On 05/30/2013 08:41 AM, Maxim Patlasov wrote:
> The bug was introduced with async_dio feature: trying to optimize short reads,
> we cut number-of-bytes-to-read to i_size boundary. Hence the following
> example:
>
> truncate --size=300 /mnt/file
> dd if=/mnt/file of=/dev/null iflag=direct
>
> led to FUSE_READ request of 300 bytes size. This turned out to be problem
> for userspace fuse implementations who rely on assumption that kernel fuse
> does not change alignment of request from client FS.
>
> The patch turns off the optimization if async_dio is disabled. And, if it's
> enabled, the patch fixes adjustment of number-of-bytes-to-read to preserve
> alignment.
>
> Note, that we cannot throw out short read optimization entirely because
> otherwise a direct read of a huge size issued on a tiny file would generate
> a huge amount of fuse requests and most of them would be ACKed by userspace
> with zero bytes read.
>
> Signed-off-by: Maxim Patlasov <mpatla...@parallels.com>
> ---
Looks good and passes my tests. Thanks for cooking this up Maxim!
Reviewed-by: Brian Foster <bfos...@redhat.com>
> fs/fuse/file.c | 14 ++++++++++----
> 1 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index d1c9b85..9026572 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -2365,6 +2365,11 @@ static void fuse_do_truncate(struct file *file)
> fuse_do_setattr(inode, &attr, file);
> }
>
> +static inline loff_t fuse_round_up(loff_t off)
> +{
> + return round_up(off, FUSE_MAX_PAGES_PER_REQ << PAGE_SHIFT);
> +}
> +
> static ssize_t
> fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
> loff_t offset, unsigned long nr_segs)
> @@ -2372,6 +2377,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct
> iovec *iov,
> ssize_t ret = 0;
> struct file *file = iocb->ki_filp;
> struct fuse_file *ff = file->private_data;
> + bool async_dio = ff->fc->async_dio;
> loff_t pos = 0;
> struct inode *inode;
> loff_t i_size;
> @@ -2383,10 +2389,10 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const
> struct iovec *iov,
> i_size = i_size_read(inode);
>
> /* optimization for short read */
> - if (rw != WRITE && offset + count > i_size) {
> + if (async_dio && rw != WRITE && offset + count > i_size) {
> if (offset >= i_size)
> return 0;
> - count = i_size - offset;
> + count = min_t(loff_t, count, fuse_round_up(i_size - offset));
> }
>
> io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
> @@ -2404,7 +2410,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct
> iovec *iov,
> * By default, we want to optimize all I/Os with async request
> * submission to the client filesystem if supported.
> */
> - io->async = ff->fc->async_dio;
> + io->async = async_dio;
> io->iocb = iocb;
>
> /*
> @@ -2412,7 +2418,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct
> iovec *iov,
> * to wait on real async I/O requests, so we must submit this request
> * synchronously.
> */
> - if (!is_sync_kiocb(iocb) && (offset + count > i_size))
> + if (!is_sync_kiocb(iocb) && (offset + count > i_size) && rw == WRITE)
> io->async = false;
>
> if (rw == WRITE)
>
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