From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 72F11BE65 for ; Tue, 17 Jun 2025 08:13:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750147983; cv=none; b=IvP39zp2BvSj+Jkfx+oL39E5qe12O0cTqPmKs8ArHBaQTkKh4yJpr8/hiCqUlN3ippyqZiSUcWyMPEBCea7po7IODY4251MXy0f9SfCYlNaUxOgSugww4iOlsoyi2ueUXRQgaY/mjCFceXTl+ybSEsrjnALPnPQLLSxODKP9ue0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750147983; c=relaxed/simple; bh=yZ2rudRmr33RYh9OVOX8UzfJ/YJIWEg1jDmses83dhs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UhVbGAEbQeFihpyRm0y4NCisWl//uJnUny86G3viJTFLTfciXLNGDkHT1+2OJs9RiG5dg97CjSems6MoxmEq3YFmjS9mNNtTIZhMYakfYs/zFbzT0cuyB29UmLRwE62jITGkmUfgiK+Tey1DBM8QwYA/0sbCvzPwa5GtF5MTKOQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7D8DC15A1; Tue, 17 Jun 2025 01:12:38 -0700 (PDT) Received: from [10.57.84.117] (unknown [10.57.84.117]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E31613F673; Tue, 17 Jun 2025 01:12:57 -0700 (PDT) Message-ID: Date: Tue, 17 Jun 2025 09:12:56 +0100 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 v3] arm64: Enable vmalloc-huge with ptdump Content-Language: en-GB To: Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: anshuman.khandual@arm.com, quic_zhenhuah@quicinc.com, kevin.brodsky@arm.com, yangyicong@hisilicon.com, joey.gouly@arm.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, david@redhat.com References: <20250616103310.17625-1-dev.jain@arm.com> <5f7b0a4d-fb3f-43bc-9f2a-3951222cfff2@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 17/06/2025 04:59, Dev Jain wrote: > > On 17/06/25 8:24 am, Dev Jain wrote: >> >> On 16/06/25 11:37 pm, Ryan Roberts wrote: >>> On 16/06/2025 11:33, Dev Jain wrote: >>>> arm64 disables vmalloc-huge when kernel page table dumping is enabled, >>>> because an intermediate table may be removed, potentially causing the >>>> ptdump code to dereference an invalid address. We want to be able to >>>> analyze block vs page mappings for kernel mappings with ptdump, so to >>>> enable vmalloc-huge with ptdump, synchronize between page table removal in >>>> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We >>>> use mmap_read_lock and not write lock because we don't need to synchronize >>>> between two different vm_structs; two vmalloc objects running this same >>>> code path will point to different page tables, hence there is no race. >>>> >>>> For pud_free_pmd_page(), we isolate the PMD table to avoid taking the lock >>>> 512 times again via pmd_free_pte_page(). >>>> >>>> We implement the locking mechanism using static keys, since the chance >>>> of a race is very small. Observe that the synchronization is needed >>>> to avoid the following race: >>>> >>>> CPU1                            CPU2 >>>>                         take reference of PMD table >>>> pud_clear() >>>> pte_free_kernel() >>>>                         walk freed PMD table >>>> >>>> and similar race between pmd_free_pte_page and ptdump_walk_pgd. >>>> >>>> Therefore, there are two cases: if ptdump sees the cleared PUD, then >>>> we are safe. If not, then the patched-in read and write locks help us >>>> avoid the race. >>>> >>>> To implement the mechanism, we need the static key access from mmu.c and >>>> ptdump.c. Note that in case !CONFIG_PTDUMP_DEBUGFS, ptdump.o won't be a >>>> target in the Makefile, therefore we cannot initialize the key there, as >>>> is being done, for example, in the static key implementation of >>>> hugetlb-vmemmap. Therefore, include asm/cpufeature.h, which includes >>>> the jump_label mechanism. Declare the key there and define the key to false >>>> in mmu.c. >>>> >>>> No issues were observed with mm-selftests. No issues were observed while >>>> parallelly running test_vmalloc.sh and dumping the kernel pagetable through >>>> sysfs in a loop. >>>> >>>> v2->v3: >>>>   - Use static key mechanism >>>> >>>> v1->v2: >>>>   - Take lock only when CONFIG_PTDUMP_DEBUGFS is on >>>>   - In case of pud_free_pmd_page(), isolate the PMD table to avoid taking >>>>     the lock 512 times again via pmd_free_pte_page() >>>> >>>> Signed-off-by: Dev Jain >>>> --- >>>>   arch/arm64/include/asm/cpufeature.h |  1 + >>>>   arch/arm64/mm/mmu.c                 | 51 ++++++++++++++++++++++++++--- >>>>   arch/arm64/mm/ptdump.c              |  5 +++ >>>>   3 files changed, 53 insertions(+), 4 deletions(-) >>>> [...] >>>> +    pud_clear(pudp); >>> How can this possibly be correct; you're clearing the pud without any >>> synchronisation. So you could have this situation: >>> >>> CPU1 (vmalloc)            CPU2 (ptdump) >>> >>>                 static_branch_enable() >>>                   mmap_write_lock() >>>                     pud = pudp_get() >> >> When you do pudp_get(), you won't be dereferencing a NULL pointer. >> pud_clear() will nullify the pud entry. So pudp_get() will boil >> down to retrieving a NULL entry. Or, pudp_get() will retrieve an >> entry pointing to the now isolated PMD table. Correct me if I am >> wrong. >> >>> pud_free_pmd_page() >>>    pud_clear() >>>                     access the table pointed to by pud >>>                     BANG! > > I am also confused thoroughly now : ) This should not go bang as the > > table pointed to by pud is still there, and our sequence guarantees that > > if the ptdump walk is using the pmd table, then pud_free_pmd_page won't > > free the PMD table yet. You're right... I'm not sure what I was smoking last night. For some reason I read the pXd_clear() as "free". This approach looks good to me - very clever! And you even managed to ensure the WRITE_ONCE() in pXd_clear() doesn't get reordered after taking the lock via the existing dsb in the tlb maintenance operation - I like it! I'll send a separate review with some nits, but I'm out today, so that might have to wait until tomorrow. Thanks, and sorry again for the noise! Ryan