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@xxxxxxxxxx>
>> ---
>> 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
>>
>>
> .
>
