[PATCH] tmpfs: change shmem_mapping() to test shmem_aops

[PATCH] tmpfs: change shmem_mapping() to test shmem_aops

[Hugh Dickins]

Callers of shmem_mapping() are interested in whether the mapping is
swap backed - except for uprobes, which is interested in whether it
should use shmem_read_mapping_page().  All these callers are better
served by a shmem_mapping() which checks for shmem_aops, than the
current version which goes through several indirections to find where
the inode lives - and has the surprising effect that a private mmap of
/dev/zero satisfies both vma_is_anonymous() and shmem_mapping(), when
that device node is on devtmpfs.  I don't think anything in the tree
suffers from that surprise, but it caught me out, and is better fixed.

Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/shmem.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- 4.9-rc8/mm/shmem.c	2016-11-13 11:44:43.052622519 -0800
+++ linux/mm/shmem.c	2016-12-05 18:54:25.348596732 -0800
@@ -2131,10 +2131,7 @@ static struct inode *shmem_get_inode(str
 
 bool shmem_mapping(struct address_space *mapping)
 {
-	if (!mapping->host)
-		return false;
-
-	return mapping->host->i_sb->s_op == &shmem_ops;
+	return mapping->a_ops == &shmem_aops;
 }
 
 #ifdef CONFIG_TMPFS

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Re: [PATCH] tmpfs: change shmem_mapping() to test shmem_aops

[Johannes Weiner]

On Mon, Dec 05, 2016 at 09:52:36PM -0800, Hugh Dickins wrote:
> Callers of shmem_mapping() are interested in whether the mapping is
> swap backed - except for uprobes, which is interested in whether it
> should use shmem_read_mapping_page().  All these callers are better
> served by a shmem_mapping() which checks for shmem_aops, than the
> current version which goes through several indirections to find where
> the inode lives - and has the surprising effect that a private mmap of
> /dev/zero satisfies both vma_is_anonymous() and shmem_mapping(), when
> that device node is on devtmpfs.  I don't think anything in the tree
> suffers from that surprise, but it caught me out, and is better fixed.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

There are actually some existing sites that check for shmemness this
way. Do you see value in converting them?

---