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

Liam R. Howlett
2025-01-15T03:45:26Z

* Shakeel Butt <shakeel.butt@linux.dev> [250114 13:14]:
> Ccing relevant folks.

Thanks Shakeel.

>
> On Fri, Jan 10, 2025 at 04:46:18PM -0800, 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);

We are dropping externs when it is not needed as things are changed.

Also, please don't use a flags for this. It will have a single user of
true, probably ever.

It might be better to break do_madvise up into more parts:
1. is_madvise_valid(), which does the checking.
2. madivse_do_behavior()

The locking type is already extracted to madivse_need_mmap_write().

What do you think?

> >
> > #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);
> > 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;
> > --
> > 2.39.5
>