Re: [PATCH v3 2/3] mm/zswap: bugfix: memory leak when invalidate andreclaim occur concurrently
From: Bob Liu
Date: Fri Oct 11 2013 - 22:50:10 EST
On Fri, Oct 11, 2013 at 3:13 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Thu, Sep 26, 2013 at 11:42:17AM +0800, Weijie Yang wrote:
>> On Tue, Sep 24, 2013 at 9:03 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> > On Mon, Sep 23, 2013 at 04:21:49PM +0800, Weijie Yang wrote:
>> > >
>> > > 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.
>>
>> I refactor the zswap refcount routine according to Minchan's idea.
>> Here is the new patch, Any suggestion is welcomed.
>>
>> To Seth and Bob, would you please review it again?
>
> Yeah, Seth, Bob. You guys are right persons to review this because this
> scheme was suggested by me who is biased so it couldn't be a fair. ;-)
> But anyway, I will review code itself.
>
>>
>> mm/zswap.c | 116
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++----------------------------------------------------------------
>> 1 file changed, 52 insertions(+), 64 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> old mode 100644
>> new mode 100755
>> index deda2b6..bd04910
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -217,6 +217,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
>> if (!entry)
>> return NULL;
>> entry->refcount = 1;
>> + RB_CLEAR_NODE(&entry->rbnode);
>> return entry;
>> }
>>
>> @@ -232,10 +233,20 @@ static void zswap_entry_get(struct zswap_entry *entry)
>> }
>>
>> /* caller must hold the tree lock */
>> -static int zswap_entry_put(struct zswap_entry *entry)
>> +static int zswap_entry_put(struct zswap_tree *tree, struct zswap_entry *entry)
>
> Why should we have return value? If we really need it, it mitigates
> get/put semantic's whole point so I'd like to just return void.
>
> Let me see.
>
>> {
>> - entry->refcount--;
>> - return entry->refcount;
>> + int refcount = --entry->refcount;
>> +
>> + if (refcount <= 0) {
>
> Hmm, I don't like minus refcount, really.
> I hope we could do following as
>
> BUG_ON(refcount < 0);
> if (refcount == 0) {
> ...
> }
>
>
>
>> + if (!RB_EMPTY_NODE(&entry->rbnode)) {
>> + rb_erase(&entry->rbnode, &tree->rbroot);
>> + RB_CLEAR_NODE(&entry->rbnode);
>
> Minor,
> You could make new function zswap_rb_del or zswap_rb_remove which detach the node
> from rb tree and clear node because we have already zswap_rb_insert.
>
>
>> + }
>> +
>> + zswap_free_entry(tree, entry);
>> + }
>> +
>> + return refcount;
>> }
>>
>> /*********************************
>> @@ -258,6 +269,17 @@ static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
>> return NULL;
>> }
>>
>
> Add function description.
>
>> +static struct zswap_entry *zswap_entry_find_get(struct rb_root *root, pgoff_t offset)
>> +{
>> + struct zswap_entry *entry = NULL;
>> +
>> + entry = zswap_rb_search(root, offset);
>> + if (entry)
>> + zswap_entry_get(entry);
>> +
>> + return entry;
>> +}
>> +
>> /*
>> * In the case that a entry with the same offset is found, a pointer to
>> * the existing entry is stored in dupentry and the function returns -EEXIST
>> @@ -387,7 +409,7 @@ 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 +423,9 @@ 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 +497,7 @@ 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;
>> }
>> @@ -517,23 +539,22 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
>>
>> /* find and ref zswap entry */
>> spin_lock(&tree->lock);
>> - entry = zswap_rb_search(&tree->rbroot, offset);
>> + entry = zswap_entry_find_get(&tree->rbroot, offset);
>> if (!entry) {
>> /* entry was invalidated */
>> spin_unlock(&tree->lock);
>> return 0;
>> }
>> - zswap_entry_get(entry);
>> spin_unlock(&tree->lock);
>> BUG_ON(offset != entry->offset);
>>
>> /* 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:
>
> Why did you remove comment?
>
>> /* page is already in the swap cache, ignore for now */
>> page_cache_release(page);
>> ret = -EEXIST;
>> @@ -562,38 +583,28 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
>> zswap_written_back_pages++;
>>
>> spin_lock(&tree->lock);
>> -
>> /* drop local reference */
>> - zswap_entry_put(entry);
>> + refcount = zswap_entry_put(tree, entry);
>> /* drop the initial reference from entry creation */
>> - refcount = zswap_entry_put(entry);
>> -
>> - /*
>> - * There are three possible values for refcount here:
>> - * (1) refcount is 1, load is in progress, unlink from rbtree,
>> - * load will free
>> - * (2) refcount is 0, (normal case) entry is valid,
>> - * remove from rbtree and free entry
>> - * (3) refcount is -1, invalidate happened during writeback,
>> - * free entry
>> - */
>> - if (refcount >= 0) {
>> - /* no invalidate yet, remove from rbtree */
>> + if (refcount > 0) {
>> rb_erase(&entry->rbnode, &tree->rbroot);
>> + RB_CLEAR_NODE(&entry->rbnode);
>> + refcount = zswap_entry_put(tree, entry);
>
> Now, I see why you need return in zswap_entry_put but let's consider again
> because it's really mess to me and it hurts get/put semantic a lot so
> How about this?
>
> spin_lock(&tree->lock);
> /* drop local reference */
> zswap_entry_put(tree, entry);
> /*
> * In here, we want to free entry but invalidation may free earlier
> * under us so that we should check it again
> */
> if (entry == zswap_rb_search(&tree->rb_root, offset))
Then where is the place unlink entry from rbtree if load was in progress ?
And in the following fail path, return value from zswap_entry_put() is
also used.
> /* Yes, it's stable so we should free it */
> zswap_entry_put(tree, entry);
>
> /*
> * Whether it would be freed by invalidation or writeback, it doesn't
> * matter. Important thing is that it will be freed so there
> * is no point to return -EAGAIN.
> */
> spin_unlock(&tree->lock);
> return 0;
>
--
Regards,
--Bob
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