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

[David Hildenbrand]

On 27.11.23 14:03, Alexandru Elisei wrote:
> 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?

Yes, please add some of that to the patch description!

--
Cheers,

David / dhildenb