Re: [RFC PATCH] mm: support large folio numa balancing

[Baolin Wang]

On 11/13/2023 6:53 PM, David Hildenbrand wrote:
> On 13.11.23 11:45, Baolin Wang wrote:
> > Currently, the file pages already support large folio, and supporting for
> > anonymous pages is also under discussion[1]. Moreover, the numa balancing
> > code are converted to use a folio by previous thread[2], and the 
> > migrate_pages
> > function also already supports the large folio migration.
> >
> > So now I did not see any reason to continue restricting NUMA balancing 
> > for
> > large folio.
>
> I recall John wanted to look into that. CCing him.
>
> I'll note that the "head page mapcount" heuristic to detect sharers will
> now strike on the PTE path and make us believe that a large folios is
> exclusive, although it isn't.
>
> As spelled out in the commit you are referencing:
>
> commit 6695cf68b15c215d33b8add64c33e01e3cbe236c
> Author: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> Date:   Thu Sep 21 15:44:14 2023 +0800
>
>      mm: memory: use a folio in do_numa_page()
>      Numa balancing only try to migrate non-compound page in 
> do_numa_page(),
>      use a folio in it to save several compound_head calls, note we use
>      folio_estimated_sharers(), it is enough to check the folio sharers 
> since
>      only normal page is handled, if large folio numa balancing is 
> supported, a
>      precise folio sharers check would be used, no functional change 
> intended.

Thanks for pointing out the part I missed.

I saw the migrate_pages() syscall is also using 
folio_estimated_sharers() to check if the folio is shared, and I wonder 
it will bring about any significant issues?

> I'll send WIP patches for one approach that can improve the situation 
> soonish.

Great. Look forward to seeing this:)