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Date: Tue, 30 Jan 2024 09:47:30 +0100
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Subject: Re: [PATCH v1 3/9] mm/memory: further separate anon and pagecache
 folio handling in zap_present_pte()
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To: Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
 Nick Piggin <npiggin@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
 Michael Ellerman <mpe@ellerman.id.au>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, Arnd Bergmann <arnd@arndb.de>,
 linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
 linux-s390@vger.kernel.org
References: <20240129143221.263763-1-david@redhat.com>
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <a50f2ee0-680d-4506-93f0-af22adda1b3b@arm.com>
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On 30.01.24 09:45, Ryan Roberts wrote:
> On 30/01/2024 08:37, David Hildenbrand wrote:
>> On 30.01.24 09:31, Ryan Roberts wrote:
>>> On 29/01/2024 14:32, David Hildenbrand wrote:
>>>> We don't need up-to-date accessed-dirty information for anon folios and can
>>>> simply work with the ptent we already have. Also, we know the RSS counter
>>>> we want to update.
>>>>
>>>> We can safely move arch_check_zapped_pte() + tlb_remove_tlb_entry() +
>>>> zap_install_uffd_wp_if_needed() after updating the folio and RSS.
>>>>
>>>> While at it, only call zap_install_uffd_wp_if_needed() if there is even
>>>> any chance that pte_install_uffd_wp_if_needed() would do *something*.
>>>> That is, just don't bother if uffd-wp does not apply.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>    mm/memory.c | 16 +++++++++++-----
>>>>    1 file changed, 11 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 69502cdc0a7d..20bc13ab8db2 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1552,12 +1552,9 @@ static inline void zap_present_pte(struct mmu_gather
>>>> *tlb,
>>>>        folio = page_folio(page);
>>>>        if (unlikely(!should_zap_folio(details, folio)))
>>>>            return;
>>>> -    ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>> -    arch_check_zapped_pte(vma, ptent);
>>>> -    tlb_remove_tlb_entry(tlb, pte, addr);
>>>> -    zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
>>>>          if (!folio_test_anon(folio)) {
>>>> +        ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>>            if (pte_dirty(ptent)) {
>>>>                folio_mark_dirty(folio);
>>>>                if (tlb_delay_rmap(tlb)) {
>>>> @@ -1567,8 +1564,17 @@ static inline void zap_present_pte(struct mmu_gather
>>>> *tlb,
>>>>            }
>>>>            if (pte_young(ptent) && likely(vma_has_recency(vma)))
>>>>                folio_mark_accessed(folio);
>>>> +        rss[mm_counter(folio)]--;
>>>> +    } else {
>>>> +        /* We don't need up-to-date accessed/dirty bits. */
>>>> +        ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
>>>> +        rss[MM_ANONPAGES]--;
>>>>        }
>>>> -    rss[mm_counter(folio)]--;
>>>> +    arch_check_zapped_pte(vma, ptent);
>>>
>>> Isn't the x86 (only) implementation of this relying on the dirty bit? So doesn't
>>> that imply you still need get_and_clear for anon? (And in hindsight I think that
>>> logic would apply to the previous patch too?)
>>
>> x86 uses the encoding !writable && dirty to indicate special shadow stacks. That
>> is, the hw dirty bit is set by software (to create that combination), not by
>> hardware.
>>
>> So you don't have to sync against any hw changes of the hw dirty bit. What you
>> had in the original PTE you read is sufficient.
>>
> 
> Right, got it. In that case:

Thanks a lot for paying that much attention during your reviews! Highly 
appreciated!

> 
> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
> 
> 

-- 
Cheers,

David / dhildenb