Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO

[Dave Hansen]

On 1/4/21 11:27 AM, Matthew Wilcox wrote:
> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
>> On 12/21/20 8:30 AM, Liang Li wrote:
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -137,6 +137,9 @@ enum pageflags {
>>>  #endif
>>>  #ifdef CONFIG_64BIT
>>>  	PG_arch_2,
>>> +#endif
>>> +#ifdef CONFIG_PREZERO_PAGE
>>> +	PG_zero,
>>>  #endif
>>>  	__NR_PAGEFLAGS,
>>
>> I don't think this is worth a generic page->flags bit.
>>
>> There's a ton of space in 'struct page' for pages that are in the
>> allocator.  Can't we use some of that space?
> 
> I was going to object to that too, but I think the entire approach is
> flawed and needs to be thrown out.  It just nukes the caches in extremely
> subtle and hard to measure ways, lowering overall system performance.

Yeah, it certainly can't be the default, but it *is* useful for thing
where we know that there are no cache benefits to zeroing close to where
the memory is allocated.

The trick is opting into it somehow, either in a process or a VMA.