Re: + mips-c-r4k-do-no-use-mmap_sem-for-gup_fast.patch added to -mm tree

[Paul Burton <paul.burton@mips.com>]

Hi Andrew,

On Thu, Feb 07, 2019 at 09:44:07PM -0800, akpm@linux-foundation.org wrote:
> 
> The patch titled
>      Subject: arch/mips/mm/c-r4k.c: do not use mmap_sem for gup_fast()
> has been added to the -mm tree.  Its filename is
>      mips-c-r4k-do-no-use-mmap_sem-for-gup_fast.patch
> 
> This patch should soon appear at
>     http://ozlabs.org/~akpm/mmots/broken-out/mips-c-r4k-do-no-use-mmap_sem-for-gup_fast.patch
> and later at
>     http://ozlabs.org/~akpm/mmotm/broken-out/mips-c-r4k-do-no-use-mmap_sem-for-gup_fast.patch
> 
> Before you just go and hit "reply", please:
>    a) Consider who else should be cc'ed
>    b) Prefer to cc a suitable mailing list as well
>    c) Ideally: find the original patch on the mailing list and do a
>       reply-to-all to that, adding suitable additional cc's
> 
> *** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
> 
> The -mm tree is included into linux-next and is updated
> there every 3-4 working days
> 
> ------------------------------------------------------
> From: Davidlohr Bueso <dave@stgolabs.net>
> Subject: arch/mips/mm/c-r4k.c: do not use mmap_sem for gup_fast()
> 
> It is well known that because the mm can internally call the regular
> gup_unlocked if the lockless approach fails and take the sem there, the
> caller must not hold the mmap_sem already.
> 
> Link: http://lkml.kernel.org/r/20190207053740.26915-3-dave@stgolabs.net
> Fixes: e523f289fe4d ("MIPS: c-r4k: Fix sigtramp SMP call to use kmap")
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Paul Burton <paul.burton@mips.com>
> Cc: James Hogan <jhogan@kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Whilst I don't object to you merging this too strongly, I thought I'd
point out again that as I already replied to Davidlohr [1] the code
being changed here is unused in mainline & all affected stable branches.
In mips-next it's entirely removed. As such this patch will have no
effect on the kernel's behaviour & cause a minor conflict with
mips-next.

Thanks,
    Paul

[1] https://lore.kernel.org/linux-mips/20190207190007.jz4rz6e6qxwazxm7@pburton-laptop/