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

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Lorenzo,

On 6/5/25 10:04 PM, Lorenzo Stoakes wrote:
> On Thu, Jun 05, 2025 at 11:23:18AM +0800, Qi Zheng wrote:
>>
>>
>> On 6/5/25 1:50 AM, Lorenzo Stoakes wrote:
>>> On Wed, Jun 04, 2025 at 02:02:12PM +0800, Qi Zheng wrote:
>>>> Hi Lorenzo,
>>>>
>>>> On 6/3/25 5:54 PM, Lorenzo Stoakes wrote:
>>>>> 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.
>>>
>>> Seems to me that we don't need the VMA locks then unless I'm missing
>>> something? :) Jann?
>>>
>>> Would this RCU-lock-acquired-by-pte_offset_map also save us from the
>>> munmap() downgraded read lock scenario also? Or is the problem there
>>> intermediate page table teardown I guess?
>>>
>>
>> Right. Currently, page table pages other than PTE pages are not
>> protected by RCU, so mmap write lock still needed in the munmap path
>> to wait for all readers of the page table pages to exit the critical
>> section.
>>
>> In other words, once we have achieved that all page table pages are
>> protected by RCU, we can completely remove the page table pages from
>> the protection of mmap locks.
> 
> Interesting - so on reclaim/migrate we are just clearing PTE entries with
> the rmap lock right? Would this lead to a future where we could also tear
> down page tables there?
> 
> Another point to remember is that when we are clearing down higher level
> page tables in the general case, the logic assumes nothing else can touch
> anything... we hold both rmap lock AND mmap/vma locks at this point.
> 
> But I guess if we're RCU-safe, we're same even from rmap right?

Yeah, and we have already done something similar. For more details,
please refer to retract_page_tables(). It only holds i_mmap_rwsem read
lock and then calls pte_free_defer() to free the PTE page through RCU.

For migrate case, the pte entry will store a migrate entry, right? And a
new physical page will be installed soon through a page fault, so I
don't think it is necessary to free the corresponding PTE page.

For reclaim case, there is a problem that only PTE entries that mapped
to a physical page are operated each time. If we want to free the entire
PTE page, we need to check the adjacent PTE entries. Maybe MGLRU can
help with this. I remember that MGLRU has an optimization that will 
check the adjacent PTE entries.

> 
>>
>> Here are some of my previous thoughts:
>>
>> ```
>> Another plan
>> ============
>>
>> Currently, page table modification are protected by page table locks
>> (page_table_lock or split pmd/pte lock), but the life cycle of page
>> table pages are protected by mmap_lock (and vma lock). For more details,
>> please refer to the latest added Documentation/mm/process_addrs.rst file.
>>
>> Currently we try to free the PTE pages through RCU when
>> CONFIG_PT_RECLAIM is turned on. In this case, we will no longer
>> need to hold mmap_lock for the read/write op on the PTE pages.
>>
>> So maybe we can remove the page table from the protection of the mmap
>> lock (which is too big), like this:
>>
>> 1. free all levels of page table pages by RCU, not just PTE pages, but
>>     also pmd, pud, etc.
>> 2. similar to pte_offset_map/pte_unmap, add
>>     [pmd|pud]_offset_map/[pmd|pud]_unmap, and make them all contain
>>     rcu_read_lock/rcu_read_unlcok, and make them accept failure.
>>
>> In this way, we no longer need the mmap lock. For readers, such as page
>> table wallers, we are already in the critical section of RCU. For
>> writers, we only need to hold the page table lock.
>>
>> But there is a difficulty here, that is, the RCU critical section is not
>> allowed to sleep, but it is possible to sleep in the callback function
>> of .pmd_entry, such as mmu_notifier_invalidate_range_start().
>>
>> Use SRCU instead? Or use RCU + refcount method? Not sure. But I think
>> it's an interesting thing to try.
> 
> Thanks for the information, RCU freeing of page tables is something of a

RCU-freeing is relatively simple, tlb_remove_table() can be easily
changed to free all levels of page table pages through RCU. The more
difficult is to protect the page table pages above PTE level through RCU
lock.

> long-term TODO discussed back and forth :) might take a look myself if
> somebody else hasn't grabbed when I have a second...

This is awesome, I'm stuck with some other stuff at the moment, I'll
also take a look at it later when I have time.

> 
> Is it _only_ the mmu notifier sleeping in this scenario? Or are there other
> examples?

I'm not sure, need some investigation.

> 
> We could in theory always add another callback .pmd_entry_sleep or
> something for this one case and document the requirement...

Maybe, but the SRCU critical section cannot prevent the PTE page from
being freed via RCU. :(

Thanks!

> 
>> ```
>>
>> Thanks!
>>
>>
> 
> Cheers, Lorenzo