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

[Suren Baghdasaryan]

On Tue, Jun 3, 2025 at 11:43 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> Hi Barry,
>
> As promised, I enclose a patch to give a sense of how I think we might
> thread state through this operation.
>
> There's a todo on the untagged stuff so you can figure that out. This is
> based on the v1 so it might not encompass everything you addressed in the
> v2.
>
> Passing in madv_behavior to madvise_walk_vmas() twice kinda sucks, I
> _despise_ the void *arg function ptr stuff there added just for the anon
> vma name stuff (ughhh) so might be the only sensible way of threading
> state.
>
> I don't need any attribution, so please use this patch as you see
> fit/adapt/delete/do whatever with it, just an easier way for me to show the
> idea!
>
> I did some very basic testing and it seems to work, but nothing deeper.
>
> Cheers, Lorenzo
>
> ----8<----
> From ff4ba0115cb31a0630b6f8c02c68f11b3fb71f7a Mon Sep 17 00:00:00 2001
> From: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Date: Tue, 3 Jun 2025 18:22:55 +0100
> Subject: [PATCH] mm/madvise: support VMA read locks for MADV_DONTNEED[_LOCKED]
>
> Refactor the madvise() code to retain state about the locking mode utilised
> for traversing VMAs.
>
> Then use this mechanism to permit VMA locking to be done later in the
> madvise() logic and also to allow altering of the locking mode to permit
> falling back to an mmap read lock if required.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
>  mm/madvise.c | 174 +++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 127 insertions(+), 47 deletions(-)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 5f7a66a1617e..a3a6d73d0bd5 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -48,38 +48,19 @@ struct madvise_walk_private {
>         bool pageout;
>  };
>
> +enum madvise_lock_mode {
> +       MADVISE_NO_LOCK,
> +       MADVISE_MMAP_READ_LOCK,
> +       MADVISE_MMAP_WRITE_LOCK,
> +       MADVISE_VMA_READ_LOCK,
> +};
> +
>  struct madvise_behavior {
>         int behavior;
>         struct mmu_gather *tlb;
> +       enum madvise_lock_mode lock_mode;
>  };
>
> -/*
> - * Any behaviour which results in changes to the vma->vm_flags needs to
> - * take mmap_lock for writing. Others, which simply traverse vmas, need
> - * to only take it for reading.
> - */
> -static int madvise_need_mmap_write(int behavior)
> -{
> -       switch (behavior) {
> -       case MADV_REMOVE:
> -       case MADV_WILLNEED:
> -       case MADV_DONTNEED:
> -       case MADV_DONTNEED_LOCKED:
> -       case MADV_COLD:
> -       case MADV_PAGEOUT:
> -       case MADV_FREE:
> -       case MADV_POPULATE_READ:
> -       case MADV_POPULATE_WRITE:
> -       case MADV_COLLAPSE:
> -       case MADV_GUARD_INSTALL:
> -       case MADV_GUARD_REMOVE:
> -               return 0;
> -       default:
> -               /* be safe, default to 1. list exceptions explicitly */
> -               return 1;
> -       }
> -}
> -
>  #ifdef CONFIG_ANON_VMA_NAME
>  struct anon_vma_name *anon_vma_name_alloc(const char *name)
>  {
> @@ -1486,6 +1467,43 @@ static bool process_madvise_remote_valid(int behavior)
>         }
>  }
>
> +/*
> + * Try to acquire a VMA read lock if possible.
> + *
> + * We only support this lock over a single VMA, which the input range must
> + * span.either partially or fully.
> + *
> + * This function always returns with an appropriate lock held. If a VMA read
> + * lock could be acquired, we return the locked VMA.
> + *
> + * If a VMA read lock could not be acquired, we return NULL and expect caller to

Worth mentioning that the function itself will fall back to taking
mmap_read_lock in such a case.

> + * fallback to mmap lock behaviour.
> + */
> +static struct vm_area_struct *try_vma_read_lock(struct mm_struct *mm,
> +               struct madvise_behavior *madv_behavior,
> +               unsigned long start, unsigned long end)
> +{
> +       struct vm_area_struct *vma;
> +
> +       if (!madv_behavior || madv_behavior->lock_mode != MADVISE_VMA_READ_LOCK)

nit: I think it would be better to do this check before calling
try_vma_read_lock(). IMHO it does not make sense to call
try_vma_read_lock() when lock_mode != MADVISE_VMA_READ_LOCK. It also
makes reading this function easier. The first time I looked at it and
saw "return NULL" in one place that takes mmap_read_lock() and another
place which returns the same NULL but does not take mmap_lock really
confused me.

> +               return NULL;
> +
> +       vma = lock_vma_under_rcu(mm, start);
> +       if (!vma)
> +               goto take_mmap_read_lock;
> +       /* We must span only a single VMA, uffd unsupported. */
> +       if (end > vma->vm_end || userfaultfd_armed(vma)) {

vma->vm_end is not inclusive, so the above condition I think should be
(end >= vma->vm_end || ...)

> +               vma_end_read(vma);
> +               goto take_mmap_read_lock;
> +       }
> +       return vma;
> +
> +take_mmap_read_lock:
> +       mmap_read_lock(mm);
> +       madv_behavior->lock_mode = MADVISE_MMAP_READ_LOCK;
> +       return NULL;
> +}
> +
>  /*
>   * Walk the vmas in range [start,end), and call the visit function on each one.
>   * The visit function will get start and end parameters that cover the overlap
> @@ -1496,7 +1514,8 @@ static bool process_madvise_remote_valid(int behavior)
>   */
>  static
>  int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
> -                     unsigned long end, void *arg,
> +                     unsigned long end, struct madvise_behavior *madv_behavior,
> +                     void *arg,
>                       int (*visit)(struct vm_area_struct *vma,
>                                    struct vm_area_struct **prev, unsigned long start,
>                                    unsigned long end, void *arg))
> @@ -1505,6 +1524,15 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
>         struct vm_area_struct *prev;
>         unsigned long tmp;
>         int unmapped_error = 0;
> +       int error;
> +
> +       /* If VMA read lock supported, we apply advice to a single VMA only. */
> +       vma = try_vma_read_lock(mm, madv_behavior, start, end);
> +       if (vma) {
> +               error = visit(vma, &prev, start, end, arg);
> +               vma_end_read(vma);
> +               return error;
> +       }
>
>         /*
>          * If the interval [start,end) covers some unmapped address
> @@ -1516,8 +1544,6 @@ int madvise_walk_vmas(struct mm_struct *mm, unsigned long start,
>                 prev = vma;
>
>         for (;;) {
> -               int error;
> -
>                 /* Still start < end. */
>                 if (!vma)
>                         return -ENOMEM;
> @@ -1598,34 +1624,86 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
>         if (end == start)
>                 return 0;
>
> -       return madvise_walk_vmas(mm, start, end, anon_name,
> +       return madvise_walk_vmas(mm, start, end, anon_name, NULL,
>                                  madvise_vma_anon_name);
>  }
>  #endif /* CONFIG_ANON_VMA_NAME */
>
> -static int madvise_lock(struct mm_struct *mm, int behavior)
> +
> +/*
> + * Any behaviour which results in changes to the vma->vm_flags needs to
> + * take mmap_lock for writing. Others, which simply traverse vmas, need
> + * to only take it for reading.
> + */
> +static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavior)
>  {
> +       int behavior = madv_behavior->behavior;
> +
>         if (is_memory_failure(behavior))
> -               return 0;
> +               return MADVISE_NO_LOCK;
>
> -       if (madvise_need_mmap_write(behavior)) {
> +       switch (behavior) {
> +       case MADV_REMOVE:
> +       case MADV_WILLNEED:
> +       case MADV_COLD:
> +       case MADV_PAGEOUT:
> +       case MADV_FREE:
> +       case MADV_POPULATE_READ:
> +       case MADV_POPULATE_WRITE:
> +       case MADV_COLLAPSE:
> +       case MADV_GUARD_INSTALL:
> +       case MADV_GUARD_REMOVE:
> +               return MADVISE_MMAP_READ_LOCK;
> +       case MADV_DONTNEED:
> +       case MADV_DONTNEED_LOCKED:
> +               return MADVISE_VMA_READ_LOCK;
> +       default:
> +               return MADVISE_MMAP_WRITE_LOCK;
> +       }
> +}
> +
> +static int madvise_lock(struct mm_struct *mm,
> +               struct madvise_behavior *madv_behavior)
> +{
> +       enum madvise_lock_mode lock_mode = get_lock_mode(madv_behavior);
> +
> +       switch (lock_mode) {
> +       case MADVISE_NO_LOCK:
> +               break;
> +       case MADVISE_MMAP_WRITE_LOCK:
>                 if (mmap_write_lock_killable(mm))
>                         return -EINTR;
> -       } else {
> +               break;
> +       case MADVISE_MMAP_READ_LOCK:
>                 mmap_read_lock(mm);
> +               break;
> +       case MADVISE_VMA_READ_LOCK:
> +               /* We will acquire the lock per-VMA in madvise_walk_vmas(). */
> +               break;
>         }
> +
> +       madv_behavior->lock_mode = lock_mode;
>         return 0;
>  }
>
> -static void madvise_unlock(struct mm_struct *mm, int behavior)
> +static void madvise_unlock(struct mm_struct *mm,
> +               struct madvise_behavior *madv_behavior)
>  {
> -       if (is_memory_failure(behavior))
> +       switch (madv_behavior->lock_mode) {
> +       case  MADVISE_NO_LOCK:
>                 return;
> -
> -       if (madvise_need_mmap_write(behavior))
> +       case MADVISE_MMAP_WRITE_LOCK:
>                 mmap_write_unlock(mm);
> -       else
> +               break;
> +       case MADVISE_MMAP_READ_LOCK:
>                 mmap_read_unlock(mm);
> +               break;
> +       case MADVISE_VMA_READ_LOCK:
> +               /* We will drop the lock per-VMA in madvise_walk_vmas(). */
> +               break;
> +       }
> +
> +       madv_behavior->lock_mode = MADVISE_NO_LOCK;
>  }
>
>  static bool madvise_batch_tlb_flush(int behavior)
> @@ -1721,6 +1799,8 @@ static int madvise_do_behavior(struct mm_struct *mm,
>
>         if (is_memory_failure(behavior))
>                 return madvise_inject_error(behavior, start, start + len_in);
> +
> +       // TODO: handle untagged stuff here...
>         start = untagged_addr(start); //untagged_addr_remote(mm, start);
>         end = start + PAGE_ALIGN(len_in);
>
> @@ -1729,7 +1809,7 @@ static int madvise_do_behavior(struct mm_struct *mm,
>                 error = madvise_populate(mm, start, end, behavior);
>         else
>                 error = madvise_walk_vmas(mm, start, end, madv_behavior,
> -                                         madvise_vma_behavior);
> +                                         madv_behavior, madvise_vma_behavior);
>         blk_finish_plug(&plug);
>         return error;
>  }
> @@ -1817,13 +1897,13 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
>
>         if (madvise_should_skip(start, len_in, behavior, &error))
>                 return error;
> -       error = madvise_lock(mm, behavior);
> +       error = madvise_lock(mm, &madv_behavior);
>         if (error)
>                 return error;
>         madvise_init_tlb(&madv_behavior, mm);
>         error = madvise_do_behavior(mm, start, len_in, &madv_behavior);
>         madvise_finish_tlb(&madv_behavior);
> -       madvise_unlock(mm, behavior);
> +       madvise_unlock(mm, &madv_behavior);
>
>         return error;
>  }
> @@ -1847,7 +1927,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>
>         total_len = iov_iter_count(iter);
>
> -       ret = madvise_lock(mm, behavior);
> +       ret = madvise_lock(mm, &madv_behavior);
>         if (ret)
>                 return ret;
>         madvise_init_tlb(&madv_behavior, mm);
> @@ -1880,8 +1960,8 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>
>                         /* Drop and reacquire lock to unwind race. */
>                         madvise_finish_tlb(&madv_behavior);
> -                       madvise_unlock(mm, behavior);
> -                       ret = madvise_lock(mm, behavior);
> +                       madvise_unlock(mm, &madv_behavior);
> +                       ret = madvise_lock(mm, &madv_behavior);
>                         if (ret)
>                                 goto out;
>                         madvise_init_tlb(&madv_behavior, mm);
> @@ -1892,7 +1972,7 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
>                 iov_iter_advance(iter, iter_iov_len(iter));
>         }
>         madvise_finish_tlb(&madv_behavior);
> -       madvise_unlock(mm, behavior);
> +       madvise_unlock(mm, &madv_behavior);
>
>  out:
>         ret = (total_len - iov_iter_count(iter)) ? : ret;
> --
> 2.49.0