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Date: Sat, 12 Oct 2013 17:14:12 +0800
From: Weijie Yang <weijie.yang.kh@...il.com>
To: Bob Liu <lliubbo@...il.com>
Cc: Minchan Kim <minchan@...nel.org>,
Weijie Yang <weijie.yang@...sung.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Seth Jennings <sjenning@...ux.vnet.ibm.com>,
Bob Liu <bob.liu@...cle.com>, Linux-MM <linux-mm@...ck.org>,
Linux-Kernel <linux-kernel@...r.kernel.org>,
stable@...r.kernel.org, d.j.shin@...sung.com,
heesub.shin@...sung.com, Kyungmin Park <kyungmin.park@...sung.com>,
hau.chen@...sung.com, bifeng.tong@...sung.com, rui.xie@...sung.com
Subject: Re: [PATCH v3 2/3] mm/zswap: bugfix: memory leak when invalidate and
reclaim occur concurrently
On Sat, Oct 12, 2013 at 10:50 AM, Bob Liu <lliubbo@...il.com> wrote:
> On Fri, Oct 11, 2013 at 3:13 PM, Minchan Kim <minchan@...nel.org> 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@...nel.org> 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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