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[95.203.1.180]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-553ac1ab12csm1877346e87.113.2025.06.17.04.51.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Jun 2025 04:51:33 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 17 Jun 2025 13:51:31 +0200 To: Ryan Roberts Cc: Dev Jain , catalin.marinas@arm.com, will@kernel.org, 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 Subject: Re: [PATCH v3] arm64: Enable vmalloc-huge with ptdump Message-ID: References: <20250616103310.17625-1-dev.jain@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jun 16, 2025 at 10:20:29PM +0100, Ryan Roberts wrote: > On 16/06/2025 19:07, 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(-) > >> > >> 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); > >> /* > >> * 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; > >> 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); > >> + > >> 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; > >> 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); > > > > 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() > > pud_free_pmd_page() > > pud_clear() > > access the table pointed to by pud > > BANG! > > > > Surely the logic needs to be: > > > > if (static_branch_unlikely(&ptdump_lock_key)) { > > mmap_read_lock(&init_mm); > > lock_taken = true; > > } > > pud_clear(pudp); > > if (unlikely(lock_taken)) > > mmap_read_unlock(&init_mm); > > > > That fixes your first case, I think? But doesn't fix your second case. You could > > still have: > > > > CPU1 (vmalloc) CPU2 (ptdump) > > > > pud_free_pmd_page() > > > > static_branch_enable() > > mmap_write_lock() > > pud = pudp_get() > > pud_clear() > > access the table pointed to by pud > > BANG! > > > > I think what you need is some sort of RCU read-size critical section in the > > vmalloc side that you can then synchonize on in the ptdump side. But you would > > need to be in the read side critical section when you sample the static key, but > > you can't sleep waiting for the mmap lock while in the critical section. This > > feels solvable, and there is almost certainly a well-used pattern, but I'm not > > quite sure what the answer is. Perhaps others can help... > > Just taking a step back here, I found the "percpu rw semaphore". From the > documentation: > > """ > Percpu rw semaphores is a new read-write semaphore design that is > optimized for locking for reading. > > The problem with traditional read-write semaphores is that when multiple > cores take the lock for reading, the cache line containing the semaphore > is bouncing between L1 caches of the cores, causing performance > degradation. > > Locking for reading is very fast, it uses RCU and it avoids any atomic > instruction in the lock and unlock path. On the other hand, locking for > writing is very expensive, it calls synchronize_rcu() that can take > hundreds of milliseconds. > """ > > Perhaps this provides the properties we are looking for? Could just define one > of these and lock it in read mode around pXd_clear() on the vmalloc side. Then > lock it in write mode around ptdump_walk_pgd() on the ptdump side. No need for > static key or other hoops. Given its a dedicated lock, there is no risk of > accidental contention because no other code is using it. > Write-lock indeed is super expensive, as you noted it blocks on synchronize_rcu(). If that write-lock interferes with a critical vmalloc fast path, where a read-lock could be injected, then it is definitely a problem. I have not analysed this patch series. I need to have a look what "ptdump" does. -- Uladzislau Rezki