On Sat, 23 Nov 2024 at 00:24, Joanne Koong <joannelkoong@xxxxxxxxx> 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@xxxxxxxxxx>
I think this should go via the mm tree.
Thanks,
Miklos
