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

Alexandru Elisei
2023-11-28T17:18:36Z

Hi,

On Tue, Nov 28, 2023 at 05:58:55PM +0100, David Hildenbrand wrote:
> 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!

Will do!

Thanks,
Alex

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