linux-kernel - Re: fs/ocfs2/dlm: Use GFP_ATOMIC under spin

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Message-Id: <20110127170948.9a0d8b60.akpm@linux-foundation.org>
Date:	Thu, 27 Jan 2011 17:09:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	dsterba@...e.cz
Cc:	ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org,
	mfasheh@...e.com, joel.becker@...cle.com
Subject: Re: fs/ocfs2/dlm: Use GFP_ATOMIC under spin_lock

Blast from the past...

On Tue, 2 Nov 2010 23:36:02 +0100
David Sterba <dsterba@...e.cz> wrote:

> coccinelle check scripts/coccinelle/locks/call_kern.cocci found that
> in fs/ocfs2/dlm/dlmdomain.c an allocation with GFP_KERNEL is done
> with locks held:
> 
> dlm_query_region_handler
>   spin_lock(dlm_domain_lock)
>     dlm_match_regions
>       kmalloc(GFP_KERNEL)
> 
> Change it to GFP_ATOMIC.
> 
> Signed-off-by: David Sterba <dsterba@...e.cz>
> CC: Joel Becker <joel.becker@...cle.com>
> CC: Mark Fasheh <mfasheh@...e.com>
> CC: ocfs2-devel@....oracle.com
> 
> --
> Exists in v2.6.37-rc1 and current linux-next.
> 
> diff -u -p a/fs/ocfs2/dlm/dlmdomain.c b/fs/ocfs2/dlm/dlmdomain.c
> --- a/fs/ocfs2/dlm/dlmdomain.c 2010-10-22 10:23:23.502434402 +0200
> +++ b/fs/ocfs2/dlm/dlmdomain.c 2010-11-02 17:11:06.000000000 +0100
> @@ -959,7 +959,7 @@ static int dlm_match_regions(struct dlm_
>                 r += O2HB_MAX_REGION_NAME_LEN;
>         }
> 
> -       local = kmalloc(sizeof(qr->qr_regions), GFP_KERNEL);
> +       local = kmalloc(sizeof(qr->qr_regions), GFP_ATOMIC);
>         if (!local) {
>                 status = -ENOMEM;
>                 goto bail;
> 

Switching to GFP_ATOMIC is the worst of all possible ways of "fixing"
this bug.  GFP_ATOMIC is *unreliable*.  We don't want to introduce
unreliability deep down inside our filesytems.  And fs maintainers who
don't want to make their filesystems less reliable shouldn't blindly
apply patches that do so :(


A reliable way of fixing this bug is below.  As an added bonus, it
makes the fs faster.

--- a/fs/ocfs2/dlm/dlmdomain.c~a
+++ a/fs/ocfs2/dlm/dlmdomain.c
@@ -926,9 +926,9 @@ static int dlm_assert_joined_handler(str
 }
 
 static int dlm_match_regions(struct dlm_ctxt *dlm,
-			     struct dlm_query_region *qr)
+			     struct dlm_query_region *qr, u8 *local)
 {
-	char *local = NULL, *remote = qr->qr_regions;
+	char *remote = qr->qr_regions;
 	char *l, *r;
 	int localnr, i, j, foundit;
 	int status = 0;
@@ -957,12 +957,6 @@ static int dlm_match_regions(struct dlm_
 		r += O2HB_MAX_REGION_NAME_LEN;
 	}
 
-	local = kmalloc(sizeof(qr->qr_regions), GFP_ATOMIC);
-	if (!local) {
-		status = -ENOMEM;
-		goto bail;
-	}
-
 	localnr = o2hb_get_all_regions(local, O2NM_MAX_REGIONS);
 
 	/* compare local regions with remote */
@@ -1012,8 +1006,6 @@ static int dlm_match_regions(struct dlm_
 	}
 
 bail:
-	kfree(local);
-
 	return status;
 }
 
@@ -1077,6 +1069,7 @@ static int dlm_query_region_handler(stru
 	struct dlm_ctxt *dlm = NULL;
 	int status = 0;
 	int locked = 0;
+	static u8 local[sizeof(qr->qr_regions)]; /* locked by dlm_domain_lock */
 
 	qr = (struct dlm_query_region *) msg->buf;
 
@@ -1112,7 +1105,7 @@ static int dlm_query_region_handler(stru
 		goto bail;
 	}
 
-	status = dlm_match_regions(dlm, qr);
+	status = dlm_match_regions(dlm, qr, local);
 
 bail:
 	if (locked)
_


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