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 B3B0129B23C for ; Wed, 25 Jun 2025 10:35:19 +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=1750847723; cv=none; b=XLlOGnFYzu4YJGXg07H1vCbNhTzjlHG2amU2XT/RzHHmG/kUedzjL+b27wSasiWxZXUXzJ3w1ZkqluyRndmbwKvaEX6RHPcTsSwzI+D7JWV4QC6PWVAidgyBFpIJwMOqPqKJV1m9egx0u92wte/kinLJEfgCrDqrVIT+TuQVXNk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750847723; c=relaxed/simple; bh=kcAcK5jd09oWV4gJibbPb6KcqEYlBE9/nXmilJck/7w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Rk8o/wpoLuzmQvvNhWvfv0LIhaZ/rUWQuPDFgPXQe47bHoTq3GPjSGCgOjLftW1GyiymWZYQ6wSbl57Ts2AO2tLCus9cm0Mbh/6Y0b3of8xGWiDgInl9rgx4MGLdPGndRSdJ1NkxliJXaO7WbDgO2x9URQw6q9qZ37Q/kG8LS78= 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 D3E1C106F; Wed, 25 Jun 2025 03:34:54 -0700 (PDT) Received: from [10.57.84.221] (unknown [10.57.84.221]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 34EFA3F63F; Wed, 25 Jun 2025 03:35:11 -0700 (PDT) Message-ID: <01b48f6c-81cf-499c-97f7-0b537364c5e7@arm.com> Date: Wed, 25 Jun 2025 11:35:09 +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> From: Ryan Roberts In-Reply-To: <20250616103310.17625-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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(-) > > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h > index c4326f1cb917..3e386563b587 100644 > --- a/arch/arm64/include/asm/cpufeature.h > +++ b/arch/arm64/include/asm/cpufeature.h > @@ -26,6 +26,7 @@ > #include > #include > > +DECLARE_STATIC_KEY_FALSE(ptdump_lock_key); Is this really the correct header file for this declaration? Perhaps it would be better in arch/arm64/include/asm/ptdump.h ? > /* > * CPU feature register tracking > * > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index 8fcf59ba39db..e242ba428820 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -41,11 +41,14 @@ > #include > #include > #include > +#include > > #define NO_BLOCK_MAPPINGS BIT(0) > #define NO_CONT_MAPPINGS BIT(1) > #define NO_EXEC_MAPPINGS BIT(2) /* assumes FEAT_HPDS is not used */ > > +DEFINE_STATIC_KEY_FALSE(ptdump_lock_key); > + > enum pgtable_type { > TABLE_PTE, > TABLE_PMD, > @@ -1267,8 +1270,9 @@ int pmd_clear_huge(pmd_t *pmdp) > return 1; > } > > -int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) > +static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr, bool lock) > { > + bool lock_taken = false; As David commented, no need for this. > pte_t *table; > pmd_t pmd; > > @@ -1279,15 +1283,29 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) > return 1; > } > > + /* See comment in pud_free_pmd_page for static key logic */ > table = pte_offset_kernel(pmdp, addr); > pmd_clear(pmdp); > __flush_tlb_kernel_pgtable(addr); > + if (static_branch_unlikely(&ptdump_lock_key) && lock) { > + mmap_read_lock(&init_mm); > + lock_taken = true; > + } > + if (unlikely(lock_taken)) > + mmap_read_unlock(&init_mm); > + As per David's comment this can just be: if (static_branch_unlikely(&ptdump_lock_key) && lock) { mmap_read_lock(&init_mm); mmap_read_unlock(&init_mm); } > pte_free_kernel(NULL, table); > return 1; > } > > +int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr) > +{ > + return __pmd_free_pte_page(pmdp, addr, true); > +} > + > int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > { > + bool lock_taken = false; Same comment. > pmd_t *table; > pmd_t *pmdp; > pud_t pud; > @@ -1301,15 +1319,40 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) > } > > table = pmd_offset(pudp, addr); > + /* > + * Isolate the PMD table; in case of race with ptdump, this helps > + * us to avoid taking the lock in __pmd_free_pte_page(). > + * > + * Static key logic: > + * > + * Case 1: If ptdump does static_branch_enable(), and after that we > + * execute the if block, then this patches in the read lock, ptdump has > + * the write lock patched in, therefore ptdump will never read from > + * a potentially freed PMD table. > + * > + * Case 2: If the if block starts executing before ptdump's > + * static_branch_enable(), then no locking synchronization > + * will be done. However, pud_clear() + the dsb() in > + * __flush_tlb_kernel_pgtable will ensure that ptdump observes an > + * empty PUD. Thus, it will never walk over a potentially freed > + * PMD table. > + */ > + pud_clear(pudp); > + __flush_tlb_kernel_pgtable(addr); > + if (static_branch_unlikely(&ptdump_lock_key)) { > + mmap_read_lock(&init_mm); > + lock_taken = true; > + } > + if (unlikely(lock_taken)) > + mmap_read_unlock(&init_mm); Same comment. > + > pmdp = table; > next = addr; > end = addr + PUD_SIZE; > do { > - pmd_free_pte_page(pmdp, next); > + __pmd_free_pte_page(pmdp, next, false); > } while (pmdp++, next += PMD_SIZE, next != end); > > - pud_clear(pudp); > - __flush_tlb_kernel_pgtable(addr); > pmd_free(NULL, table); > return 1; > } > diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c > index 421a5de806c6..f75e12a1d068 100644 > --- a/arch/arm64/mm/ptdump.c > +++ b/arch/arm64/mm/ptdump.c > @@ -25,6 +25,7 @@ > #include > #include > > +#include > > #define pt_dump_seq_printf(m, fmt, args...) \ > ({ \ > @@ -311,7 +312,9 @@ void ptdump_walk(struct seq_file *s, struct ptdump_info *info) > } > }; > > + static_branch_enable(&ptdump_lock_key); > ptdump_walk_pgd(&st.ptdump, info->mm, NULL); > + static_branch_disable(&ptdump_lock_key); > } > > static void __init ptdump_initialize(void) > @@ -353,7 +356,9 @@ bool ptdump_check_wx(void) > } > }; > > + static_branch_enable(&ptdump_lock_key); > ptdump_walk_pgd(&st.ptdump, &init_mm, NULL); > + static_branch_disable(&ptdump_lock_key); > > if (st.wx_pages || st.uxn_pages) { > pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found, %lu non-UXN pages found\n", With the improvements as suggested, LGTM: Reviewed-by: Ryan Roberts