Re: [PATCH -mm 0/2] mm: page_ext: split page_ext flags

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Sat, Dec 17, 2022 at 10:58:31AM +0000, Yafang Shao wrote:
> On 64bit system, page extension is for debugging purpose only currently,
> because of its overhead, in particular the memory overhead.
> 
> Once a page_ext is enabled, at least it will take 0.2% of the total
> memory because of the page_ext flags, no matter this page_ext uses it or
> not. Currently this page_ext flags is only used for page_owner on 64bit
> system. So it doesn't make sense to allocate this flags for all page_ext
> by default. We'd better move it into page_owner's structure, then when
> someone wants to introduce a new page_ext which may be memory-overhead
> sensitive, it will save this unneeded overhead.

I'm still worried about the approach of using page_ext for BPF memory
accounting.

Let's say we finally accepted the approach of using page_ext for BPF memory
accounting, and if it's not enabled by default, you need to rebuild the kernel
when you want to check BPF memory usage. (Or make everyone bear the overhead
by enabling page_ext default for BPF memory accounting that one may not be interested)

How the problem of accounting kfree_rcu() is going?
Doesn't call_rcu() work for the problem?
I still think it's much more feasible.

> On 32bit system, there's page_idle running on production envrionment,
> which also uses this page_ext flags. So it will take another 0.2% of
> total memory if the user enable both page_idle and page_owner after this
> change, but considering page_owner is for debugging purpose only, the
> memory overhead in this case won't be a problem.
> 
> So, let split the page_ext flags.
> 
> Yafang Shao (2):
>   mm: page_owner: split page_owner's flag from the comm flags
>   mm: page_idle: split 32bit page_idle's flag from the common flags
> 
>  include/linux/page_ext.h  | 14 +-------------
>  include/linux/page_idle.h | 39 +++++++++++++++++++++++++++++++++------
>  mm/page_ext.c             | 10 ----------
>  mm/page_idle.c            | 12 ++++++++++++
>  mm/page_owner.c           | 36 ++++++++++++++++++++++--------------
>  5 files changed, 68 insertions(+), 43 deletions(-)
> 
> -- 
> 2.30.1 (Apple Git-130)
> 

-- 
Thanks,
Hyeonggon