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[2003:d8:2f4a:8900:884a:b3af:e3c9:ec88]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a1f57ddd53sm22111507f8f.1.2025.05.15.02.32.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 May 2025 02:32:23 -0700 (PDT) Message-ID: Date: Thu, 15 May 2025 11:32:22 +0200 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: Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: 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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250515_023229_827976_45607ADC X-CRM114-Status: GOOD ( 20.29 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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(). > Also we should BUG_ON(!pmd_present(pmd)) to avoid the null dereference? Definitely no BUG_ON :) > >> >>         table = pte_offset_kernel(pmdp, addr); >>         pmd_clear(pmdp); >> @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long >> addr) >>         next = addr; >>         end = addr + PUD_SIZE; >>         do { >> -               pmd_free_pte_page(pmdp, next); >> +               if (pmd_present(*pmdp)) >> +                       pmd_free_pte_page(pmdp, next); > > Ah yes, the "caller" of pmd_free_pte_page() is not only > vmap_try_huge_pmd but this also...my mind has been foggy lately... > need to solve a math problem or two to sharpen it :) Haha. -- Cheers, David / dhildenb