On Mon, Sep 23, 2013 at 04:21:49PM +0800, Weijie Yang wrote: > Consider the following scenario: > thread 0: reclaim entry x (get refcount, but not call > zswap_get_swap_cache_page) > thread 1: call zswap_frontswap_invalidate_page to invalidate entry x. > finished, entry x and its zbud is not freed as its refcount != 0 > now, the swap_map[x] = 0 > thread 0: now call zswap_get_swap_cache_page > swapcache_prepare return -ENOENT because entry x is not used any more > zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM > zswap_writeback_entry do nothing except put refcount > Now, the memory of zswap_entry x and its zpage leak. > > Modify: > - check the refcount in fail path, free memory if it is not referenced.
Hmm, I don't like this because zswap refcount routine is already mess for me. I'm not sure why it was designed from the beginning. I hope we should fix it first. 1. zswap_rb_serach could include zswap_entry_get semantic if it founds a entry from the tree. Of course, we should ranme it as find_get_zswap_entry like find_get_page. 2. zswap_entry_put could hide resource free function like zswap_free_entry so that all of caller can use it easily following pattern. find_get_zswap_entry ... ... zswap_entry_put Of course, zswap_entry_put have to check the entry is in the tree or not so if someone already removes it from the tree, it should avoid double remove. One of the concern I can think is that approach extends critical section but I think it would be no problem because more bottleneck would be [de]compress functions. If it were really problem, we can mitigate a problem with moving unnecessary functions out of zswap_free_entry because it seem to be rather over-enginnering. > - use ZSWAP_SWAPCACHE_FAIL instead of ZSWAP_SWAPCACHE_NOMEM as the fail path > can be not only caused by nomem but also by invalidate. > > Signed-off-by: Weijie Yang <[email protected]> > Reviewed-by: Bob Liu <[email protected]> > Cc: Minchan Kim <[email protected]> > Cc: [email protected] > Acked-by: Seth Jennings <[email protected]> > --- > mm/zswap.c | 21 +++++++++++++-------- > 1 file changed, 13 insertions(+), 8 deletions(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index cbd9578..1be7b90 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -387,7 +387,7 @@ static void zswap_free_entry(struct zswap_tree *tree, > struct zswap_entry *entry) > enum zswap_get_swap_ret { > ZSWAP_SWAPCACHE_NEW, > ZSWAP_SWAPCACHE_EXIST, > - ZSWAP_SWAPCACHE_NOMEM > + ZSWAP_SWAPCACHE_FAIL, > }; > > /* > @@ -401,9 +401,9 @@ enum zswap_get_swap_ret { > * added to the swap cache, and returned in retpage. > * > * If success, the swap cache page is returned in retpage > - * Returns 0 if page was already in the swap cache, page is not locked > - * Returns 1 if the new page needs to be populated, page is locked > - * Returns <0 on error > + * Returns ZSWAP_SWAPCACHE_EXIST if page was already in the swap cache > + * Returns ZSWAP_SWAPCACHE_NEW if the new page needs to be populated, page > is locked > + * Returns ZSWAP_SWAPCACHE_FAIL on error > */ > static int zswap_get_swap_cache_page(swp_entry_t entry, > struct page **retpage) > @@ -475,7 +475,7 @@ static int zswap_get_swap_cache_page(swp_entry_t entry, > if (new_page) > page_cache_release(new_page); > if (!found_page) > - return ZSWAP_SWAPCACHE_NOMEM; > + return ZSWAP_SWAPCACHE_FAIL; > *retpage = found_page; > return ZSWAP_SWAPCACHE_EXIST; > } > @@ -529,11 +529,11 @@ static int zswap_writeback_entry(struct zbud_pool > *pool, unsigned long handle) > > /* try to allocate swap cache page */ > switch (zswap_get_swap_cache_page(swpentry, &page)) { > - case ZSWAP_SWAPCACHE_NOMEM: /* no memory */ > + case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */ > ret = -ENOMEM; > goto fail; > > - case ZSWAP_SWAPCACHE_EXIST: /* page is unlocked */ > + case ZSWAP_SWAPCACHE_EXIST: > /* page is already in the swap cache, ignore for now */ > page_cache_release(page); > ret = -EEXIST; > @@ -591,7 +591,12 @@ static int zswap_writeback_entry(struct zbud_pool *pool, > unsigned long handle) > > fail: > spin_lock(&tree->lock); > - zswap_entry_put(entry); > + refcount = zswap_entry_put(entry); > + if (refcount <= 0) { > + /* invalidate happened, consider writeback as success */ > + zswap_free_entry(tree, entry); > + ret = 0; > + } > spin_unlock(&tree->lock); > return ret; > } > -- > 1.7.10.4 > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to [email protected]. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"[email protected]";> [email protected] </a> -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
