On Thu, Oct 31, 2024 at 02:52:57PM GMT, Joanne Koong wrote: > 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. 3) Add WARN_ONCE() in folio_wait_writeback() if folio->mapping has AS_NO_WRITEBACK_WAIT set and return without waiting. > > Does this sound good? Yes.
