linux-kernel - Re: [PATCH 06/15] mm/swap: remove buggy cache->nr check in refill_swap_slots

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Message-ID: <8b03aa8a-5023-6915-64c6-f6f298f19f36@huawei.com>
Date:   Mon, 16 May 2022 10:00:43 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     David Hildenbrand <david@...hat.com>, <akpm@...ux-foundation.org>
CC:     <willy@...radead.org>, <vbabka@...e.cz>, <dhowells@...hat.com>,
        <neilb@...e.de>, <apopple@...dia.com>, <surenb@...gle.com>,
        <peterx@...hat.com>, <naoya.horiguchi@....com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/15] mm/swap: remove buggy cache->nr check in
 refill_swap_slots_cache

On 2022/5/12 21:37, David Hildenbrand wrote:
> On 09.05.22 15:14, Miaohe Lin wrote:
>> refill_swap_slots_cache is always called when cache->nr is 0. And if
>> cache->nr != 0, we should return cache->nr instead of 0. So remove
>> such buggy and confusing check.
> 
> Not sure about the "cache->nr != 0, we should return cache->nr instead
> of 0" part, I'd just drop that from the patch description. We'd actually
> end up overwriting cache->nr after your change, which doesn't sound
> right and also different to what you describe here.

Will do.

> 
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/swap_slots.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
>> index 2f877e6f87d7..2a65a89b5b4d 100644
>> --- a/mm/swap_slots.c
>> +++ b/mm/swap_slots.c
>> @@ -258,7 +258,7 @@ void enable_swap_slots_cache(void)
>>  /* called with swap slot cache's alloc lock held */
>>  static int refill_swap_slots_cache(struct swap_slots_cache *cache)
>>  {
>> -	if (!use_swap_slot_cache || cache->nr)
>> +	if (!use_swap_slot_cache)
>>  		return 0;
>>  
>>  	cache->cur = 0;
> 
> I feel like if this function would be called with cache->nr, it would be
> a BUG. So I'm fine with removing it, but we could also think about
> turning it into some sort of WARN/BG to make it clearer that this is
> unexpected.

Since refill_swap_slots_cache is only called by folio_alloc_swap when cache->nr == 0. I think
it might be too overkill to add a WARN/BG.

> 
> 
> Anyhow,
> 
> Acked-by: David Hildenbrand <david@...hat.com>

Many thanks for comment and Acked-by tag!

>