On 4/10/25 7:47 AM, Joanne Koong wrote:
> On Tue, Apr 8, 2025 at 7:43 PM Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote:
>>
>> Hi Joanne,
>>
>> On 4/5/25 2:14 AM, Joanne Koong wrote:
>>> 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 in order to mitigate the following deadlock scenario
>>> that may arise if reclaim waits on writeback on the dirty page to complete:
>>> * 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
>>>
>>> With a recent change that added AS_WRITEBACK_INDETERMINATE and mitigates
>>> the situations described above, FUSE writeback does not need to use
>>> temp pages if it sets AS_WRITEBACK_INDETERMINATE on its inode mappings.
>>>
>>> This commit sets AS_WRITEBACK_INDETERMINATE on the inode mappings
>>> and removes the temporary pages + extra copying and the internal rb
>>> tree.
>>>
>>> fio benchmarks --
>>> (using averages observed from 10 runs, throwing away outliers)
>>>
>>> Setup:
>>> sudo mount -t tmpfs -o size=30G tmpfs ~/tmp_mount
>>> ./libfuse/build/example/passthrough_ll -o writeback -o max_threads=4 -o source=~/tmp_mount ~/fuse_mount
>>>
>>> fio --name=writeback --ioengine=sync --rw=write --bs={1k,4k,1M} --size=2G
>>> --numjobs=2 --ramp_time=30 --group_reporting=1 --directory=/root/fuse_mount
>>>
>>> bs = 1k 4k 1M
>>> Before 351 MiB/s 1818 MiB/s 1851 MiB/s
>>> After 341 MiB/s 2246 MiB/s 2685 MiB/s
>>> % diff -3% 23% 45%
>>>
>>> Signed-off-by: Joanne Koong <joannelkoong@xxxxxxxxx>
>>> Reviewed-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>
>>> Acked-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
>>
>
> Hi Jingbo,
>
> Thanks for sharing your analysis for this.
>
>> Overall this patch LGTM.
>>
>> Apart from that, IMO the fi->writectr and fi->queued_writes mechanism is
>> also unneeded then, at least the DIRECT IO routine (i.e.
>
> I took a look at fi->writectr and fi->queued_writes and my
> understanding is that we do still need this. For example, for
> truncates (I'm looking at fuse_do_setattr()), I think we still need to
> prevent concurrent writeback or else the setattr request and the
> writeback request could race which would result in a mismatch between
> the file's reported size and the actual data written to disk.
I haven't looked into the truncate routine yet. I will see it later.
>
>> fuse_direct_io()) doesn't need fuse_sync_writes() anymore. That is
>> because after removing the temp page, the DIRECT IO routine has already
>> been waiting for all inflight WRITE requests, see
>>
>> # DIRECT read
>> generic_file_read_iter
>> kiocb_write_and_wait
>> filemap_write_and_wait_range
>
> Where do you see generic_file_read_iter() getting called for direct io reads?
# DIRECT read
fuse_file_read_iter
fuse_cache_read_iter
generic_file_read_iter
kiocb_write_and_wait
filemap_write_and_wait_range
a_ops->direct_IO(),i.e. fuse_direct_IO()
> Similarly, where do you see generic_file_write_iter() getting called
> for direct io writes?
# DIRECT read
fuse_file_write_iter
fuse_cache_write_iter
generic_file_write_iter
generic_file_direct_write
kiocb_invalidate_pages
filemap_invalidate_pages
filemap_write_and_wait_range
a_ops->direct_IO(),i.e. fuse_direct_IO()
> Where do you see fi->writectr / fi->queued-writes preventing this
> race?
IMO overall fi->writectr / fi->queued-writes are introduced to prevent
DIRECT IO and writeback from sending duplicate (inflight) WRITE requests
for the same page.
For the DIRECT write routine:
# non-FOPEN_DIRECT_IO DIRECT write
fuse_cache_write_iter
fuse_direct_IO
fuse_direct_io
fuse_sync_writes
# FOPEN_DIRECT_IO DIRECT write
fuse_direct_write_iter
fuse_direct_IO
fuse_direct_io
fuse_sync_writes
For the writeback routine:
fuse_writepages()
fuse_writepages_fill
fuse_writepages_send
# buffer the WRITE request in queued_writes list
fuse_flush_writepages
# flush WRITE only when fi->writectr >= 0
> It looks to me like in the existing code, this race condition
> you described of direct write invalidating the page cache, then
> another buffer write reads the page cache and dirties it, then
> writeback is called on that, and the 2 write requests racing, could
> still happen?
>
>
>> However it seems that the writeback
>> won't wait for previous inflight DIRECT WRITE requests, so I'm not much
>> sure about that. Maybe other folks could offer more insights...
>
> My understanding is that these lines
>
> if (!cuse && filemap_range_has_writeback(...)) {
> ...
> fuse_sync_writes(inode);
> ...
> }
>
> in fuse_direct_io() is what waits on previous inflight direct write
> requests to complete before the direct io happens.
Right.
>
>
>>
>> Also fuse_sync_writes() is not needed in fuse_flush() anymore, with
>> which I'm pretty sure.
>
> Why don't we still need this for fuse_flush()?
>
> If a caller calls close(), this will call
>
> filp_close()
> filp_flush()
> filp->f_op->flush()
> fuse_flush()
>
> it seems like we should still be waiting for all writebacks to finish
> before sending the fuse server the fuse_flush request, no?
>
filp_close()
filp_flush()
filp->f_op->flush()
fuse_flush()
write_inode_now
writeback_single_inode(WB_SYNC_ALL)
do_writepages
# flush dirty page
filemap_fdatawait
# wait for WRITE completion
>>
>>> ---
>>> fs/fuse/file.c | 360 ++++-------------------------------------------
>>> fs/fuse/fuse_i.h | 3 -
>>> 2 files changed, 28 insertions(+), 335 deletions(-)
>>>
>>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
>>> index 754378dd9f71..91ada0208863 100644
>>> --- a/fs/fuse/file.c
>>> +++ b/fs/fuse/file.c
>>> @@ -415,89 +415,11 @@ u64 fuse_lock_owner_id(struct fuse_conn *fc, fl_owner_t id)
>>>
>>> struct fuse_writepage_args {
>>> struct fuse_io_args ia;
>>> - struct rb_node writepages_entry;
>>> struct list_head queue_entry;
>>> - struct fuse_writepage_args *next;
>>> struct inode *inode;
>>> struct fuse_sync_bucket *bucket;
>>> };
>>>
>>> -static struct fuse_writepage_args *fuse_find_writeback(struct fuse_inode *fi,
>>> - pgoff_t idx_from, pgoff_t idx_to)
>>> -{
>>> - struct rb_node *n;
>>> -
>>> - n = fi->writepages.rb_node;
>>> -
>>> - while (n) {
>>> - struct fuse_writepage_args *wpa;
>>> - pgoff_t curr_index;
>>> -
>>
>> --
>> Thanks,
>> Jingbo
--
Thanks,
Jingbo
