Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, 14 Jan 2025 at 19:08, Joanne Koong <joannelkoong@gmail.com> wrote:

> - my understanding is that the majority of use cases do use splice (eg
> iirc, libfuse does as well), in which case there's no point to this
> patchset then

If it turns out that non-splice writes are more performant, then
libfuse can be fixed to use non-splice by default.   It's not as clear
cut though, since write through (which is also the default in libfuse,
AFAIK) should not be affected by all this, since that never used tmp
pages.

> - codewise, imo this gets messy (eg we would still need the rb tree
> and would now need to check writeback against folio writeback state
> and against the rb tree)

I'm thinking of something slightly different: remove the current tmp
page mess, but instead of duplicating a page ref on splice, fall back
to copying the cache page (see the user_pages case in
fuse_copy_page()).  This should have very similar performance to what
we have today, but allows us to deal with page accesses the same way
for both regular and splice I/O on /dev/fuse.

Thanks,
Miklos