Add workaround for broken Linux pipes

Linux has an unresolved hang if you resize a pipe with bytes in it. Since there's no obvious way to detect this happening, added a workaround to disable resizing the pipe buffer if you set an environment variable. Reviewed-by: Brian Behlendorf <[email protected]> Signed-off-by: Rich Ercolani <[email protected]> Closes #13309
openzfs · May 9, 2022 · a30927f · a30927f
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diff --git a/lib/libzfs_core/libzfs_core.c b/lib/libzfs_core/libzfs_core.c
@@ -607,6 +607,19 @@ max_pipe_buffer(int infd)
 	}
 
 	unsigned int cur = fcntl(infd, F_GETPIPE_SZ);
+	/*
+	 * Sadly, Linux has an unfixed deadlock if you do SETPIPE_SZ on a pipe
+	 * with data in it.
+	 * cf. #13232, https://bugzilla.kernel.org/show_bug.cgi?id=212295
+	 *
+	 * And since the problem is in waking up the writer, there's nothing
+	 * we can do about it from here.
+	 *
+	 * So if people want to, they can set this, but they
+	 * may regret it...
+	 */
+	if (getenv("ZFS_SET_PIPE_MAX") == NULL)
+		return (cur);
 	if (cur < max && fcntl(infd, F_SETPIPE_SZ, max) != -1)
 		cur = max;
 	return (cur);

diff --git a/man/man8/zfs.8 b/man/man8/zfs.8
@@ -745,6 +745,14 @@ to use
 to mount ZFS datasets.
 This option is provided for backwards compatibility with older ZFS versions.
 .El
+.Bl -tag -width "ZFS_SET_PIPE_MAX"
+.It Sy ZFS_SET_PIPE_MAX
+Tells
+.Nm zfs
+to set the maximum pipe size for sends/recieves.
+Disabled by default on Linux
+due to an unfixed deadlock in Linux's pipe size handling code.
+.El
 .
 .Sh INTERFACE STABILITY
 .Sy Committed .