Re: [PATCH v3 2/3] mm/zswap: bugfix: memory leak when invalidate andreclaim occur concurrently

[Weijie Yang]

On Sat, Oct 12, 2013 at 10:50 AM, Bob Liu <lliubbo@xxxxxxxxx> wrote:
> 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 ?

zswap_entry_put() have the unlink handle logic

> And in the following fail path, return value from zswap_entry_put() is
> also used.

It is okay even if we return -EAGAIN in the fail path

>>                 /* 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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