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Date:   Tue, 28 Nov 2023 17:58:55 +0100
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Subject: Re: [PATCH RFC v2 06/27] mm: page_alloc: Allow an arch to hook early
 into free_pages_prepare()
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To:     Alexandru Elisei <alexandru.elisei@arm.com>
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References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-7-alexandru.elisei@arm.com>
 <45466b05-d620-41e5-8a2b-05c420b8fa7b@redhat.com> <ZWSTiCghf8nMFy4G@raptor>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <ZWSTiCghf8nMFy4G@raptor>
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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