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mm: remove worrying dead code from find_get_pages() 

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>
Cc: Nick Piggin <npiggin@kernel.dk>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Salman Qazi <sqazi@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2011-03-22 16:33:06 -07:00 committed by Linus Torvalds
parent c033a93c0d
commit 9d8aa4ea85
1 changed files with 16 additions and 2 deletions
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18
mm/filemap.c
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@ -863,9 +863,13 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
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;
}
@ -915,6 +919,11 @@ unsigned find_get_pages_contig(struct address_space *mapping, pgoff_t index,
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;
@ -975,6 +984,11 @@ unsigned find_get_pages_tag(struct address_space *mapping, pgoff_t *index,
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;