[PATCH] pipe: increase minimum default pipe size to 2 pages

["Alex Xu (Hello71)" <alex_y_xu@xxxxxxxx>]

Before this patch, the following program prints 4096 and hangs.
Afterwards, it prints 8192 and exits successfully. Note that you may
need to increase your RLIMIT_NOFILE before running the program.

int main() {
    int pipefd[2];
    for (int i = 0; i < 1025; i++)
        if (pipe(pipefd) == -1)
            return 1;
    size_t bufsz = fcntl(pipefd[1], F_GETPIPE_SZ);
    printf("%zd\n", bufsz);
    char *buf = calloc(bufsz, 1);
    write(pipefd[1], buf, bufsz);
    read(pipefd[0], buf, bufsz-1);
    write(pipefd[1], buf, 1);
}

Signed-off-by: Alex Xu (Hello71) <alex_y_xu@xxxxxxxx>
---

See discussion at https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.none@localhost/.

 fs/pipe.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 9ef4231cce61..8e6ef62aeb1c 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -31,6 +31,21 @@
 
 #include "internal.h"
 
+/*
+ * New pipe buffers will be restricted to this size while the user is exceeding
+ * their pipe buffer quota. The general pipe use case needs at least two
+ * buffers: one for data yet to be read, and one for new data. If this is less
+ * than two, then a write to a non-empty pipe may block even if the pipe is not
+ * full. This can occur with GNU make jobserver or similar uses of pipes as
+ * semaphores: multiple processes may be waiting to write tokens back to the
+ * pipe before reading tokens: https://lore.kernel.org/lkml/1628086770.5rn8p04n6j.none@localhost/.
+ *
+ * Users can reduce their pipe buffers with F_SETPIPE_SZ below this at their
+ * own risk, namely: pipe writes to non-full pipes may block until the pipe is
+ * emptied.
+ */
+#define PIPE_MIN_DEF_BUFFERS 2
+
 /*
  * The max size that a non-root user is allowed to grow the pipe. Can
  * be set by root in /proc/sys/fs/pipe-max-size
@@ -781,8 +796,8 @@ struct pipe_inode_info *alloc_pipe_info(void)
 	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
 
 	if (too_many_pipe_buffers_soft(user_bufs) && pipe_is_unprivileged_user()) {
-		user_bufs = account_pipe_buffers(user, pipe_bufs, 1);
-		pipe_bufs = 1;
+		user_bufs = account_pipe_buffers(user, pipe_bufs, PIPE_MIN_DEF_BUFFERS);
+		pipe_bufs = PIPE_MIN_DEF_BUFFERS;
 	}
 
 	if (too_many_pipe_buffers_hard(user_bufs) && pipe_is_unprivileged_user())
-- 
2.32.0