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mm: clarify the radix_tree exceptional cases

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Make the radix_tree exceptional cases, mostly in filemap.c, clearer.

It's hard to devise a suitable snappy name that illuminates the use by
shmem/tmpfs for swap, while keeping filemap/pagecache/radix_tree
generality.  And akpm points out that /* radix_tree_deref_retry(page) */
comments look like calls that have been commented out for unknown
reason.

Skirt the naming difficulty by rearranging these blocks to handle the
transient radix_tree_deref_retry(page) case first; then just explain the
remaining shmem/tmpfs swap case in a comment.

Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Hugh Dickins 2011-08-03 16:21:28 -07:00 • committed by Linus Torvalds
commit 8079b1c859
3 changed files with 53 additions and 26 deletions
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66
mm/filemap.c
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@ -700,10 +700,14 @@ repeat:
if (unlikely(!page))
goto out;
if (radix_tree_exception(page)) {
if (radix_tree_exceptional_entry(page))
goto out;
/* radix_tree_deref_retry(page) */
goto repeat;
if (radix_tree_deref_retry(page))
goto repeat;
/*
* Otherwise, shmem/tmpfs must be storing a swap entry
* here as an exceptional entry: so return it without
* attempting to raise page count.
*/
goto out;
}
if (!page_cache_get_speculative(page))
goto repeat;
@ -838,15 +842,21 @@ repeat:
continue;
if (radix_tree_exception(page)) {
if (radix_tree_exceptional_entry(page))
continue;
if (radix_tree_deref_retry(page)) {
/*
* Transient condition which can only trigger
* when entry at index 0 moves out of or back
* to root: none yet gotten, safe to restart.
*/
WARN_ON(start | i);
goto restart;
}
/*
* radix_tree_deref_retry(page):
* can only trigger when entry at index 0 moves out of
* or back to root: none yet gotten, safe to restart.
* Otherwise, shmem/tmpfs must be storing a swap entry
* here as an exceptional entry: so skip over it -
* we only reach this from invalidate_mapping_pages().
*/
WARN_ON(start | i);
goto restart;
continue;
}
if (!page_cache_get_speculative(page))
@ -904,14 +914,20 @@ repeat:
continue;
if (radix_tree_exception(page)) {
if (radix_tree_exceptional_entry(page))
break;
if (radix_tree_deref_retry(page)) {
/*
* Transient condition which can only trigger
* when entry at index 0 moves out of or back
* to root: none yet gotten, safe to restart.
*/
goto restart;
}
/*
* radix_tree_deref_retry(page):
* can only trigger when entry at index 0 moves out of
* or back to root: none yet gotten, safe to restart.
* Otherwise, shmem/tmpfs must be storing a swap entry
* here as an exceptional entry: so stop looking for
* contiguous pages.
*/
goto restart;
break;
}
if (!page_cache_get_speculative(page))
@ -973,13 +989,19 @@ repeat:
continue;
if (radix_tree_exception(page)) {
BUG_ON(radix_tree_exceptional_entry(page));
if (radix_tree_deref_retry(page)) {
/*
* Transient condition which can only trigger
* when entry at index 0 moves out of or back
* to root: none yet gotten, safe to restart.
*/
goto restart;
}
/*
* radix_tree_deref_retry(page):
* can only trigger when entry at index 0 moves out of
* or back to root: none yet gotten, safe to restart.
* This function is never used on a shmem/tmpfs
* mapping, so a swap entry won't be found here.
*/
goto restart;
BUG();
}
if (!page_cache_get_speculative(page))