Re: [RFC PATCH 0/4] Multiple consecutive page for anonymous mapping

["Yin, Fengwei" <fengwei.yin@intel.com>]

On 1/9/2023 4:37 PM, Kirill A. Shutemov wrote:
> On Mon, Jan 09, 2023 at 03:22:28PM +0800, Yin Fengwei wrote:
>> In a nutshell:  4k is too small and 2M is too big.  We started
>> asking ourselves whether there was something in the middle that
>> we could do.  This series shows what that middle ground might
>> look like.  It provides some of the benefits of THP while
>> eliminating some of the downsides.
>>
>> This series uses "multiple consecutive pages" (mcpages) of
>> between 8K and 2M of base pages for anonymous user space mappings.
>> This will lead to less internal fragmentation versus 2M mappings
>> and thus less memory consumption and wasted CPU time zeroing
>> memory which will never be used.
>>
>> In the implementation, we allocate high order page with order of
>> mcpage (e.g., order 2 for 16KB mcpage). This makes sure the
>> physical contiguous memory is used and benefit sequential memory
>> access latency.
>>
>> Then split the high order page. By doing this, the sub-page of
>> mcpage is just 4K normal page. The current kernel page
>> management is applied to "mc" pages without any changes. Batching
>> page faults is allowed with mcpage and reduce page faults number.
>>
>> There are costs with mcpage. Besides no TLB benefit THP brings, it
>> increases memory consumption and latency of allocation page
>> comparing to 4K base page.
>>
>> This series is the first step of mcpage. The furture work can be
>> enable mcpage for more components like page cache, swapping etc.
>> Finally, most pages in system will be allocated/free/reclaimed
>> with mcpage order.
> 
> It doesn't worth adding a new path in page fault handing. We need to make
> existing mechanisms more flexible.
> 
> I think it has to be done on top of folios:
> 
> 1. Converts anonymous memory to folios. Only order-9 (HPAGE_PMD_ORDER) and
>    order-0 at first.
> 2. Remove assumption of THP being order-9.
> 3. Start allocating THPs <order-9.
Thanks a lot for the comments. Really appreciate it.


Regards
Yin, Fengwei

>