Re: [PATCH RFC v2] mm: use per_vma lock for MADV_DONTNEED

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

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.