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7am8kxhmzdgug51ipu9mnbsrgo7nf8cg X-Rspam-User: X-HE-Tag: 1748944492-850499 X-HE-Meta: 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 RBcpr4eZ rIPg5FqTrNXH2S2jAmkt0MWATT8gVAPieHuEPWjLWFUDfzlrXUJlSZ5jHIopxSLdAKAnElEMP9SJbdmu28Flzbsj6FA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 03, 2025 at 03:24:28PM +0800, Qi Zheng wrote: > Hi Jann, > > On 5/30/25 10:06 PM, Jann Horn wrote: > > On Fri, May 30, 2025 at 12:44 PM Barry Song <21cnbao@gmail.com> wrote: > > > Certain madvise operations, especially MADV_DONTNEED, occur far more > > > frequently than other madvise options, particularly in native and Java > > > heaps for dynamic memory management. > > > > > > Currently, the mmap_lock is always held during these operations, even when > > > unnecessary. This causes lock contention and can lead to severe priority > > > inversion, where low-priority threads—such as Android's HeapTaskDaemon— > > > hold the lock and block higher-priority threads. > > > > > > This patch enables the use of per-VMA locks when the advised range lies > > > entirely within a single VMA, avoiding the need for full VMA traversal. In > > > practice, userspace heaps rarely issue MADV_DONTNEED across multiple VMAs. > > > > > > Tangquan’s testing shows that over 99.5% of memory reclaimed by Android > > > benefits from this per-VMA lock optimization. After extended runtime, > > > 217,735 madvise calls from HeapTaskDaemon used the per-VMA path, while > > > only 1,231 fell back to mmap_lock. > > > > > > To simplify handling, the implementation falls back to the standard > > > mmap_lock if userfaultfd is enabled on the VMA, avoiding the complexity of > > > userfaultfd_remove(). > > > > One important quirk of this is that it can, from what I can see, cause > > freeing of page tables (through pt_reclaim) without holding the mmap > > lock at all: > > > > do_madvise [behavior=MADV_DONTNEED] > > madvise_lock > > lock_vma_under_rcu > > madvise_do_behavior > > madvise_single_locked_vma > > madvise_vma_behavior > > madvise_dontneed_free > > madvise_dontneed_single_vma > > zap_page_range_single_batched [.reclaim_pt = true] > > unmap_single_vma > > unmap_page_range > > zap_p4d_range > > zap_pud_range > > zap_pmd_range > > zap_pte_range > > try_get_and_clear_pmd > > free_pte > > > > This clashes with the assumption in walk_page_range_novma() that > > holding the mmap lock in write mode is sufficient to prevent > > concurrent page table freeing, so it can probably lead to page table > > UAF through the ptdump interface (see ptdump_walk_pgd()). > > Maybe not? The PTE page is freed via RCU in zap_pte_range(), so in the > following case: > > cpu 0 cpu 1 > > ptdump_walk_pgd > --> walk_pte_range > --> pte_offset_map (hold RCU read lock) > zap_pte_range > --> free_pte (via RCU) > walk_pte_range_inner > --> ptdump_pte_entry (the PTE page is not freed at this time) > > IIUC, there is no UAF issue here? > > If I missed anything please let me know. > > Thanks, > Qi > > I forgot about that interesting placement of RCU lock acquisition :) I will obviously let Jann come back to you on this, but I wonder if I need to update the doc to reflect this actually.