Re: [PATCH v2 2/2] fuse: remove tmp folio for writebacks and internal rb tree

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On Thu, Oct 31, 2024 at 1:06 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
>
> On Thu, Oct 31, 2024 at 12:06:49PM GMT, Joanne Koong wrote:
> > On Wed, Oct 30, 2024 at 5:30 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> [...]
> > >
> > > Memory pool is a bit confusing term here. Most probably you are asking
> > > about the migrate type of the page block from which tmp page is
> > > allocated from. In a normal system, tmp page would be allocated from page
> > > block with MIGRATE_UNMOVABLE migrate type while the page cache page, it
> > > depends on what gfp flag was used for its allocation. What does fuse fs
> > > use? GFP_HIGHUSER_MOVABLE or something else? Under low memory situation
> > > allocations can get mixed up with different migrate types.
> > >
> >
> > I believe it's GFP_HIGHUSER_MOVABLE for the page cache pages since
> > fuse doesn't set any additional gfp masks on the inode mapping.
> >
> > Could we just allocate the fuse writeback pages with GFP_HIGHUSER
> > instead of GFP_HIGHUSER_MOVABLE? That would be in fuse_write_begin()
> > where we pass in the gfp mask to __filemap_get_folio(). I think this
> > would give us the same behavior memory-wise as what the tmp pages
> > currently do,
>
> I don't think it would be the same behavior. From what I understand the
> liftime of the tmp page is from the start of the writeback till the ack
> from the fuse server that writeback is done. While the lifetime of the
> page of the page cache can be arbitrarily large. We should just make it
> unmovable for its lifetime. I think it is fine to make the page
> unmovable during the writeback. We should not try to optimize for the
> bad or buggy behavior of fuse server.
>
> Regarding the avoidance of wait on writeback for fuse folios, I think we
> can handle the migration similar to how you are handling reclaim and in
> addition we can add a WARN() in folio_wait_writeback() if the kernel ever
> sees a fuse folio in that function.

Awesome, this is what I'm planning to do in v3 to address migration then:

1) in migrate_folio_unmap(), only call "folio_wait_writeback(src);" if
src->mapping does not have the AS_NO_WRITEBACK_WAIT bit set on it (eg
fuse folios will have that AS_NO_WRITEBACK_WAIT bit set)

2) in the fuse filesystem's implementation of the
mapping->a_ops->migrate_folio callback, return -EAGAIN if the folio is
under writeback.

Does this sound good?


Thanks,
Joanne





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