4l2: disable filesystem-dax mapping support

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Nov 7, 2017 at 12:33 AM, Mauro Carvalho Chehab
<mchehab@s-opensource.com> wrote:
> Em Mon, 06 Nov 2017 16:57:28 -0800
> Dan Williams <dan.j.williams@intel.com> escreveu:
>
>> V4L2 memory registrations are incompatible with filesystem-dax that
>> needs the ability to revoke dma access to a mapping at will, or
>> otherwise allow the kernel to wait for completion of DMA. The
>> filesystem-dax implementation breaks the traditional solution of
>> truncate of active file backed mappings since there is no page-cache
>> page we can orphan to sustain ongoing DMA.
>>
>> If v4l2 wants to support long lived DMA mappings it needs to arrange to
>> hold a file lease or use some other mechanism so that the kernel can
>> coordinate revoking DMA access when the filesystem needs to truncate
>> mappings.
>
>
> Not sure if I understand this your comment here... what happens
> if FS_DAX is enabled? The new err = get_user_pages_longterm()
> would cause DMA allocation to fail?

Correct, any attempt to specify a filesystem-dax mapping range to
get_user_pages_longterm will fail with EOPNOTSUPP. In the future we
want to add something like a 'struct file_lock *' argument to
get_user_pages_longterm so that the kernel has a handle to revoke
access to the returned pages. Once we have a safe way for the kernel
to undo elevated page counts we can stop failing the longterm vs
filesystem-dax case.

Here is more background on why _longterm gup is a problem for filesystem-dax:

    https://lwn.net/Articles/737273/

> If so, that doesn't sound
> right. Instead, mm should somehow mark this mapping to be out
> of FS_DAX control range.

DAX is currently global setting for the entire backing device of the
filesystem, so any mapping of any file when the "-o dax" mount option
is set is in the "FS_DAX control range". In other words there's
currently no way to prevent FS_DAX mappings from being exposed to V4L2
outside of CONFIG_FS_DAX=n.

> Also, it is not only videobuf-dma-sg.c that does long lived
> DMA mappings. VB2 also does that (and videobuf-vmalloc).

Without finding the code videobuf-vmalloc sounds like it should be ok
if the kernel is allocating memory separate from a file-backed DAX
mapping. Where is the VB2 get_user_pages call?

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