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 1641E21D3EC for ; Fri, 30 May 2025 11:50:44 +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=1748605847; cv=none; b=Kda7i0s8klxW2Rs8Iq985gfqNzagsclAjAAE7ArIdgMHq4o9VMEUgltWwIXPYG6UgnO8bhuu23pc6BtPwzXo1nuD+5sDWNcR5bG2b+Hfe/XyB8Xo0ukVBHWKZrKFfT2aBiWXVUIEhC3c32mngBBnauE3cpolFZlTrTgYHTfhzrM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748605847; c=relaxed/simple; bh=8J7MijuSmv8BbECeoJk6Iux4Xa8Uk9BEZ3d9RPs87d8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=O2QuP9OHzJTsFoFjKc5KHkLRRj+fKpwCCgneH3oqVPNKnjduRpcTX5bErmAC8HE0+BCVUr0a4vx93tQ7gEF4GjcUDDg83H+OwEiV1hJivXlsGqb2yKJlMz+V2lynHzl9wH+cyw4RG/akrq4TgmgJI11dOo7+TP/Ikk2cj04hAHA= 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 C679A16F2; Fri, 30 May 2025 04:50:27 -0700 (PDT) Received: from [10.57.95.14] (unknown [10.57.95.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7EB2C3F5A1; Fri, 30 May 2025 04:50:42 -0700 (PDT) Message-ID: <4202a03d-dacd-429c-91e6-81a5d05726a4@arm.com> Date: Fri, 30 May 2025 12:50:40 +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] 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: <20250530082021.18182-1-dev.jain@arm.com> <832e84a9-4303-4e21-a88b-94395898fa3e@arm.com> From: Ryan Roberts In-Reply-To: <832e84a9-4303-4e21-a88b-94395898fa3e@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 30/05/2025 10:14, Dev Jain wrote: > > On 30/05/25 2:10 pm, Ryan Roberts wrote: >> On 30/05/2025 09:20, 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. > > My "correction" from race->no problem was incorrect after all :) There will > be no race too since the vm_struct object has exclusive access to whatever > table it is clearing. > >>> >>> Signed-off-by: Dev Jain >>> --- >>>   arch/arm64/include/asm/vmalloc.h | 6 ++---- >>>   arch/arm64/mm/mmu.c              | 7 +++++++ >>>   2 files changed, 9 insertions(+), 4 deletions(-) >>> >>> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h >>> index 38fafffe699f..28b7173d8693 100644 >>> --- a/arch/arm64/include/asm/vmalloc.h >>> +++ b/arch/arm64/include/asm/vmalloc.h >>> @@ -12,15 +12,13 @@ static inline bool arch_vmap_pud_supported(pgprot_t prot) >>>       /* >>>        * SW table walks can't handle removal of intermediate entries. >>>        */ >>> -    return pud_sect_supported() && >>> -           !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>> +    return pud_sect_supported(); >>>   } >>>     #define arch_vmap_pmd_supported arch_vmap_pmd_supported >>>   static inline bool arch_vmap_pmd_supported(pgprot_t prot) >>>   { >>> -    /* See arch_vmap_pud_supported() */ >>> -    return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS); >>> +    return true; >>>   } >>>     #endif >>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>> index ea6695d53fb9..798cebd9e147 100644 >>> --- a/arch/arm64/mm/mmu.c >>> +++ b/arch/arm64/mm/mmu.c >>> @@ -1261,7 +1261,11 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) >>>       } >>>         table = pte_offset_kernel(pmdp, addr); >>> + >>> +    /* Synchronize against ptdump_walk_pgd() */ >>> +    mmap_read_lock(&init_mm); >>>       pmd_clear(pmdp); >>> +    mmap_read_unlock(&init_mm); >> So this works because ptdump_walk_pgd() takes the write_lock (which is mutually >> exclusive with any read_lock holders) for the duration of the table walk, so it >> will either consistently see the pgtables before or after this removal. It will >> never disappear during the walk, correct? >> >> I guess there is a risk of this showing up as contention with other init_mm >> write_lock holders. But I expect that pmd_free_pte_page()/pud_free_pmd_page() >> are called sufficiently rarely that the risk is very small. Let's fix any perf >> problem if/when we see it. > > We can avoid all of that by my initial approach - to wrap the lock around > CONFIG_PTDUMP_DEBUGFS. > I don't have a strong opinion, just putting it out there. (I wrote then failed to send earlier): It's ugly though. Personally I'd prefer to keep it simple unless we have clear evidence that its needed. I was just laying out my justification for not needing to doing the conditional wrapping in this comment. > >> >>>       __flush_tlb_kernel_pgtable(addr); >> And the tlbi doesn't need to be serialized because there is no security issue. >> The walker can be trusted to only dereference memory that it sees as it walks >> the pgtable (obviously). >> >>>       pte_free_kernel(NULL, table); >>>       return 1; >>> @@ -1289,7 +1293,10 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) >>>           pmd_free_pte_page(pmdp, next); >>>       } while (pmdp++, next += PMD_SIZE, next != end); >>>   +    /* Synchronize against ptdump_walk_pgd() */ >>> +    mmap_read_lock(&init_mm); >>>       pud_clear(pudp); >>> +    mmap_read_unlock(&init_mm); >> Hmm, so pud_free_pmd_page() is now going to cause us to acquire and release the >> (upto) lock 513 times (for a 4K kernel). I wonder if there is an argument for >> clearing the pud first (under the lock), then the pmds can all be cleared >> without a lock, since the walker won't be able to see the pmds once the pud is >> cleared. > > Yes, we can isolate the PMD table in case the caller of pmd_free_pte_page is > pud_free_pmd_page. In this case, vm_struct_1 has exclusive access to the entire > pmd page, hence no race will occur. But, in case of vmap_try_huge_pmd() being the > caller, we cannot drop the locks around pmd_free_pte_page. So we can have something > like > > #ifdef CONFIG_PTDUMP_DEBUGFS > static inline void ptdump_synchronize_lock(bool flag) > { >     if (flag) >         mmap_read_lock(&init_mm); > } > > and pass false when the caller is pud_free_pmd_page. > >> >> Thanks, >> Ryan >> >>>       __flush_tlb_kernel_pgtable(addr); >>>       pmd_free(NULL, table); >>>       return 1;