On 04/03/2024 08:13, Barry Song wrote:
> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>
> While swapping in a large folio, we need to free swaps related to the whole
> folio. To avoid frequently acquiring and releasing swap locks, it is better
> to introduce an API for batched free.
>
> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
> include/linux/swap.h | 6 ++++++
> mm/swapfile.c | 35 +++++++++++++++++++++++++++++++++++
> 2 files changed, 41 insertions(+)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2955f7a78d8d..d6ab27929458 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -481,6 +481,7 @@ extern void swap_shmem_alloc(swp_entry_t);
> extern int swap_duplicate(swp_entry_t);
> extern int swapcache_prepare(swp_entry_t);
> extern void swap_free(swp_entry_t);
> +extern void swap_nr_free(swp_entry_t entry, int nr_pages);
nit: In my swap-out v4 series, I've created a batched version of
free_swap_and_cache() and called it free_swap_and_cache_nr(). Perhaps it is
preferable to align the naming schemes - i.e. call this swap_free_nr(). Your
scheme doesn't really work when applied to free_swap_and_cache().
> extern void swapcache_free_entries(swp_entry_t *entries, int n);
> extern int free_swap_and_cache(swp_entry_t);
> int swap_type_of(dev_t device, sector_t offset);
> @@ -561,6 +562,11 @@ static inline void swap_free(swp_entry_t swp)
> {
> }
>
> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> +{
> +
> +}
> +
> static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
> {
> }
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 3f594be83b58..244106998a69 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1341,6 +1341,41 @@ void swap_free(swp_entry_t entry)
> __swap_entry_free(p, entry);
> }
>
> +/*
> + * Called after swapping in a large folio, batched free swap entries
> + * for this large folio, entry should be for the first subpage and
> + * its offset is aligned with nr_pages
> + */
> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> +{
> + int i;
> + struct swap_cluster_info *ci;
> + struct swap_info_struct *p;
> + unsigned type = swp_type(entry);
nit: checkpatch.py will complain about bare "unsigned", preferring "unsigned
int" or at least it did for me when I did something similar in my swap-out patch
set.
> + unsigned long offset = swp_offset(entry);
> + DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
I don't love this, as it could blow the stack if SWAPFILE_CLUSTER ever
increases. But the only other way I can think of is to explicitly loop over
fixed size chunks, and that's not much better.
> +
> + /* all swap entries are within a cluster for mTHP */
> + VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
> +
> + if (nr_pages == 1) {
> + swap_free(entry);
> + return;
> + }
> +
> + p = _swap_info_get(entry);
You need to handle this returning NULL, like swap_free() does.
> +
> + ci = lock_cluster(p, offset);
The existing swap_free() calls lock_cluster_or_swap_info(). So if swap is backed
by rotating media, and clusters are not in use, it will lock the whole swap
info. But your new version only calls lock_cluster() which won't lock anything
if clusters are not in use. So I think this is a locking bug.
> + for (i = 0; i < nr_pages; i++) {
> + if (__swap_entry_free_locked(p, offset + i, 1))
> + __bitmap_set(usage, i, 1);
> + }
> + unlock_cluster(ci);
> +
> + for_each_clear_bit(i, usage, nr_pages)
> + free_swap_slot(swp_entry(type, offset + i));
> +}
> +
> /*
> * Called after dropping swapcache to decrease refcnt to swap entries.
> */
Thanks,
Ryan
