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[2003:d8:2f4a:8900:884a:b3af:e3c9:ec88]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a1f58ebd43sm23421175f8f.37.2025.05.15.06.04.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 May 2025 06:04:52 -0700 (PDT) Message-ID: <23042cdf-e0fc-4b3a-92f6-688689728cc7@redhat.com> Date: Thu, 15 May 2025 15:04:50 +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] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables To: Will Deacon Cc: Dev Jain , catalin.marinas@arm.com, ryan.roberts@arm.com, anshuman.khandual@arm.com, mark.rutland@arm.com, yang@os.amperecomputing.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org References: <20250515063450.86629-1-dev.jain@arm.com> <332ecda7-14c4-4dc3-aeff-26801b74ca04@redhat.com> <4904d02f-6595-4230-a321-23327596e085@arm.com> <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> <35ef7691-7eac-4efa-838d-c504c88c042b@arm.com> <91fc96c3-4931-4f07-a0a9-507ac7b5ae6d@arm.com> <20250515125606.GA11878@willie-the-truck> 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 15.05.25 14:56, Will Deacon wrote: > On Thu, May 15, 2025 at 11:32:22AM +0200, David Hildenbrand wrote: >> On 15.05.25 11:27, Dev Jain wrote: >>> >>> >>> On 15/05/25 2:23 pm, David Hildenbrand wrote: >>>> On 15.05.25 10:47, Dev Jain wrote: >>>>> >>>>> >>>>> On 15/05/25 2:06 pm, David Hildenbrand wrote: >>>>>> On 15.05.25 10:22, Dev Jain wrote: >>>>>>> >>>>>>> >>>>>>> On 15/05/25 1:43 pm, David Hildenbrand wrote: >>>>>>>> On 15.05.25 08:34, Dev Jain wrote: >>>>>>>>> Commit 9c006972c3fe removes the pxd_present() checks because the >>>>>>>>> caller >>>>>>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller >>>>>>>>> only >>>>>>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd >>>>>>>>> through >>>>>>>>> pmd_free_pte_page(), wherein the pmd may be none. >>>>>>>> The commit states: "The core code already has a check for pXd_none()", >>>>>>>> so I assume that assumption was not true in all cases? >>>>>>>> >>>>>>>> Should that one problematic caller then check for pmd_none() instead? >>>>>>> >>>>>>>    From what I could gather of Will's commit message, my >>>>>>> interpretation is >>>>>>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. >>>>>>> These individually check for pxd_present(): >>>>>>> >>>>>>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>>>>>>      return 0; >>>>>>> >>>>>>> The problem is that vmap_try_huge_pud will also iterate on pte entries. >>>>>>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page >>>>>>> may encounter a none pmd and trigger a WARN. >>>>>> >>>>>> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. >>>>>> >>>>>> I assume we should either have an explicit pmd_none() check in >>>>>> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in >>>>>> pmd_free_pte_page(). >>>>>> >>>>>> With your patch, we'd be calling pte_free_kernel() on a NULL pointer, >>>>>> which sounds wrong -- unless I am missing something important. >>>>> >>>>> Ah thanks, you seem to be right. We will be extracting table from a none >>>>> pmd. Perhaps we should still bail out for !pxd_present() but without the >>>>> warning, which the fix commit used to do. >>>> >>>> Right. We just make sure that all callers of pmd_free_pte_page() already >>>> check for it. >>>> >>>> I'd just do something like: >>>> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index 8fcf59ba39db7..e98dd7af147d5 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -1274,10 +1274,8 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long >>>> addr) >>>> >>>>         pmd = READ_ONCE(*pmdp); >>>> >>>> -       if (!pmd_table(pmd)) { >>>> -               VM_WARN_ON(1); >>>> -               return 1; >>>> -       } >>>> +       VM_WARN_ON(!pmd_present(pmd)); >>>> +       VM_WARN_ON(!pmd_table(pmd)); >>> >>> And also return 1? >> >> I'll leave that to Catalin + Will. >> >> I'm not a friend for adding runtime-overhead for soemthing that should not >> happen and be caught early during testing -> VM_WARN_ON_ONCE(). > > I definitely think we should return early if the pmd isn't a table. > Otherwise, we could end up descending into God-knows-what! The question is: how did something that is not a table end up here, and why is it valid to check exactly that at runtime. Not strong opinion, it just feels a bit arbitrary to test for exactly that at runtime if it is completely unexpected. -- Cheers, David / dhildenb