Re: [PATCH 1/2] mm: pgtable: add missing flag and statistics for kernel PTE page

[Qi Zheng]

Hi Matthew,

On 2024/2/5 02:51, Matthew Wilcox wrote:
> On Thu, Feb 01, 2024 at 04:05:40PM +0800, Qi Zheng wrote:
> > For kernel PTE page, we do not need to allocate and initialize its split
> > ptlock, but as a page table page, it's still necessary to add PG_table
> > flag and NR_PAGETABLE statistics for it.
>
> No, this is wrong.
>
> We do not account _kernel_ page tables to the _user_.  Just because
> the kernel, say, called vmalloc() doesn't mean we should charge the
> task for it.  Moreover, one task may call vmalloc() and a different task
> would then call vfree().

Got it. Thanks for providing this information!

> This is a can of worms you don't want to open.  Why did you want to do
> this?

Ah, just because generic {pmd|pud}_alloc_one() has opened it. ;) And
When I looked through the commits (e.g. commit 1d40a5ea01d5), I couldn't
find the information you provided above. And that is why I CC'd you to
double check this, in case I might have overlooked some important
background information.

So we should actually fix generic {pmd|pud}_alloc_one() (and maybe some
implementation in the arch), right? And it would be better to add some
comments to clarify.

Thanks.