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Date:   Tue, 28 Nov 2023 17:17:59 +0000
From:   Alexandru Elisei <alexandru.elisei@arm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v2 06/27] mm: page_alloc: Allow an arch to hook early
 into free_pages_prepare()
Message-ID: <ZWYgx28LB28x5Pkb@raptor>
References: <20231119165721.9849-1-alexandru.elisei@arm.com>
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 <45466b05-d620-41e5-8a2b-05c420b8fa7b@redhat.com>
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 <2e2d3f51-34e3-4982-b982-ab7b555cba21@redhat.com>
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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
>