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[2003:cb:c749:9100:c078:eec6:f2f4:dd3b]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43118305c6bsm39740645e9.22.2024.10.11.04.40.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Oct 2024 04:40:35 -0700 (PDT) Message-ID: Date: Fri, 11 Oct 2024 13:40:33 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] mm: don't install PMD mappings when THPs are disabled by the hw/process/vma To: Ryan Roberts , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, kvm@vger.kernel.org, Andrew Morton , Hugh Dickins , Thomas Huth , "Matthew Wilcox (Oracle)" , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Kefeng Wang , Leo Fu References: <20241011102445.934409-1-david@redhat.com> <20241011102445.934409-3-david@redhat.com> <4fd20101-d15c-4f9b-93c1-c780734a2294@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 11.10.24 13:36, Ryan Roberts wrote: > On 11/10/2024 12:33, David Hildenbrand wrote: >> On 11.10.24 13:29, Ryan Roberts wrote: >>> On 11/10/2024 11:24, David Hildenbrand wrote: >>>> We (or rather, readahead logic :) ) might be allocating a THP in the >>>> pagecache and then try mapping it into a process that explicitly disabled >>>> THP: we might end up installing PMD mappings. >>>> >>>> This is a problem for s390x KVM, which explicitly remaps all PMD-mapped >>>> THPs to be PTE-mapped in s390_enable_sie()->thp_split_mm(), before >>>> starting the VM. >>>> >>>> For example, starting a VM backed on a file system with large folios >>>> supported makes the VM crash when the VM tries accessing such a mapping >>>> using KVM. >>>> >>>> Is it also a problem when the HW disabled THP using >>>> TRANSPARENT_HUGEPAGE_UNSUPPORTED? At least on x86 this would be the case >>>> without X86_FEATURE_PSE. >>>> >>>> In the future, we might be able to do better on s390x and only disallow >>>> PMD mappings -- what s390x and likely TRANSPARENT_HUGEPAGE_UNSUPPORTED >>>> really wants. For now, fix it by essentially performing the same check as >>>> would be done in __thp_vma_allowable_orders() or in shmem code, where this >>>> works as expected, and disallow PMD mappings, making us fallback to PTE >>>> mappings. >>>> >>>> Reported-by: Leo Fu >>>> Fixes: 793917d997df ("mm/readahead: Add large folio readahead") >>> >>> Will this patch be difficult to backport given it depends on the previous patch >>> and that doesn't have a Fixes tag? >> >> "difficult" -- not really. Andrew might want to tag patch #1  with "Fixes:" as >> well, but I can also send simple stable backports that avoid patch #1. >> >> (Thinking again, I assume we want to Cc:stable) >> >>> >>>> Cc: Thomas Huth >>>> Cc: Matthew Wilcox (Oracle) >>>> Cc: Ryan Roberts >>>> Cc: Christian Borntraeger >>>> Cc: Janosch Frank >>>> Cc: Claudio Imbrenda >>>> Signed-off-by: David Hildenbrand >>>> --- >>>>   mm/memory.c | 9 +++++++++ >>>>   1 file changed, 9 insertions(+) >>>> >>>> diff --git a/mm/memory.c b/mm/memory.c >>>> index 2366578015ad..a2e501489517 100644 >>>> --- a/mm/memory.c >>>> +++ b/mm/memory.c >>>> @@ -4925,6 +4925,15 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct >>>> page *page) >>>>       pmd_t entry; >>>>       vm_fault_t ret = VM_FAULT_FALLBACK; >>>>   +    /* >>>> +     * It is too late to allocate a small folio, we already have a large >>>> +     * folio in the pagecache: especially s390 KVM cannot tolerate any >>>> +     * PMD mappings, but PTE-mapped THP are fine. So let's simply refuse any >>>> +     * PMD mappings if THPs are disabled. >>>> +     */ >>>> +    if (thp_disabled_by_hw() || vma_thp_disabled(vma, vma->vm_flags)) >>>> +        return ret; >>> >>> Why not just call thp_vma_allowable_orders()? >> >> Why call thp_vma_allowable_orders() that does a lot more work that doesn't >> really apply here? :) > > Yeah fair enough, I was just thinking it makes the code simpler to keep all the > checks in one place. But no strong opinion. > > Either way: > > Reviewed-by: Ryan Roberts Thanks! Also, I decided to not use "thp_vma_allowable_orders" because we are past the allocation phase (as indicated in the comment) and can really just change the way how we map the folio (PMD vs. PTE), not really *what* folio to use. Ideally, in the future we have a different way of just saying "no PMD mappings please", decoupling the mapping from the allocation granularity. -- Cheers, David / dhildenb