linux-kernel - Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to check SWP_STABLE

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Date:   Tue, 10 May 2022 10:21:21 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     NeilBrown <neilb@...e.de>
CC:     <akpm@...ux-foundation.org>, <willy@...radead.org>,
        <vbabka@...e.cz>, <dhowells@...hat.com>, <apopple@...dia.com>,
        <david@...hat.com>, <surenb@...gle.com>, <peterx@...hat.com>,
        <naoya.horiguchi@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/15] mm/swap: avoid calling swp_swap_info when try to
 check SWP_STABLE_WRITES

On 2022/5/10 8:28, NeilBrown wrote:
> On Mon, 09 May 2022, Miaohe Lin wrote:
>> Use flags of si directly to check SWP_STABLE_WRITES to avoid possible
>> READ_ONCE and thus save some cpu cycles.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>  mm/memory.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 9c3e7e6ac202..89dd15504f3d 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3892,7 +3892,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>  			 */
>>  			exclusive = true;
>>  		} else if (exclusive && PageWriteback(page) &&
>> -			  (swp_swap_info(entry)->flags & SWP_STABLE_WRITES)) {
>> +			  (si->flags & SWP_STABLE_WRITES)) {
> 
> Should this have a data_race() annotation.  Other bit-tests of si->flags
> do because scan_swap_map_slots can update it asynchronously, but we know
> that won't matter in practice.

Yes, you're right. scan_swap_map_slots can update si->flags asynchronously while
do_swap_page tests SWP_STABLE_WRITES here. We know this is harmless because
SWP_STABLE_WRITES is only changed at swapon/swapoff.

Will add data_race() annotation in next version to avoid possible KCSAN data-race complaint.

Many thanks for pointing this out! :)

> 
> Thanks,
> NeilBrown
> 
> 
>>  			/*
>>  			 * This is tricky: not all swap backends support
>>  			 * concurrent page modifications while under writeback.
>> -- 
>> 2.23.0
>>
>>
> .
>