Re: [RFC PATCH] mm/madvise: remove redundant mmap_lock operations from process_madvise()

Honggyu Kim
2025-01-15T04:50:56Z

Hi SeongJae,

I have a simple comment on this.

On 1/11/2025 9:46 AM, SeongJae Park wrote:
> process_madvise() calls do_madvise() for each address range. Then, each
> do_madvise() invocation holds and releases same mmap_lock. Optimize the
> redundant lock operations by doing the locking in process_madvise(), and
> inform do_madvise() that the lock is already held and therefore can be
> skipped.
>
> Evaluation
> ==========
>
> I measured the time to apply MADV_DONTNEED advice to 256 MiB memory
> using multiple madvise() calls, 4 KiB per each call. I also do the same
> with process_madvise(), but with varying iovec size from 1 to 1024.
> The source code for the measurement is available at GitHub[1].
>
> The measurement results are as below. 'sz_batches' column shows the
> iovec size of process_madvise() calls. '0' is for madvise() calls case.
> 'before' and 'after' columns are the measured time to apply
> MADV_DONTNEED to the 256 MiB memory buffer in nanoseconds, on kernels
> that built without and with this patch, respectively. So lower value
> means better efficiency. 'after/before' column is the ratio of 'after'
> to 'before'.
>
> sz_batches before after after/before
> 0 124062365 96670188 0.779206393494111
> 1 136341258 113915688 0.835518827323714
> 2 105314942 78898211 0.749164453796119
> 4 82012858 59778998 0.728897875989153
> 8 82562651 51003069 0.617749895167489
> 16 71474930 47575960 0.665631431888076
> 32 71391211 42902076 0.600943385033768
> 64 68225932 41337835 0.605896230190011
> 128 71053578 42467240 0.597679120395598
> 256 85094126 41630463 0.489228398679364
> 512 68531628 44049763 0.6427654542221
> 1024 79338892 43370866 0.546653285755491
>
> The measurement shows this patch reduces the process_madvise() latency,
> proportional to the batching size, from about 25% with the batch size 2
> to about 55% with the batch size 1,024. The trend is somewhat we can
> expect.
>
> Interestingly, this patch has also optimize madvise() and single batch
> size process_madvise(), though. I ran this test multiple times, but the
> results are consistent. I'm still investigating if there are something
> I'm missing. But I believe the investigation may not necessarily be a
> blocker of this RFC, so just posting this. I will add updates of the
> madvise() and single batch size process_madvise() investigation later.
>
> [1] https://github.com/sjp38/eval_proc_madvise
>
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
> include/linux/mm.h | 3 ++-
> io_uring/advise.c | 2 +-
> mm/damon/vaddr.c | 2 +-
> mm/madvise.c | 54 +++++++++++++++++++++++++++++++++++-----------
> 4 files changed, 45 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 612b513ebfbd..e3ca5967ebd4 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3459,7 +3459,8 @@ int do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> unsigned long end, struct list_head *uf, bool unlock);
> extern int do_munmap(struct mm_struct *, unsigned long, size_t,
> struct list_head *uf);
> -extern int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int behavior);
> +extern int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in,
> + int behavior, bool lock_held);
>
> #ifdef CONFIG_MMU
> extern int __mm_populate(unsigned long addr, unsigned long len,
> diff --git a/io_uring/advise.c b/io_uring/advise.c
> index cb7b881665e5..010b55d5a26e 100644
> --- a/io_uring/advise.c
> +++ b/io_uring/advise.c
> @@ -56,7 +56,7 @@ int io_madvise(struct io_kiocb *req, unsigned int issue_flags)
>
> WARN_ON_ONCE(issue_flags & IO_URING_F_NONBLOCK);
>
> - ret = do_madvise(current->mm, ma->addr, ma->len, ma->advice);
> + ret = do_madvise(current->mm, ma->addr, ma->len, ma->advice, false);

I feel like this doesn't look good in terms of readability. Can we
introduce an enum for this?

> io_req_set_res(req, ret, 0);
> return IOU_OK;
> #else
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index a6174f725bd7..30b5a251d73e 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -646,7 +646,7 @@ static unsigned long damos_madvise(struct damon_target *target,
> if (!mm)
> return 0;
>
> - applied = do_madvise(mm, start, len, behavior) ? 0 : len;
> + applied = do_madvise(mm, start, len, behavior, false) ? 0 : len;
> mmput(mm);
>
> return applied;
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 49f3a75046f6..c107376db9d5 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1637,7 +1637,8 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
> * -EAGAIN - a kernel resource was temporarily unavailable.
> * -EPERM - memory is sealed.
> */
> -int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int behavior)
> +int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in,
> + int behavior, bool lock_held)
> {
> unsigned long end;
> int error;
> @@ -1668,12 +1669,14 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
> return madvise_inject_error(behavior, start, start + len_in);
> #endif
>
> - write = madvise_need_mmap_write(behavior);
> - if (write) {
> - if (mmap_write_lock_killable(mm))
> - return -EINTR;
> - } else {
> - mmap_read_lock(mm);
> + if (!lock_held) {
> + write = madvise_need_mmap_write(behavior);
> + if (write) {
> + if (mmap_write_lock_killable(mm))
> + return -EINTR;
> + } else {
> + mmap_read_lock(mm);
> + }
> }
>
> start = untagged_addr_remote(mm, start);
> @@ -1692,17 +1695,19 @@ int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int beh
> }
> blk_finish_plug(&plug);
>
> - if (write)
> - mmap_write_unlock(mm);
> - else
> - mmap_read_unlock(mm);
> + if (!lock_held) {
> + if (write)
> + mmap_write_unlock(mm);
> + else
> + mmap_read_unlock(mm);
> + }
>
> return error;
> }
>
> SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
> {
> - return do_madvise(current->mm, start, len_in, behavior);
> + return do_madvise(current->mm, start, len_in, behavior, false);
> }
>
> /* Perform an madvise operation over a vector of addresses and lengths. */
> @@ -1711,12 +1716,28 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> {
> ssize_t ret = 0;
> size_t total_len;
> + bool hold_lock = true;
> + int write;
>
> total_len = iov_iter_count(iter);
>
> +#ifdef CONFIG_MEMORY_FAILURE
> + if (behavior == MADV_HWPOISON || behavior == MADV_SOFT_OFFLINE)
> + hold_lock = false;
> +#endif
> + if (hold_lock) {
> + write = madvise_need_mmap_write(behavior);
> + if (write) {
> + if (mmap_write_lock_killable(mm))
> + return -EINTR;
> + } else {
> + mmap_read_lock(mm);
> + }
> + }
> +
> while (iov_iter_count(iter)) {
> ret = do_madvise(mm, (unsigned long)iter_iov_addr(iter),
> - iter_iov_len(iter), behavior);
> + iter_iov_len(iter), behavior, hold_lock);
> /*
> * An madvise operation is attempting to restart the syscall,
> * but we cannot proceed as it would not be correct to repeat
> @@ -1739,6 +1760,13 @@ static ssize_t vector_madvise(struct mm_struct *mm, struct iov_iter *iter,
> iov_iter_advance(iter, iter_iov_len(iter));
> }
>
> + if (hold_lock) {
> + if (write)
> + mmap_write_unlock(mm);
> + else
> + mmap_read_unlock(mm);
> + }
> +
> ret = (total_len - iov_iter_count(iter)) ? : ret;
>
> return ret;