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Message-ID: <CAGsJ_4zNd5oCG1vpWRJxOQgPRvyO3AbjGM5nt9SxGjm=YTcrdg@mail.gmail.com>
Date: Tue, 6 Aug 2024 14:07:42 +1200
From: Barry Song <21cnbao@...il.com>
To: zhiguojiang <justinjiang@...o.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Chris Li <chrisl@...nel.org>,
opensource.kernel@...o.com
Subject: Re: [PATCH] mm: swap: mTHP frees entries as a whole
On Tue, Aug 6, 2024 at 2:01 PM zhiguojiang <justinjiang@...o.com> wrote:
>
>
>
> 在 2024/8/6 6:09, Barry Song 写道:
> > On Tue, Aug 6, 2024 at 4:08 AM Zhiguo Jiang <justinjiang@...o.com> wrote:
> >> Support mTHP's attempt to free swap entries as a whole, which can avoid
> >> frequent swap_info locking for every individual entry in
> >> swapcache_free_entries(). When the swap_map count values corresponding
> >> to all contiguous entries are all zero excluding SWAP_HAS_CACHE, the
> >> entries will be freed directly by skippping percpu swp_slots caches.
> >>
> > No, this isn't quite good. Please review the work done by Chris and Kairui[1];
> > they have handled it better. On a different note, I have a patch that can
> > handle zap_pte_range() for swap entries in batches[2][3].
> I'm glad to see your optimized submission about batch freeing swap
> entries for
> zap_pte_range(), sorry, I didn't see it before. My this patch can be
> ignored.
no worries, please help test and review the formal patch I sent:
https://lore.kernel.org/linux-mm/20240806012409.61962-1-21cnbao@gmail.com/
Please note that I didn't use a bitmap to avoid a large stack, and
there is a real possibility of the below can occur, your patch can
crash if the below is true:
nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER
Additionally, I quickly skip the case where
swap_count(data_race(si->swap_map[start_offset]) != 1) to avoid regressions
in cases that can't be batched.
>
> Thanks
> Zhiguo
>
> >
> > [1] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-5-cb9c148b9297@kernel.org/
> > [2] https://lore.kernel.org/linux-mm/20240803091118.84274-1-21cnbao@gmail.com/
> > [3] https://lore.kernel.org/linux-mm/CAGsJ_4wPnQqKOHx6iQcwO8bQzoBXKr2qY2AgSxMwTQCj3-8YWw@mail.gmail.com/
> >
> >> Signed-off-by: Zhiguo Jiang <justinjiang@...o.com>
> >> ---
> >> mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >> 1 file changed, 61 insertions(+)
> >>
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index ea023fc25d08..829fb4cfb6ec
> >> --- a/mm/swapfile.c
> >> +++ b/mm/swapfile.c
> >> @@ -1493,6 +1493,58 @@ static void swap_entry_range_free(struct swap_info_struct *p, swp_entry_t entry,
> >> swap_range_free(p, offset, nr_pages);
> >> }
> >>
> >> +/*
> >> + * Free the contiguous swap entries as a whole, caller have to
> >> + * ensure all entries belong to the same folio.
> >> + */
> >> +static void swap_entry_range_check_and_free(struct swap_info_struct *p,
> >> + swp_entry_t entry, int nr, bool *any_only_cache)
> >> +{
> >> + const unsigned long start_offset = swp_offset(entry);
> >> + const unsigned long end_offset = start_offset + nr;
> >> + unsigned long offset;
> >> + DECLARE_BITMAP(to_free, SWAPFILE_CLUSTER) = { 0 };
> >> + struct swap_cluster_info *ci;
> >> + int i = 0, nr_setbits = 0;
> >> + unsigned char count;
> >> +
> >> + /*
> >> + * Free and check swap_map count values corresponding to all contiguous
> >> + * entries in the whole folio range.
> >> + */
> >> + WARN_ON_ONCE(nr > SWAPFILE_CLUSTER);
> >> + ci = lock_cluster_or_swap_info(p, start_offset);
> >> + for (offset = start_offset; offset < end_offset; offset++, i++) {
> >> + if (data_race(p->swap_map[offset])) {
> >> + count = __swap_entry_free_locked(p, offset, 1);
> >> + if (!count) {
> >> + bitmap_set(to_free, i, 1);
> >> + nr_setbits++;
> >> + } else if (count == SWAP_HAS_CACHE) {
> >> + *any_only_cache = true;
> >> + }
> >> + } else {
> >> + WARN_ON_ONCE(1);
> >> + }
> >> + }
> >> + unlock_cluster_or_swap_info(p, ci);
> >> +
> >> + /*
> >> + * If the swap_map count values corresponding to all contiguous entries are
> >> + * all zero excluding SWAP_HAS_CACHE, the entries will be freed directly by
> >> + * skippping percpu swp_slots caches, which can avoid frequent swap_info
> >> + * locking for every individual entry.
> >> + */
> >> + if (nr > 1 && nr_setbits == nr) {
> >> + spin_lock(&p->lock);
> >> + swap_entry_range_free(p, entry, nr);
> >> + spin_unlock(&p->lock);
> >> + } else {
> >> + for_each_set_bit(i, to_free, SWAPFILE_CLUSTER)
> >> + free_swap_slot(swp_entry(p->type, start_offset + i));
> >> + }
> >> +}
> >> +
> >> static void cluster_swap_free_nr(struct swap_info_struct *sis,
> >> unsigned long offset, int nr_pages,
> >> unsigned char usage)
> >> @@ -1808,6 +1860,14 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> >> if (WARN_ON(end_offset > si->max))
> >> goto out;
> >>
> >> + /*
> >> + * Try to free all contiguous entries about mTHP as a whole.
> >> + */
> >> + if (IS_ENABLED(CONFIG_THP_SWAP) && nr > 1) {
> >> + swap_entry_range_check_and_free(si, entry, nr, &any_only_cache);
> >> + goto free_cache;
> >> + }
> >> +
> >> /*
> >> * First free all entries in the range.
> >> */
> >> @@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> >> }
> >> }
> >>
> >> +free_cache:
> >> /*
> >> * Short-circuit the below loop if none of the entries had their
> >> * reference drop to zero.
> >> --
> >> 2.39.0
> >>
Thanks
Barry
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