On Wed, Aug 7, 2024 at 9:29 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > mm/swapfile.c | 78 +++++++++++++++++++++++++++++++++++++++++++--------
> > 1 file changed, 67 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 35cb58373493..25c3f98fa8d5 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si,
> > return true;
> > }
> >
> > +static bool swap_is_last_map(struct swap_info_struct *si,
> > + unsigned long offset, int nr_pages,
> > + bool *has_cache)
>
> Please use double tabs for indenting parameters on 2nd line on
> new/changed code:
>
> unsigned long offset, int nr_pages, bool *has_cache)
>
> Results in less churn when renaming functions and we can frequently
> avoid some lines.
ack.
>
> > +{
> > + unsigned char *map = si->swap_map + offset;
> > + unsigned char *map_end = map + nr_pages;
> > + bool cached = false;
> > +
> > + do {
> > + if ((*map & ~SWAP_HAS_CACHE) != 1)
> > + return false;
> > + if (*map & SWAP_HAS_CACHE)
> > + cached = true;
> > + } while (++map < map_end);
> > +
> > + *has_cache = cached;
> > + return true;
> > +}
> > +
> > /*
> > * returns number of pages in the folio that backs the swap entry. If positive,
> > * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
> > @@ -1469,6 +1488,53 @@ static unsigned char __swap_entry_free(struct swap_info_struct *si,
> > return usage;
> > }
> >
> > +static bool __swap_entries_free(struct swap_info_struct *si,
> > + swp_entry_t entry, int nr)
>
> Dito.
ack.
>
> > +{
> > + unsigned long offset = swp_offset(entry);
> > + unsigned int type = swp_type(entry);
> > + struct swap_cluster_info *ci;
> > + bool has_cache = false;
> > + unsigned char count;
> > + bool can_batch;
> > + int i;
> > +
> > + if (nr <= 1 || swap_count(data_race(si->swap_map[offset])) != 1)
> > + goto fallback;
> > + /* cross into another cluster */
> > + if (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)
> > + goto fallback;
> > +
> > + ci = lock_cluster_or_swap_info(si, offset);
> > + can_batch = swap_is_last_map(si, offset, nr, &has_cache);
> > + if (can_batch) {
> > + for (i = 0; i < nr; i++)
> > + WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
> > + }
> > + unlock_cluster_or_swap_info(si, ci);
> > +
> > + if (!can_batch)
> > + goto fallback;
>
> I'd avoid "can_batch" and just do:
>
> ci = lock_cluster_or_swap_info(si, offset);
> if (!swap_is_last_map(si, offset, nr, &has_cache)) {
> unlock_cluster_or_swap_info(si, ci);
> goto fallback;
> }
> for (i = 0; i < nr; i++)
> WRITE_ONCE(si->swap_map[offset + i], SWAP_HAS_CACHE);
> unlock_cluster_or_swap_info(si, ci);
ack.
>
> > + if (!has_cache) {
> > + spin_lock(&si->lock);
>
> I'm no expert on that code, but we might drop the cluster lock the take
> the swap_info lock and then retake the cluster lock. I assume there are
> no races we are worrying about here, right?
I suppose so. Even the original single-entry code follows the same pattern:
static unsigned char __swap_entry_free(struct swap_info_struct *p,
swp_entry_t entry)
{
struct swap_cluster_info *ci;
unsigned long offset = swp_offset(entry);
unsigned char usage;
ci = lock_cluster_or_swap_info(p, offset);
usage = __swap_entry_free_locked(p, offset, 1);
unlock_cluster_or_swap_info(p, ci);
if (!usage)
free_swap_slot(entry);
return usage;
}
I assume that once we mark them as SWAP_HAS_CACHE, no one else
will touch them except ourselves.
>
> > + swap_entry_range_free(si, entry, nr);
> > + spin_unlock(&si->lock);
> > + }
> > + return has_cache;
> > +
> > +fallback:
> > + for (i = 0; i < nr; i++) {
>
> One space too much before the "<".
ack.
>
> > + if (data_race(si->swap_map[offset + i])) {
> > + count = __swap_entry_free(si, swp_entry(type, offset + i));
> > + if (count == SWAP_HAS_CACHE)
> > + has_cache = true;
> > + } else {
> > + WARN_ON_ONCE(1);
> > + }
> > + }
> > + return has_cache;
> > +}
> > +
>
> --
> Cheers,
>
> David / dhildenb
>
Thanks
Barry
