Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap
From: Nhat Pham
Date: Tue Jun 11 2024 - 14:39:36 EST
On Mon, Jun 10, 2024 at 5:18 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
> Approximately 10-20% of pages to be swapped out are zero pages [1].
> Rather than reading/writing these pages to flash resulting
> in increased I/O and flash wear, a bitmap can be used to mark these
> pages as zero at write time, and the pages can be filled at
> read time if the bit corresponding to the page is set.
> With this patch, NVMe writes in Meta server fleet decreased
> by almost 10% with conventional swap setup (zswap disabled).
>
> [1]https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
>
> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> ---
> include/linux/swap.h | 1 +
> mm/page_io.c | 92 +++++++++++++++++++++++++++++++++++++++++++-
> mm/swapfile.c | 21 +++++++++-
> 3 files changed, 111 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a11c75e897ec..e88563978441 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -299,6 +299,7 @@ struct swap_info_struct {
> signed char type; /* strange name for an index */
> unsigned int max; /* extent of the swap_map */
> unsigned char *swap_map; /* vmalloc'ed array of usage counts */
> + unsigned long *zeromap; /* vmalloc'ed bitmap to track zero pages */
> struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
> struct swap_cluster_list free_clusters; /* free clusters list */
> unsigned int lowest_bit; /* index of first free in swap_map */
> diff --git a/mm/page_io.c b/mm/page_io.c
> index a360857cf75d..2cac1e11fb85 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -172,6 +172,82 @@ int generic_swapfile_activate(struct swap_info_struct *sis,
> goto out;
> }
>
> +static bool is_folio_page_zero_filled(struct folio *folio, int i)
> +{
> + unsigned long *data;
> + unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1;
> + bool ret = false;
> +
> + data = kmap_local_folio(folio, i * PAGE_SIZE);
> + if (data[last_pos])
> + goto out;
> + for (pos = 0; pos < PAGE_SIZE / sizeof(*data); pos++) {
> + if (data[pos])
> + goto out;
> + }
> + ret = true;
> +out:
> + kunmap_local(data);
> + return ret;
> +}
> +
> +static bool is_folio_zero_filled(struct folio *folio)
> +{
> + unsigned int i;
> +
> + for (i = 0; i < folio_nr_pages(folio); i++) {
> + if (!is_folio_page_zero_filled(folio, i))
> + return false;
> + }
> + return true;
> +}
> +
> +static void folio_zero_fill(struct folio *folio)
> +{
> + unsigned int i;
> +
> + for (i = 0; i < folio_nr_pages(folio); i++)
> + clear_highpage(folio_page(folio, i));
> +}
> +
> +static void swap_zeromap_folio_set(struct folio *folio)
> +{
> + struct swap_info_struct *sis = swp_swap_info(folio->swap);
> + swp_entry_t entry;
> + unsigned int i;
> +
> + for (i = 0; i < folio_nr_pages(folio); i++) {
> + entry = page_swap_entry(folio_page(folio, i));
> + set_bit(swp_offset(entry), sis->zeromap);
> + }
> +}
> +
> +static void swap_zeromap_folio_clear(struct folio *folio)
> +{
> + struct swap_info_struct *sis = swp_swap_info(folio->swap);
> + swp_entry_t entry;
> + unsigned int i;
> +
> + for (i = 0; i < folio_nr_pages(folio); i++) {
> + entry = page_swap_entry(folio_page(folio, i));
> + clear_bit(swp_offset(entry), sis->zeromap);
> + }
> +}
> +
> +static bool swap_zeromap_folio_test(struct folio *folio)
> +{
> + struct swap_info_struct *sis = swp_swap_info(folio->swap);
> + swp_entry_t entry;
> + unsigned int i;
> +
> + for (i = 0; i < folio_nr_pages(folio); i++) {
> + entry = page_swap_entry(folio_page(folio, i));
> + if (!test_bit(swp_offset(entry), sis->zeromap))
> + return false;
> + }
> + return true;
> +}
> +
> /*
> * We may have stale swap cache pages in memory: notice
> * them here and get rid of the unnecessary final write.
> @@ -195,6 +271,15 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> folio_unlock(folio);
> return ret;
> }
> +
> + if (is_folio_zero_filled(folio)) {
> + swap_zeromap_folio_set(folio);
> + folio_start_writeback(folio);
> + folio_unlock(folio);
> + folio_end_writeback(folio);
> + return 0;
> + }
> + swap_zeromap_folio_clear(folio);
> if (zswap_store(folio)) {
> folio_start_writeback(folio);
> folio_unlock(folio);
> @@ -515,8 +600,11 @@ void swap_read_folio(struct folio *folio, bool synchronous,
> psi_memstall_enter(&pflags);
> }
> delayacct_swapin_start();
> -
> - if (zswap_load(folio)) {
> + if (swap_zeromap_folio_test(folio)) {
> + folio_zero_fill(folio);
> + folio_mark_uptodate(folio);
> + folio_unlock(folio);
> + } else if (zswap_load(folio)) {
> folio_mark_uptodate(folio);
> folio_unlock(folio);
> } else if (data_race(sis->flags & SWP_FS_OPS)) {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index f1e559e216bd..90451174fe34 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -453,6 +453,8 @@ static unsigned int cluster_list_del_first(struct swap_cluster_list *list,
> static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> unsigned int idx)
> {
> + unsigned int i;
> +
> /*
> * If scan_swap_map_slots() can't find a free cluster, it will check
> * si->swap_map directly. To make sure the discarding cluster isn't
> @@ -461,6 +463,13 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> */
> memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> SWAP_MAP_BAD, SWAPFILE_CLUSTER);
> + /*
> + * zeromap can see updates from concurrent swap_writepage() and swap_read_folio()
> + * call on other slots, hence use atomic clear_bit for zeromap instead of the
> + * non-atomic bitmap_clear.
> + */
> + for (i = 0; i < SWAPFILE_CLUSTER; i++)
> + clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
>
> cluster_list_add_tail(&si->discard_clusters, si->cluster_info, idx);
>
> @@ -482,7 +491,7 @@ static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
> static void swap_do_scheduled_discard(struct swap_info_struct *si)
> {
> struct swap_cluster_info *info, *ci;
> - unsigned int idx;
> + unsigned int idx, i;
>
> info = si->cluster_info;
>
> @@ -498,6 +507,8 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
> __free_cluster(si, idx);
> memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> 0, SWAPFILE_CLUSTER);
> + for (i = 0; i < SWAPFILE_CLUSTER; i++)
> + clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
> unlock_cluster(ci);
> }
> }
> @@ -1336,6 +1347,7 @@ static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
> count = p->swap_map[offset];
> VM_BUG_ON(count != SWAP_HAS_CACHE);
> p->swap_map[offset] = 0;
> + clear_bit(offset, p->zeromap);
Hmm so clear_bit() is done at the swap_entry_free() point. I wonder if
we can have a problem, where:
1. The swap entry has its zeromap bit set, and is freed to the swap
slot cache (free_swap_slot() in mm/swap_slots.c). For instance, it is
reclaimed from the swap cache, and all the processes referring to it
are terminated, which decrements the swap count to 0 (swap_free() ->
__swap_entry_free() -> free_swap_slots())
2. The swap slot is then re-used in swap space allocation
(add_to_swap()) - its zeromap bit is never cleared.
3. swap_writepage() writes that non-zero page to swap
4. swap_read_folio() checks the bitmap, sees that the zeromap bit for
the entry is set, so populates a zero page for it.
zswap in the past has to carefully invalidate these leftover entries
quite carefully. Chengming then move the invalidation point to
free_swap_slot(), massively simplifying the logic.
I wonder if we need to do the same here?