If userspace calls sendfile() with a very large "count" parameter, the kernel can block for a very long time until 2 GiB (0x7ffff000 bytes) have been read from the hard disk and pushed into the socket buffer. Usually, that is not a problem, because the socket write buffer gets filled quickly, and if the socket is non-blocking, the last direct_splice_actor() call will return -EAGAIN, causing splice_direct_to_actor() to break from the loop, and sendfile() will return a partial transfer. However, if the network happens to be faster than the hard disk, and the socket buffer keeps getting drained between two generic_file_read_iter() calls, the sendfile() system call can keep running for a long time, blocking for disk I/O over and over. That is undesirable, because it can block the calling process for too long. I discovered a problem where nginx would block for so long that it would drop the HTTP connection because the kernel had just transferred 2 GiB in one call, and the HTTP socket was not writable (EPOLLOUT) for more than 60 seconds, resulting in a timeout: sendfile(4, 12, [5518919528] => [5884939344], 1813448856) = 366019816 <3.033067> sendfile(4, 12, [5884939344], 1447429040) = -1 EAGAIN (Resource temporarily unavailable) <0.000037> epoll_wait(9, [{EPOLLOUT, {u32=2181955104, u64=140572166585888}}], 512, 60000) = 1 <0.003355> gettimeofday({tv_sec=1667508799, tv_usec=201201}, NULL) = 0 <0.000024> sendfile(4, 12, [5884939344] => [8032418896], 2147480496) = 2147479552 <10.727970> writev(4, [], 0) = 0 <0.000439> epoll_wait(9, [], 512, 60000) = 0 <60.060430> gettimeofday({tv_sec=1667508869, tv_usec=991046}, NULL) = 0 <0.000078> write(5, "10.40.5.23 - - [03/Nov/2022:21:5"..., 124) = 124 <0.001097> close(12) = 0 <0.000063> close(4) = 0 <0.000091> In newer nginx versions (since 1.21.4), this problem was worked around by defaulting "sendfile_max_chunk" to 2 MiB: https://github.com/nginx/nginx/commit/5636e7f7b4 Instead of asking userspace to provide an artificial upper limit, I'd like the kernel to block for disk I/O at most once, and then pass back control to userspace. There is prior art for this kind of behavior in filemap_read(): /* * If we've already successfully copied some data, then we * can no longer safely return -EIOCBQUEUED. Hence mark * an async read NOWAIT at that point. */ if ((iocb->ki_flags & IOCB_WAITQ) && already_read) iocb->ki_flags |= IOCB_NOWAIT; This modifies the caller-provided "struct kiocb", which has an effect on repeated filemap_read() calls. This effect however vanishes because the "struct kiocb" is not persistent; splice_direct_to_actor() doesn't have one, and each generic_file_splice_read() call initializes a new one, losing the "IOCB_NOWAIT" flag that was injected by filemap_read(). There was no way to make generic_file_splice_read() aware that IOCB_NOWAIT was desired because some data had already been transferred in a previous call: - checking whether the input file has O_NONBLOCK doesn't work because this should be fixed even if the input file is not non-blocking - the SPLICE_F_NONBLOCK flag is not appropriate because it affects only whether pipe operations are non-blocking, not whether file/socket operations are non-blocking Since there are no other parameters, I suggest adding the SPLICE_F_NOWAIT flag, which is similar to SPLICE_F_NONBLOCK, but affects the "non-pipe" file descriptor passed to sendfile() or splice(). It translates to IOCB_NOWAIT for regular files, just like RWF_NOWAIT does. --- Changes v1 -> v2: - value of SPLICE_F_NOWAIT changed to 0x10 - added SPLICE_F_NOWAIT to SPLICE_F_ALL to make it part of uapi v2 -> v3: repost and rebase on linus/master Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx> --- fs/splice.c | 14 ++++++++++++++ include/linux/splice.h | 4 +++- 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/fs/splice.c b/fs/splice.c index 60aed8de21f8..6257fef93ec4 100644 --- a/fs/splice.c +++ b/fs/splice.c @@ -362,6 +362,8 @@ ssize_t copy_splice_read(struct file *in, loff_t *ppos, iov_iter_bvec(&to, ITER_DEST, bv, npages, len); init_sync_kiocb(&kiocb, in); kiocb.ki_pos = *ppos; + if (flags & SPLICE_F_NOWAIT) + kiocb.ki_flags |= IOCB_NOWAIT; ret = in->f_op->read_iter(&kiocb, &to); if (ret > 0) { @@ -1090,6 +1092,18 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd, if (unlikely(ret <= 0)) goto read_failure; + /* + * After at least one byte was read from the input + * file, don't wait for blocking I/O in the following + * loop iterations; instead of blocking for arbitrary + * amounts of time in the kernel, let userspace decide + * how to proceed. This avoids excessive latency if + * the output is being consumed faster than the input + * file can fill it (e.g. sendfile() from a slow hard + * disk to a fast network). + */ + flags |= SPLICE_F_NOWAIT; + read_len = ret; sd->total_len = read_len; diff --git a/include/linux/splice.h b/include/linux/splice.h index 9dec4861d09f..0deb87fb8055 100644 --- a/include/linux/splice.h +++ b/include/linux/splice.h @@ -21,7 +21,9 @@ #define SPLICE_F_MORE (0x04) /* expect more data */ #define SPLICE_F_GIFT (0x08) /* pages passed in are a gift */ -#define SPLICE_F_ALL (SPLICE_F_MOVE|SPLICE_F_NONBLOCK|SPLICE_F_MORE|SPLICE_F_GIFT) +#define SPLICE_F_NOWAIT (0x10) /* do not wait for data which is not immediately available */ + +#define SPLICE_F_ALL (SPLICE_F_MOVE|SPLICE_F_NONBLOCK|SPLICE_F_MORE|SPLICE_F_GIFT|SPLICE_F_NOWAIT) /* * Passed to the actors -- 2.39.2