Re: [PATCH] fuse: avoid unnecessary spinlock bump

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Apr 08, 2022 at 07:50:55PM +0800, JeffleXu wrote:
> 
> 
> On 4/8/22 7:25 PM, Vivek Goyal wrote:
> > On Fri, Apr 08, 2022 at 10:36:40AM +0800, JeffleXu wrote:
> >>
> >>
> >> On 4/7/22 10:10 PM, Vivek Goyal wrote:
> >>> On Sat, Apr 02, 2022 at 06:32:50PM +0800, Jeffle Xu wrote:
> >>>> Move dmap free worker kicker inside the critical region, so that extra
> >>>> spinlock lock/unlock could be avoided.
> >>>>
> >>>> Suggested-by: Liu Jiang <gerry@linux.alibaba.com>
> >>>> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> >>>
> >>> Looks good to me. Have you done any testing to make sure nothing is
> >>> broken.
> >>
> >> xfstests -g quick shows no regression. The tested virtiofs is mounted
> >> with "dax=always".
> > 
> > I think xfstests might not trigger reclaim. You probably will have to
> > run something like blogbench with a small dax window like 1G so that
> > heavy reclaim happens.
> 
> 
> Actually, I configured the DAX window to 8MB, i.e. 4 slots when running
> xfstests. Thus I think the reclaim path is most likely triggered.
> 
> 
> > 
> > For fun, I sometimes used to run it with a window of just say 16 dax
> > ranges so that reclaim was so heavy that if there was a bug, it will
> > show up.
> > 
> 
> Yeah, my colleague had ever reported that a DAX window of 4KB will cause
> hang in our internal OS (which is 4.19, we back ported virtiofs to
> 4.19). But then I found that this issue doesn't exist in the latest
> upstream. The reason seems that in the upstream kernel,
> devm_memremap_pages() called in virtio_fs_setup_dax() will fail directly
> since the dax window (4KB) is not aligned with the sparse memory section.

Given our default chunk size is 2MB (FUSE_DAX_SHIFT), may be it is not
a bad idea to enforce some minimum cache window size. IIRC, even one
range is not enough. Minimum 2 are required for reclaim to not deadlock.

Hence, I guess it is not a bad idea to check for cache window size and
if it is too small, reject it and disable dax.

Thanks
Vivek