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author | William Lee Irwin III <wli@holomorphy.com> | 2005-05-01 08:58:38 -0700 |
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committer | Linus Torvalds <torvalds@ppc970.osdl.org> | 2005-05-01 08:58:38 -0700 |
commit | dd1d5afca8d3bda7ff9db773fc08e648d2503dc6 (patch) | |
tree | b0cc49be9dc43558e31be18d6045181661492018 | |
parent | 93ea1d0a12623dc1a693642b5758261f35f9bf96 (diff) | |
download | linux-dd1d5afca8d3bda7ff9db773fc08e648d2503dc6.tar.bz2 |
[PATCH] sync_page() smp_mb() comment
The smp_mb() is becaus sync_page() doesn't have PG_locked while it accesses
page_mapping(page). The comments in the patch (the entire patch is the
addition of this comment) try to explain further how and why smp_mb() is
used.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
-rw-r--r-- | mm/filemap.c | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/mm/filemap.c b/mm/filemap.c index 9b74674e36ad..ee79b5d3439f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -139,7 +139,25 @@ static int sync_page(void *word) page = container_of((page_flags_t *)word, struct page, flags); /* - * FIXME, fercrissake. What is this barrier here for? + * page_mapping() is being called without PG_locked held. + * Some knowledge of the state and use of the page is used to + * reduce the requirements down to a memory barrier. + * The danger here is of a stale page_mapping() return value + * indicating a struct address_space different from the one it's + * associated with when it is associated with one. + * After smp_mb(), it's either the correct page_mapping() for + * the page, or an old page_mapping() and the page's own + * page_mapping() has gone NULL. + * The ->sync_page() address_space operation must tolerate + * page_mapping() going NULL. By an amazing coincidence, + * this comes about because none of the users of the page + * in the ->sync_page() methods make essential use of the + * page_mapping(), merely passing the page down to the backing + * device's unplug functions when it's non-NULL, which in turn + * ignore it for all cases but swap, where only page->private is + * of interest. When page_mapping() does go NULL, the entire + * call stack gracefully ignores the page and returns. + * -- wli */ smp_mb(); mapping = page_mapping(page); |