Linux Virtualization - Jan 2021 - [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO

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.

> Am 04.01.2021 um 20:52 schrieb Dave Hansen <dave.hansen at
intel.com>:
> 
> ?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.
> 
The patch set is mostly trying to optimize starting a new process. So
process/vma doesn?t really work.

I still wonder if using tmpfs/shmem cannot somehow be used to cover the original
use case of starting a new vm fast (or rebooting an existing one involving
restarting the process).