Re: [PATCH RFC v2 06/27] mm: page_alloc: Allow an arch to hook early into free_pages_prepare()

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Fri, Nov 24, 2023 at 08:36:52PM +0100, David Hildenbrand wrote:
> On 19.11.23 17:57, Alexandru Elisei wrote:
> > Add arch_free_pages_prepare() hook that is called before that page flags
> > are cleared. This will be used by arm64 when explicit management of tag
> > storage pages is enabled.
> 
> Can you elaborate a bit what exactly will be done by that code with that
> information?

Of course.

The MTE code that is in the kernel today uses the PG_arch_2 page flag, which it
renames to PG_mte_tagged, to track if a page has been mapped with tagging
enabled. That flag is cleared by free_pages_prepare() when it does:

	page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;

When tag storage management is enabled, tag storage is reserved for a page if
and only if the page is mapped as tagged. When a page is freed, the code looks
at the PG_mte_tagged flag to determine if the page was mapped as tagged, and
therefore has tag storage reserved, to determine if the corresponding tag
storage should also be freed.

I have considered using arch_free_page(), but free_pages_prepare() calls the
function after the flags are cleared.

Does that answer your question?

Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>