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

[Joanne Koong <joannelkoong@gmail.com>]

On Tue, Aug 12, 2025 at 6:20 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> On Tue, Aug 12, 2025 at 04:02:12PM -0700, Joanne Koong wrote:
> > On Tue, Aug 12, 2025 at 12:38 PM Darrick J. Wong <djwong@kernel.org> wrote:
> > >
> > My understanding of strictlimit is that it's a way of preventing
> > non-trusted filesystems from dirtying too many pages too quickly and
> > thus taking up too much bandwidth. It imposes stricter / more
>
> Oh, BDI_CAP_STRICTLIMIT.
>
> /me digs
>
> "Then wb_thresh is 1% of 20% of 16GB. This amounts to ~8K pages."
>
> Oh wow.
>
> > conservative limits on how many pages a filesystem can dirty before it
> > gets forcibly throttled (the bulk of the logic happens in
> > balance_dirty_pages()). This is needed for fuse because fuse servers
> > may be unprivileged and malicious or buggy. The feature was introduced
> > in commit 5a53748568f7 ("mm/page-writeback.c: add strictlimit
> > feature). The reason we now run into this is because with large
> > folios, the dirtying happens so much faster now (eg a 1MB folio is
> > dirtied and copied at once instead of page by page), and as a result
> > the fuse server gets throttled more while doing large writes, which
> > ends up making the write overall slower.
>
> <nod> and hence your patchset gives the number of dirty blocks (pages?)
> within the large folio to the writeback throttling code so that you
> don't get charged for 2M of dirty data if you've really only touched a
> single byte of a 2M folio, right?
>
Yeah, exactly!
> Will go have a look at that tomorrow.
Thanks!
>
> --D
>