Re: [PATCH] fuse: avoid unnecessary spinlock bump

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Apr 11, 2022 at 10:10:23AM +0800, JeffleXu wrote:
> 
> 
> On 4/8/22 8:06 PM, Vivek Goyal wrote:
> > 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.
> 
> Curiously, why minimum 1 range is not adequate? In which case minimum 2
> are required?

Frankly speaking, right now I don't remember. I have vague memories
of concluding in the past that 1 range is not sufficient. But if you
like dive deeper, and try with one range and see if you can introduce
deadlock.

Thanks
Vivek