Re: dm-writecache

[Mikulas Patocka <mpatocka@redhat.com>]

On Fri, 18 May 2018, Dan Williams wrote:

> On Fri, May 18, 2018 at 8:44 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> > On Thu, Mar 08 2018 at 12:08pm -0500,
> > Dan Williams <dan.j.williams@intel.com> wrote:
> >
> >> Mikulas sent this useful enhancement to the memcpy_flushcache API:
> >>
> >>     https://patchwork.kernel.org/patch/10217655/
> >>
> >> ...it's in my queue to either push through -tip or add it to the next
> >> libnvdimm pull request for 4.17-rc1.
> >
> > Hi Dan,
> >
> > Seems this never actually went upstream.  I've staged it in
> > linux-dm.git's "for-next" for the time being:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.18&id=a7e96990b5ff6206fefdc5bfe74396bb880f7e48
> >
> > But do you intend to pick it up for 4.18 inclusion?  If so I'll drop
> > it.. would just hate for it to get dropped on the floor by getting lost
> > in the shuffle between trees.
> >
> > Please avise, thanks!
> > Mike
> 
> Thanks for picking it up! I was hoping to resend it to get acks from
> x86 folks, and then yes it fell through the cracks in my patch
> tracking.
> 
> Now that I look at it again I don't think we need this hunk:
> 
> void memcpy_page_flushcache(char *to, struct page *page, size_t offset,
> size_t len)
> {
> char *from = kmap_atomic(page);
> - memcpy_flushcache(to, from + offset, len);
> + __memcpy_flushcache(to, from + offset, len);
> kunmap_atomic(from);
> }

Yes - this is not needed.

> ...and I wonder what the benefit is of the 16-byte case? I would
> assume the bulk of the benefit is limited to the 4 and 8 byte copy
> cases.

dm-writecache uses 16-byte writes frequently, so it is needed for that.

If we split 16-byte write to two 8-byte writes, it would degrade 
performance for architectures where memcpy_flushcache needs to flush the 
cache.

> Mikulas please resend with those comments addressed and include Ingo and 
> Thomas.

Mikulas
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