Re: [PATCH v6 0/5] fuse: remove temp page copies in writeback

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

+Andrew

On Fri, Dec 13, 2024 at 12:52:44PM +0100, Miklos Szeredi wrote:
> On Sat, 23 Nov 2024 at 00:24, Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > The purpose of this patchset is to help make writeback-cache write
> > performance in FUSE filesystems as fast as possible.
> >
> > In the current FUSE writeback design (see commit 3be5a52b30aa
> > ("fuse: support writable mmap"))), a temp page is allocated for every dirty
> > page to be written back, the contents of the dirty page are copied over to the
> > temp page, and the temp page gets handed to the server to write back. This is
> > done so that writeback may be immediately cleared on the dirty page, and this
> > in turn is done for two reasons:
> > a) in order to mitigate the following deadlock scenario that may arise if
> > reclaim waits on writeback on the dirty page to complete (more details can be
> > found in this thread [1]):
> > * single-threaded FUSE server is in the middle of handling a request
> >   that needs a memory allocation
> > * memory allocation triggers direct reclaim
> > * direct reclaim waits on a folio under writeback
> > * the FUSE server can't write back the folio since it's stuck in
> >   direct reclaim
> > b) in order to unblock internal (eg sync, page compaction) waits on writeback
> > without needing the server to complete writing back to disk, which may take
> > an indeterminate amount of time.
> >
> > Allocating and copying dirty pages to temp pages is the biggest performance
> > bottleneck for FUSE writeback. This patchset aims to get rid of the temp page
> > altogether (which will also allow us to get rid of the internal FUSE rb tree
> > that is needed to keep track of writeback status on the temp pages).
> > Benchmarks show approximately a 20% improvement in throughput for 4k
> > block-size writes and a 45% improvement for 1M block-size writes.
> >
> > With removing the temp page, writeback state is now only cleared on the dirty
> > page after the server has written it back to disk. This may take an
> > indeterminate amount of time. As well, there is also the possibility of
> > malicious or well-intentioned but buggy servers where writeback may in the
> > worst case scenario, never complete. This means that any
> > folio_wait_writeback() on a dirty page belonging to a FUSE filesystem needs to
> > be carefully audited.
> >
> > In particular, these are the cases that need to be accounted for:
> > * potentially deadlocking in reclaim, as mentioned above
> > * potentially stalling sync(2)
> > * potentially stalling page migration / compaction
> >
> > This patchset adds a new mapping flag, AS_WRITEBACK_INDETERMINATE, which
> > filesystems may set on its inode mappings to indicate that writeback
> > operations may take an indeterminate amount of time to complete. FUSE will set
> > this flag on its mappings. This patchset adds checks to the critical parts of
> > reclaim, sync, and page migration logic where writeback may be waited on.
> >
> > Please note the following:
> > * For sync(2), waiting on writeback will be skipped for FUSE, but this has no
> >   effect on existing behavior. Dirty FUSE pages are already not guaranteed to
> >   be written to disk by the time sync(2) returns (eg writeback is cleared on
> >   the dirty page but the server may not have written out the temp page to disk
> >   yet). If the caller wishes to ensure the data has actually been synced to
> >   disk, they should use fsync(2)/fdatasync(2) instead.
> > * AS_WRITEBACK_INDETERMINATE does not indicate that the folios should never be
> >   waited on when in writeback. There are some cases where the wait is
> >   desirable. For example, for the sync_file_range() syscall, it is fine to
> >   wait on the writeback since the caller passes in a fd for the operation.
> 
> Looks good, thanks.
> 
> Acked-by: Miklos Szeredi <mszeredi@redhat.com>
> 
> I think this should go via the mm tree.

Andrew, can you please pick this series up or Joanne can send an updated
version with all Acks/Review tag collected? Let us know what you prefer.

Thanks,
Shakeel