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

[Shakeel Butt <shakeel.butt@linux.dev>]

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@linux.dev> 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.