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[2001:1c00:570d:ee00:fb4a:4b87:a6e0:eb91]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-be9cfc8557csm374637466b.0.2026.06.01.11.53.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Jun 2026 11:53:00 -0700 (PDT) Date: Mon, 1 Jun 2026 20:52:59 +0200 From: Amir Goldstein To: Russ Fellows Cc: linux-fsdevel@vger.kernel.org, miklos@szeredi.hu, linux-kernel@vger.kernel.org, fuse-devel@lists.linux.dev Subject: Re: [PATCH 1/2] fuse: fix FOPEN_PARALLEL_DIRECT_WRITES being ignored for passthrough writes Message-ID: References: <20260529031918.7361-1-russ.fellows@gmail.com> <20260529031918.7361-2-russ.fellows@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260529031918.7361-2-russ.fellows@gmail.com> Removing stable list - this is definetly NOT a bug fix Please CC fuse-devel@lists.linux.dev for future fuse patches On Fri, May 29, 2026 at 03:19:15AM +0000, Russ Fellows wrote: > FOPEN_PARALLEL_DIRECT_WRITES has no effect on passthrough-backed FUSE > files due to two independent bugs that each prevent it from working. > Both must be fixed to restore parallel write concurrency. > > Bug 1: fuse_passthrough_write_iter() acquires the exclusive inode lock > directly: > > inode_lock(inode); > ret = backing_file_write_iter(...); > inode_unlock(inode); > > This serializes all concurrent writers regardless of whether the server > set FOPEN_PARALLEL_DIRECT_WRITES. The flag is checked by > fuse_dio_wr_exclusive_lock(), called from fuse_dio_lock(), called from > fuse_direct_write_iter() -- the non-passthrough O_DIRECT path. > fuse_file_write_iter() routes passthrough opens to > fuse_passthrough_write_iter() instead, bypassing the flag check entirely. > > Bug 2: fuse_file_io_open() in iomode.c strips FOPEN_PARALLEL_DIRECT_WRITES > from any open that lacks FOPEN_DIRECT_IO: > > if (!(ff->open_flags & FOPEN_DIRECT_IO)) > ff->open_flags &= ~FOPEN_PARALLEL_DIRECT_WRITES; > > This is correct for regular direct-IO opens where FOPEN_DIRECT_IO ensures > O_DIRECT is actually in effect. It is wrong for passthrough opens: a > passthrough file already bypasses the FUSE page cache by definition, so > FOPEN_DIRECT_IO is redundant and should not be required to preserve the > parallel-writes flag. > > Note: adding FOPEN_DIRECT_IO to the daemon's open flags is not a valid > workaround. fuse_file_write_iter() checks FOPEN_DIRECT_IO before > FOPEN_PASSTHROUGH, so setting both causes writes to be routed through > fuse_direct_write_iter() (requiring a userspace round-trip) instead of > fuse_passthrough_write_iter() (zero-copy kernel path). > > Combined effect: a daemon that opens with FOPEN_PASSTHROUGH | > FOPEN_PARALLEL_DIRECT_WRITES (without FOPEN_DIRECT_IO) has the parallel > flag stripped by Bug 2 before Bug 1 is even reached. Both bugs must be > fixed together. > > Fix Bug 1: make fuse_dio_lock() and fuse_dio_unlock() non-static and call > them from fuse_passthrough_write_iter(), replacing the open-coded > inode_lock/inode_unlock. This reuses the existing logic that handles > FOPEN_PARALLEL_DIRECT_WRITES, append writes, writes past EOF, and > page-cache IO mode transitions. > > Fix Bug 2: skip the FOPEN_PARALLEL_DIRECT_WRITES strip when > FOPEN_PASSTHROUGH is set. The flag remains stripped for non-passthrough > opens without FOPEN_DIRECT_IO, preserving existing behaviour. > > Safety: backing_file_write_iter() calls into the backing filesystem's > write_iter (e.g. xfs_file_write_iter), which acquires the backing inode's > own lock independently. The FUSE inode lock and the backing inode lock are > entirely separate; using inode_lock_shared on the FUSE inode does not > affect the backing filesystem's concurrency control. > > Fixes: 4d99ff8f6b85 ("fuse: implement open/create with FOPEN_PASSTHROUGH") Not a fix, because this was very intentional. It is a new feature that you are proposing to support parallel passthrough dio. Anyway, this patch has many problems > Cc: stable@vger.kernel.org > Signed-off-by: Russ Fellows > --- > fs/fuse/file.c | 6 +++--- > fs/fuse/fuse_i.h | 2 ++ > fs/fuse/iomode.c | 8 ++++++-- > fs/fuse/passthrough.c | 6 +++--- > 4 files changed, 14 insertions(+), 8 deletions(-) > > diff --git a/fs/fuse/file.c b/fs/fuse/file.c > index f94f3dc082c6..602c3f18676e 100644 > --- a/fs/fuse/file.c > +++ b/fs/fuse/file.c > @@ -1428,8 +1428,8 @@ static bool fuse_dio_wr_exclusive_lock(struct kiocb *iocb, struct iov_iter *from > return false; > } > > -static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from, > - bool *exclusive) > +void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from, > + bool *exclusive) > { > struct inode *inode = file_inode(iocb->ki_filp); > struct fuse_inode *fi = get_fuse_inode(inode); > @@ -1455,7 +1455,7 @@ static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from, > } > } > > -static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive) > +void fuse_dio_unlock(struct kiocb *iocb, bool exclusive) > { > struct inode *inode = file_inode(iocb->ki_filp); > struct fuse_inode *fi = get_fuse_inode(inode); > @@ -1469,7 +1469,7 @@ static void fuse_dio_unlock(struct kiocb *iocb, bool exclusive) > } > } > > -static const struct iomap_write_ops fuse_iomap_write_ops = { > +static const struct iomap_write_ops fuse_iomap_write_ops = { /* unchanged */ > .read_folio_range = fuse_iomap_read_folio_range, > }; > > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h > index 17423d4e3cfa..120de517cea0 100644 > --- a/fs/fuse/fuse_i.h > +++ b/fs/fuse/fuse_i.h > @@ -1541,6 +1541,8 @@ int fuse_file_io_open(struct file *file, struct inode *inode); > void fuse_file_io_release(struct fuse_file *ff, struct inode *inode); > > /* file.c */ > +void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from, bool *exclusive); > +void fuse_dio_unlock(struct kiocb *iocb, bool exclusive); > struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid, > unsigned int open_flags, bool isdir); > void fuse_file_release(struct inode *inode, struct fuse_file *ff, > diff --git a/fs/fuse/iomode.c b/fs/fuse/iomode.c > index c99e285f3..b3f51e3d1 100644 > --- a/fs/fuse/iomode.c > +++ b/fs/fuse/iomode.c > @@ -214,10 +214,14 @@ int fuse_file_io_open(struct file *file, struct inode *inode) > if (fuse_inode_backing(fi) && !(ff->open_flags & FOPEN_PASSTHROUGH)) > goto fail; > > - /* > - * FOPEN_PARALLEL_DIRECT_WRITES requires FOPEN_DIRECT_IO. > - */ > - if (!(ff->open_flags & FOPEN_DIRECT_IO)) > + /* > + * FOPEN_PARALLEL_DIRECT_WRITES requires FOPEN_DIRECT_IO, except for > + * passthrough opens which bypass the page cache regardless and do not > + * need FOPEN_DIRECT_IO to guarantee direct I/O semantics. > + */ > + if (!(ff->open_flags & FOPEN_DIRECT_IO) && > + !(ff->open_flags & FOPEN_PASSTHROUGH)) > ff->open_flags &= ~FOPEN_PARALLEL_DIRECT_WRITES; > > /* > diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c > index f2d08ac2459b..f83d0a27cfb9 100644 > --- a/fs/fuse/passthrough.c > +++ b/fs/fuse/passthrough.c > @@ -54,11 +54,11 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, > struct iov_iter *iter) > { > struct file *file = iocb->ki_filp; > - struct inode *inode = file_inode(file); > struct fuse_file *ff = file->private_data; > struct file *backing_file = fuse_file_passthrough(ff); > size_t count = iov_iter_count(iter); > ssize_t ret; > + bool exclusive; > struct backing_file_ctx ctx = { > .cred = ff->cred, > .end_write = fuse_passthrough_end_write, > @@ -70,10 +70,10 @@ ssize_t fuse_passthrough_write_iter(struct kiocb *iocb, > if (!count) > return 0; > > - inode_lock(inode); > + fuse_dio_lock(iocb, iter, &exclusive); You can't just use fuse_dio_lock() like this it plays nasty games with the iomode. It does not even check for O_DIRECT mode, so this would do parallel buffered write (at least until it meets the filesystem lock but its not something we would want. You should study this code better. > ret = backing_file_write_iter(backing_file, iter, iocb, iocb->ki_flags, > &ctx); The problem is that while backing_file_write_iter() does not seem to directly require an exclusive lock (apart from maybe file_remove_privs) the fuse_passthrough_end_write() callback does I think rely on this exclusive lock. So proving that this can work will require more research and more effort and I am not really sure how that is going to work out. Thanks, Amir.