On 7/25/23 12:11, Hao Xu wrote:
On 7/21/23 19:56, Bernd Schubert wrote:
On July 21, 2023 1:27:26 PM GMT+02:00, Hao Xu <hao.xu@xxxxxxxxx> wrote:
On 7/21/23 14:35, Jiachen Zhang wrote:
On 2023/6/30 17:46, Hao Xu wrote:
From: Hao Xu <howeyxu@xxxxxxxxxxx>
In direct_io_relax mode, there can be shared mmaped files and thus
dirty
pages in its page cache. Therefore those dirty pages should be written
back to backend before direct write to avoid data loss.
Signed-off-by: Hao Xu <howeyxu@xxxxxxxxxxx>
---
fs/fuse/file.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 176f719f8fc8..7c9167c62bf6 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1485,6 +1485,13 @@ ssize_t fuse_direct_io(struct fuse_io_priv
*io, struct iov_iter *iter,
if (!ia)
return -ENOMEM;
+ if (fopen_direct_write && fc->direct_io_relax) {
+ res = filemap_write_and_wait_range(mapping, pos, pos +
count - 1);
+ if (res) {
+ fuse_io_free(ia);
+ return res;
+ }
+ }
if (!cuse && fuse_range_is_writeback(inode, idx_from,
idx_to)) {
if (!write)
inode_lock(inode);
Tested-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
Looks good to me.
By the way, the behaviour would be a first FUSE_WRITE flushing the
page cache, followed by a second FUSE_WRITE doing the direct IO. In
the future, further optimization could be first write into the page
cache and then flush the dirty page to the FUSE daemon.
I think this makes sense, cannot think of any issue in it for now, so
I'll do that change and send next version, super thanks, Jiachen!
Thanks,
Hao
Thanks,
Jiachen
On my phone, sorry if mail formatting is not optimal.
Do I understand it right? You want DIO code path copy into pages and
then flush/invalidate these pages? That would be punish DIO for for
the unlikely case there are also dirty pages (discouraged IO pattern).
Hi Bernd,
I think I don't get what you said, why it is punishment and why it's
discouraged IO pattern?
On my first eyes seeing Jiachen's idea, I was thinking "that sounds
disobeying direct write semantics" because usually direct write is
"flush dirty page -> invalidate page -> write data through to backend"
not "write data to page -> flush dirty page/(writeback data)"
The latter in worst case write data both to page cache and backend
while the former just write to backend and load it to the page cache
when buffered reading. But seems there is no such "standard way" which
says we should implement direct IO in that way.
Hi Hao,
sorry for being brief last week, I was on vacation and reading/writing
some mails on my phone.
With 'punishment' I mean memory copies to the page cache - memory copies
are expensive and DIO should avoid it.
Right now your patch adds filemap_write_and_wait_range(), but we do not
know if it did work (i.e. if pages had to be flushed). So unless you
find a way to get that information, copy to page cache would be
unconditionally - overhead of memory copy even if there are no dirty pages.
With 'discouraged' I mean mix of page cache and direct-io. Typically one
should only do either of both (page cache or DIO), but not a mix of
them. For example see your patch, it flushes the page cache, but without
a lock - races are possible. Copying to the page cache might be a
solution, but it has the overhead above.
Thanks,
Bernd
