[PATCH] mm: remove worrying dead code from find_get_pages()

[PATCH] mm: remove worrying dead code from find_get_pages()

[Hugh Dickins]

The radix_tree_deref_retry() case in find_get_pages() has a strange
little excrescence, not seen in the other gang lookups: it looks like
the start of an abandoned attempt to guarantee forward progress in a
case that cannot arise.

ret should always be 0 here: if it isn't, then going back to restart
will leak references to pages already gotten.  There used to be a
comment saying nr_found is necessarily 1 here: that's not quite true,
but the radix_tree_deref_retry() case is peculiar to the entry at index
0, when we race with it being moved out of the radix_tree root or back.

Remove the worrisome two lines, add a brief comment here and in
find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON
in find_get_pages() should it ever be seen elsewhere than at 0.

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

 mm/filemap.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

--- 2.6.38-rc6/mm/filemap.c	2011-01-18 22:04:56.000000000 -0800
+++ linux/mm/filemap.c	2011-02-23 15:45:47.000000000 -0800
@@ -782,9 +782,13 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page)) {
-			if (ret)
-				start = pages[ret-1]->index;
+			WARN_ON(start | i);
 			goto restart;
 		}
 
@@ -834,6 +838,11 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page))
 			goto restart;
 
@@ -894,6 +903,11 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page))
 			goto restart;
 

[PATCH] mm: remove worrying dead code from find_get_pages()

[Hugh Dickins]

The radix_tree_deref_retry() case in find_get_pages() has a strange
little excrescence, not seen in the other gang lookups: it looks like
the start of an abandoned attempt to guarantee forward progress in a
case that cannot arise.

ret should always be 0 here: if it isn't, then going back to restart
will leak references to pages already gotten.  There used to be a
comment saying nr_found is necessarily 1 here: that's not quite true,
but the radix_tree_deref_retry() case is peculiar to the entry at index
0, when we race with it being moved out of the radix_tree root or back.

Remove the worrisome two lines, add a brief comment here and in
find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON
in find_get_pages() should it ever be seen elsewhere than at 0.

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

 mm/filemap.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

--- 2.6.38-rc6/mm/filemap.c	2011-01-18 22:04:56.000000000 -0800
+++ linux/mm/filemap.c	2011-02-23 15:45:47.000000000 -0800
@@ -782,9 +782,13 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page)) {
-			if (ret)
-				start = pages[ret-1]->index;
+			WARN_ON(start | i);
 			goto restart;
 		}
 
@@ -834,6 +838,11 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page))
 			goto restart;
 
@@ -894,6 +903,11 @@ repeat:
 		page = radix_tree_deref_slot((void **)pages[i]);
 		if (unlikely(!page))
 			continue;
+
+		/*
+		 * This can only trigger when the entry at index 0 moves out
+		 * of or back to the root: none yet gotten, safe to restart.
+		 */
 		if (radix_tree_deref_retry(page))
 			goto restart;
 

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Re: [PATCH] mm: remove worrying dead code from find_get_pages()

[Peter Zijlstra]

On Wed, 2011-02-23 at 21:31 -0800, Hugh Dickins wrote:
> The radix_tree_deref_retry() case in find_get_pages() has a strange
> little excrescence, not seen in the other gang lookups: it looks like
> the start of an abandoned attempt to guarantee forward progress in a
> case that cannot arise.
> 
> ret should always be 0 here: if it isn't, then going back to restart
> will leak references to pages already gotten.  There used to be a
> comment saying nr_found is necessarily 1 here: that's not quite true,
> but the radix_tree_deref_retry() case is peculiar to the entry at index
> 0, when we race with it being moved out of the radix_tree root or back.
> 
> Remove the worrisome two lines, add a brief comment here and in
> find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON
> in find_get_pages() should it ever be seen elsewhere than at 0.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Re: [PATCH] mm: remove worrying dead code from find_get_pages()

[Peter Zijlstra]

On Wed, 2011-02-23 at 21:31 -0800, Hugh Dickins wrote:
> The radix_tree_deref_retry() case in find_get_pages() has a strange
> little excrescence, not seen in the other gang lookups: it looks like
> the start of an abandoned attempt to guarantee forward progress in a
> case that cannot arise.
> 
> ret should always be 0 here: if it isn't, then going back to restart
> will leak references to pages already gotten.  There used to be a
> comment saying nr_found is necessarily 1 here: that's not quite true,
> but the radix_tree_deref_retry() case is peculiar to the entry at index
> 0, when we race with it being moved out of the radix_tree root or back.
> 
> Remove the worrisome two lines, add a brief comment here and in
> find_get_pages_contig() and find_get_pages_tag(), and a WARN_ON
> in find_get_pages() should it ever be seen elsewhere than at 0.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>


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