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

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Date: Fri, 2 Feb 2024 16:18:21 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Nhat Pham <nphamcs@...il.com>, 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 2024/2/2 01:55, Johannes Weiner wrote:
> On Thu, Feb 01, 2024 at 03:49:04PM +0000, Chengming Zhou 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?)
> 
> Probably not worth it, especially after the next patch.

You are right, not worth it.

> 
>> 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.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> 
> Either way, I agree that the WARN_ON() is more useful to point out a
> bug than a counter.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Thanks!