linux-kernel - Re: [PATCH RFC v2 05/27] mm: page_alloc: Add an arch hook to allow prep_new

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Date:   Tue, 28 Nov 2023 17:57:31 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
        maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
        yuzenghui@...wei.com, arnd@...db.de, akpm@...ux-foundation.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, mhiramat@...nel.org,
        rppt@...nel.org, hughd@...gle.com, pcc@...gle.com,
        steven.price@....com, anshuman.khandual@....com,
        vincenzo.frascino@....com, eugenis@...gle.com, kcc@...gle.com,
        hyesoo.yu@...sung.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 05/27] mm: page_alloc: Add an arch hook to allow
 prep_new_page() to fail

On 27.11.23 13:09, Alexandru Elisei wrote:
> Hi,
> 
> Thank you so much for your comments, there are genuinely useful.
> 
> On Fri, Nov 24, 2023 at 08:35:47PM +0100, David Hildenbrand wrote:
>> On 19.11.23 17:56, Alexandru Elisei wrote:
>>> Introduce arch_prep_new_page(), which will be used by arm64 to reserve tag
>>> storage for an allocated page. Reserving tag storage can fail, for example,
>>> if the tag storage page has a short pin on it, so allow prep_new_page() ->
>>> arch_prep_new_page() to similarly fail.
>>
>> But what are the side-effects of this? How does the calling code recover?
>>
>> E.g., what if we need to populate a page into user space, but that
>> particular page we allocated fails to be prepared? So we inject a signal
>> into that poor process?
> 
> When the page fails to be prepared, it is put back to the tail of the
> freelist with __free_one_page(.., FPI_TO_TAIL). If all the allocation paths
> are exhausted and no page has been found for which tag storage has been
> reserved, then that's treated like an OOM situation.
> 
> I have been thinking about this, and I think I can simplify the code by
> making tag reservation a best effort approach. The page can be allocated
> even if reserving tag storage fails, but the page is marked as invalid in
> set_pte_at() (PAGE_NONE + an extra bit to tell arm64 that it needs tag
> storage) and next time it is accessed, arm64 will reserve tag storage in
> the fault handling code (the mechanism for that is implemented in patch #19
> of the series, "mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for
> mprotect(PROT_MTE)").
> 
> With this new approach, prep_new_page() stays the way it is, and no further
> changes are required for the page allocator, as there are already arch
> callbacks that can be used for that, for example tag_clear_highpage() and
> arch_alloc_page(). The downside is extra page faults, which might impact
> performance.
> 
> What do you think?

That sounds a lot more robust, compared to intermittent failures to 
allocate pages.

-- 
Cheers,

David / dhildenb