Re: [PATCH RFC] mm: madvise: use per_vma lock for MADV_FREE

From: Lorenzo Stoakes
Date: Tue Jun 10 2025 - 14:53:09 EST

On Tue, Jun 10, 2025 at 05:59:20PM +1200, Barry Song wrote:
> From: Barry Song <v-songbaohua@xxxxxxxx>
>
> MADV_FREE is another option, besides MADV_DONTNEED, for dynamic memory
> freeing in user-space native or Java heap memory management. For example,
> jemalloc can be configured to use MADV_FREE, and recent versions of the
> Android Java heap have also increasingly adopted MADV_FREE. Supporting
> per-VMA locking for MADV_FREE thus appears increasingly necessary.
>
> We have replaced walk_page_range() with walk_page_range_vma(). Along with
> the proposed madvise_lock_mode by Lorenzo, the necessary infrastructure is
> now in place to begin exploring per-VMA locking support for MADV_FREE and
> potentially other madvise using walk_page_range_vma().

Thanks :)

>
> This patch adds support for the PGWALK_VMA_RDLOCK walk_lock mode in
> walk_page_range_vma(), and leverages madvise_lock_mode from
> madv_behavior to select the appropriate walk_lock—either mmap_lock or
> per-VMA lock—based on the context.
>
> To ensure thread safety, madvise_free_walk_ops is now defined as a stack
> variable instead of a global constant.

A nit but I'd add 'because we now dynamically update the walk_ops->walk_lock
field we must make sure this is thread safe' or something like this to clarify
the need for this

Did we not have to worry about this before I guess because the mmap lock would
exclude other threads?

An aside, but I wonder if we have this implicit assumption elsewhere that VMA
locks defeat... hm :)

>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Tangquan Zheng <zhengtangquan@xxxxxxxx>
> Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>

Looks good to me, kinda neat how the previous work for the MADV_DONTNEED under
VMA lock stuff made this pretty straightforward :)

So:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
> include/linux/pagewalk.h | 2 ++
> mm/madvise.c | 20 ++++++++++++++------
> mm/pagewalk.c | 6 ++++++
> 3 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
> index 9700a29f8afb..a4afa64ef0ab 100644
> --- a/include/linux/pagewalk.h
> +++ b/include/linux/pagewalk.h
> @@ -14,6 +14,8 @@ enum page_walk_lock {
> PGWALK_WRLOCK = 1,
> /* vma is expected to be already write-locked during the walk */
> PGWALK_WRLOCK_VERIFY = 2,
> + /* vma is expected to be already read-locked during the walk */
> + PGWALK_VMA_RDLOCK_VERIFY = 3,
> };
>
> /**
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 381eedde8f6d..23d58eb31c8f 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -775,10 +775,14 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> return 0;
> }
>
> -static const struct mm_walk_ops madvise_free_walk_ops = {
> - .pmd_entry = madvise_free_pte_range,
> - .walk_lock = PGWALK_RDLOCK,
> -};
> +static inline enum page_walk_lock get_walk_lock(enum madvise_lock_mode mode)
> +{
> + /* Other modes don't require fixing up the walk_lock. */
> + VM_WARN_ON_ONCE(mode != MADVISE_VMA_READ_LOCK &&
> + mode != MADVISE_MMAP_READ_LOCK);

I find this a bit hard to parse...

> + return mode == MADVISE_VMA_READ_LOCK ?
> + PGWALK_VMA_RDLOCK_VERIFY : PGWALK_RDLOCK;

...might be better as something like:

switch (mode) {
case MADVISE_VMA_READ_LOCK:
return PGWALK_VMA_RDLOCK_VERIFY;
case MADVISE_MMAP_READ_LOCK:
return PGWALK_RDLOCK;
default:
/* Invalid. */
WARN_ON_ONCE(1);
return PGWALK_RDLOCK;
}

> +}
>
> static int madvise_free_single_vma(struct madvise_behavior *madv_behavior,
> struct vm_area_struct *vma,
> @@ -787,6 +791,9 @@ static int madvise_free_single_vma(struct madvise_behavior *madv_behavior,
> struct mm_struct *mm = vma->vm_mm;
> struct mmu_notifier_range range;
> struct mmu_gather *tlb = madv_behavior->tlb;
> + struct mm_walk_ops walk_ops = {
> + .pmd_entry = madvise_free_pte_range,
> + };
>
> /* MADV_FREE works for only anon vma at the moment */
> if (!vma_is_anonymous(vma))
> @@ -806,8 +813,9 @@ static int madvise_free_single_vma(struct madvise_behavior *madv_behavior,
>
> mmu_notifier_invalidate_range_start(&range);
> tlb_start_vma(tlb, vma);
> + walk_ops.walk_lock = get_walk_lock(madv_behavior->lock_mode);
> walk_page_range_vma(vma, range.start, range.end,
> - &madvise_free_walk_ops, tlb);
> + &walk_ops, tlb);
> tlb_end_vma(tlb, vma);
> mmu_notifier_invalidate_range_end(&range);
> return 0;
> @@ -1653,7 +1661,6 @@ static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavi
> case MADV_WILLNEED:
> case MADV_COLD:
> case MADV_PAGEOUT:
> - case MADV_FREE:
> case MADV_POPULATE_READ:
> case MADV_POPULATE_WRITE:
> case MADV_COLLAPSE:
> @@ -1662,6 +1669,7 @@ static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavi
> return MADVISE_MMAP_READ_LOCK;
> case MADV_DONTNEED:
> case MADV_DONTNEED_LOCKED:
> + case MADV_FREE:
> return MADVISE_VMA_READ_LOCK;
> default:
> return MADVISE_MMAP_WRITE_LOCK;
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index e478777c86e1..c984aacc5552 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -420,6 +420,9 @@ static int __walk_page_range(unsigned long start, unsigned long end,
> static inline void process_mm_walk_lock(struct mm_struct *mm,
> enum page_walk_lock walk_lock)
> {
> + if (walk_lock == PGWALK_VMA_RDLOCK_VERIFY)
> + return;
> +
> if (walk_lock == PGWALK_RDLOCK)
> mmap_assert_locked(mm);
> else
> @@ -437,6 +440,9 @@ static inline void process_vma_walk_lock(struct vm_area_struct *vma,
> case PGWALK_WRLOCK_VERIFY:
> vma_assert_write_locked(vma);
> break;
> + case PGWALK_VMA_RDLOCK_VERIFY:
> + vma_assert_locked(vma);
> + break;
> case PGWALK_RDLOCK:
> /* PGWALK_RDLOCK is handled by process_mm_walk_lock */
> break;
> --
> 2.39.3 (Apple Git-146)
>