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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov