Re: [PATCH] fuse: enable large folios (if writeback cache is unused)

[Joanne Koong <joannelkoong@gmail.com>]

On Mon, Aug 11, 2025 at 8:26 PM Chunsheng Luo <luochunsheng@ustc.edu> wrote:
>
> On Mon, Aug 11, 2025 Joanne Koong <joannelkoong@gmail.com> wrote:
> >>
> >> Large folios are only enabled if the writeback cache isn't on.
> >> (Strictlimiting needs to be turned off if the writeback cache is used in
> >> conjunction with large folios, else this tanks performance.)
> >
> > Some ideas for having this work with the writeback cache are
> > a) add a fuse sysctl sysadmins can set to turn off strictlimiting for
> > all fuse servers mounted after, in the kernel turn on large folios for
> > writeback if that sysctl is on
> > b) if the fuse server is privileged automatically turn off
> > strictlimiting and enable large folios for writeback
> >
> > Any thoughts?
>
> Should large folios be enabled based on mount options? Consider adding an
> option in fuse_init_out to explicitly turn on large folios.
>

Hi Chunsheng,

Personally I'm not a fan of doing it through the init request because
it is tied hand-in-hand with disabling strictlimiting (which requires
admin privileges) and imo
a) it feels clunky that the user needs to opt into it for writeback
(for non-writeback cases, ideally large folios are the status quo) and
then also find the bdi that corresponds to that fuse mount, then go
into /sys/class/bdi/* for that bdi to disable strictlimiting, all
while making sure this happens before write workloads start
b) I think users (who most are not familiar with kernel internals)
will likely be confused by what large folios are and whether/when they
should opt into it or not

imo if the fuse server is mounted as a privileged server, I think it's
reasonable that strictlimiting could be turned off by default.

Thanks,
Joanne

> Thanks
> Chunsheng Luo