linux-kernel - [PATCH 3.2 51/79] autofs: don't fail mount for transient error

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Message-ID: <lsq.1518322806.393313279@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:20:06 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "NeilBrown" <neilb@...e.com>,
        "Ian Kent" <raven@...maw.net>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.2 51/79] autofs: don't fail mount for transient error

3.2.99-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: NeilBrown <neilb@...e.com>

commit ecc0c469f27765ed1e2b967be0aa17cee1a60b76 upstream.

Currently if the autofs kernel module gets an error when writing to the
pipe which links to the daemon, then it marks the whole moutpoint as
catatonic, and it will stop working.

It is possible that the error is transient.  This can happen if the
daemon is slow and more than 16 requests queue up.  If a subsequent
process tries to queue a request, and is then signalled, the write to
the pipe will return -ERESTARTSYS and autofs will take that as total
failure.

So change the code to assess -ERESTARTSYS and -ENOMEM as transient
failures which only abort the current request, not the whole mountpoint.

It isn't a crash or a data corruption, but having autofs mountpoints
suddenly stop working is rather inconvenient.

Ian said:

: And given the problems with a half dozen (or so) user space applications
: consuming large amounts of CPU under heavy mount and umount activity this
: could happen more easily than we expect.

Link: http://lkml.kernel.org/r/87y3norvgp.fsf@notabene.neil.brown.name
Signed-off-by: NeilBrown <neilb@...e.com>
Acked-by: Ian Kent <raven@...maw.net>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: autofs4_write() doesn't take an autofs_sb_info
 pointer]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/autofs4/waitq.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

--- a/fs/autofs4/waitq.c
+++ b/fs/autofs4/waitq.c
@@ -88,7 +88,8 @@ static int autofs4_write(struct file *fi
 		spin_unlock_irqrestore(&current->sighand->siglock, flags);
 	}

-	return (bytes > 0);
+	/* if 'wr' returned 0 (impossible) we assume -EIO (safe) */
+	return bytes == 0 ? 0 : wr < 0 ? wr : -EIO;
 }

 static void autofs4_notify_daemon(struct autofs_sb_info *sbi,
@@ -102,6 +103,7 @@ static void autofs4_notify_daemon(struct
 	} pkt;
 	struct file *pipe = NULL;
 	size_t pktsz;
+	int ret;

 	DPRINTK("wait id = 0x%08lx, name = %.*s, type=%d",
 		(unsigned long) wq->wait_queue_token, wq->name.len, wq->name.name, type);
@@ -180,7 +182,18 @@ static void autofs4_notify_daemon(struct
 	mutex_unlock(&sbi->wq_mutex);

 	if (autofs4_write(pipe, &pkt, pktsz))
+	switch (ret = autofs4_write(pipe, &pkt, pktsz)) {
+	case 0:
+		break;
+	case -ENOMEM:
+	case -ERESTARTSYS:
+		/* Just fail this one */
+		autofs4_wait_release(sbi, wq->wait_queue_token, ret);
+		break;
+	default:
 		autofs4_catatonic_mode(sbi);
+		break;
+	}
 	fput(pipe);
 }