linux-kernel - Re: [PATCH 4/6] mm/zswap: remove duplicate

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Message-ID: <80460fe5-baec-4e56-9e10-a233e5a3c8d3@bytedance.com>
Date: Sat, 3 Feb 2024 12:29:16 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Yosry Ahmed <yosryahmed@...gle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH 4/6] mm/zswap: remove duplicate_entry debug value

On 2024/2/3 06:28, Nhat Pham wrote:
> On Thu, Feb 1, 2024 at 7:50 AM Chengming Zhou
> <zhouchengming@...edance.com> wrote:
>>
>> cat /sys/kernel/debug/zswap/duplicate_entry
>> 2086447
>>
>> When testing, the duplicate_entry value is very high, but no warning
>> message in the kernel log. From the comment of duplicate_entry
>> "Duplicate store was encountered (rare)", it seems something goes wrong.
>>
>> Actually it's incremented in the beginning of zswap_store(), which found
>> its zswap entry has already on the tree. And this is a normal case,
>> since the folio could leave zswap entry on the tree after swapin,
>> later it's dirtied and swapout/zswap_store again, found its original
>> zswap entry. (Maybe we can reuse it instead of invalidating it?)
> 
> Interesting. So if we make invalidate load the only mode, this oddity
> is gone as well?

Good point!
This oddity is why we need to invalidate it first at the beginning.

But there is another oddity that a stored folio maybe dirtied again,
so that folio needs to be writeback/stored for the second time, in
which case, we still need to invalidate it first to avoid WARN_ON later.

Thanks.

>>
>> So duplicate_entry should be only incremented in the real bug case,
>> which already have "WARN_ON(1)", it looks redundant to count bug case,
>> so this patch just remove it.
> 
> But yeah, I have literally never checked this value (maybe I should
> ha). I'm fine with removing it, unless someone has a strong case for
> this counter?
> 
> For now:
> Reviewed-by: Nhat Pham <nphamcs@...il.com>
> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  mm/zswap.c | 9 +--------
>>  1 file changed, 1 insertion(+), 8 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 4381b7a2d4d6..3fbb7e2c8b8d 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -71,8 +71,6 @@ static u64 zswap_reject_compress_poor;
>>  static u64 zswap_reject_alloc_fail;
>>  /* Store failed because the entry metadata could not be allocated (rare) */
>>  static u64 zswap_reject_kmemcache_fail;
>> -/* Duplicate store was encountered (rare) */
>> -static u64 zswap_duplicate_entry;
>>
>>  /* Shrinker work queue */
>>  static struct workqueue_struct *shrink_wq;
>> @@ -1571,10 +1569,8 @@ bool zswap_store(struct folio *folio)
>>          */
>>         spin_lock(&tree->lock);
>>         entry = zswap_rb_search(&tree->rbroot, offset);
>> -       if (entry) {
>> +       if (entry)
>>                 zswap_invalidate_entry(tree, entry);
>> -               zswap_duplicate_entry++;
>> -       }
>>         spin_unlock(&tree->lock);
>>         objcg = get_obj_cgroup_from_folio(folio);
>>         if (objcg && !obj_cgroup_may_zswap(objcg)) {
>> @@ -1661,7 +1657,6 @@ bool zswap_store(struct folio *folio)
>>          */
>>         while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
>>                 WARN_ON(1);
>> -               zswap_duplicate_entry++;
>>                 zswap_invalidate_entry(tree, dupentry);
>>         }
>>         if (entry->length) {
>> @@ -1822,8 +1817,6 @@ static int zswap_debugfs_init(void)
>>                            zswap_debugfs_root, &zswap_reject_compress_poor);
>>         debugfs_create_u64("written_back_pages", 0444,
>>                            zswap_debugfs_root, &zswap_written_back_pages);
>> -       debugfs_create_u64("duplicate_entry", 0444,
>> -                          zswap_debugfs_root, &zswap_duplicate_entry);
>>         debugfs_create_u64("pool_total_size", 0444,
>>                            zswap_debugfs_root, &zswap_pool_total_size);
>>         debugfs_create_atomic_t("stored_pages", 0444,
>>
>> --
>> b4 0.10.1