Re: [PATCH v6 2/3] mm: zswap: zswap_store() extended to handle mTHP folios.

Barry Song
2024-09-16T05:55:25Z

On Fri, Aug 30, 2024 at 5:27 AM Kanchana P Sridhar
<kanchana.p.sridhar@intel.com> wrote:
>
> zswap_store() will now process and store mTHP and PMD-size THP folios.
>
> A new config variable CONFIG_ZSWAP_STORE_THP_DEFAULT_ON (off by default)
> will enable/disable zswap storing of (m)THP.
>
> This change reuses and adapts the functionality in Ryan Roberts' RFC
> patch [1]:
>
> "[RFC,v1] mm: zswap: Store large folios without splitting"
>
> [1] https://lore.kernel.org/linux-mm/20231019110543.3284654-1-ryan.roberts@arm.com/T/#u
>
> This patch provides a sequential implementation of storing an mTHP in
> zswap_store() by iterating through each page in the folio to compress
> and store it in the zswap zpool.
>
> Towards this goal, zswap_compress() is modified to take a page instead
> of a folio as input.
>
> Each page's swap offset is stored as a separate zswap entry.
>
> If an error is encountered during the store of any page in the mTHP,
> all previous pages/entries stored will be invalidated. Thus, an mTHP
> is either entirely stored in ZSWAP, or entirely not stored in ZSWAP.
>
> This forms the basis for building batching of pages during zswap store
> of large folios, by compressing batches of up to say, 8 pages in an
> mTHP in parallel in hardware, with the Intel In-Memory Analytics
> Accelerator (Intel IAA).

Hi Kanchana,
I'm not opposed to this patch, but I don't understand how iterating
through each page within an mTHP supports the use of Intel IAA,
as it involves compressing pages individually.

In the document 'by_n compression and decompression with Intel IAA' by
Andre Glover
(https://lore.kernel.org/all/cover.1714581792.git.andre.glover@linux.intel.com),
it appears
that zsmalloc/zram needs to support multi-page compression and
decompression to fully
leverage the hardware's capabilities. Could you clarify how this
approach fits in?

In patch2/3 of that series, you have:
"Add the 'by_n' attribute to the acomp_req. The 'by_n' attribute can be
used a directive by acomp crypto algorithms for splitting compress and
decompress operations into "n" separate jobs."

How can you apply 'by_n' to a single page rather than to a large folio?

>
> Also, addressed some of the RFC comments from the discussion in [1].
>
> Made a minor edit in the comments for "struct zswap_entry" to delete
> the comments related to "value" since same-filled page handling has
> been removed from zswap.
>
> Co-developed-by: Ryan Roberts
> Signed-off-by:
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> ---
> mm/Kconfig | 8 ++
> mm/zswap.c | 243 +++++++++++++++++++++++++++++++++++++++--------------
> 2 files changed, 190 insertions(+), 61 deletions(-)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index b23913d4e47e..68c7b01120bd 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -59,6 +59,14 @@ config ZSWAP_SHRINKER_DEFAULT_ON
> reducing the chance that cold pages will reside in the zswap pool
> and consume memory indefinitely.
>
> +config ZSWAP_STORE_THP_DEFAULT_ON
> + bool "Store mTHP and THP folios in zswap"
> + depends on ZSWAP
> + default n
> + help
> + If selected, zswap will process mTHP and THP folios by
> + compressing and storing each 4K page in the large folio.
> +
> choice
> prompt "Default compressor"
> depends on ZSWAP
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 449914ea9919..3abf9810f0b7 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -127,6 +127,14 @@ static bool zswap_shrinker_enabled = IS_ENABLED(
> CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
> module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
>
> +/*
> + * Enable/disable zswap processing of mTHP folios.
> + * For now, only zswap_store will process mTHP folios.
> + */
> +static bool zswap_mthp_enabled = IS_ENABLED(
> + CONFIG_ZSWAP_STORE_THP_DEFAULT_ON);
> +module_param_named(mthp_enabled, zswap_mthp_enabled, bool, 0644);
> +
> bool zswap_is_enabled(void)
> {
> return zswap_enabled;
> @@ -190,7 +198,6 @@ static struct shrinker *zswap_shrinker;
> * section for context.
> * pool - the zswap_pool the entry's data is in
> * handle - zpool allocation handle that stores the compressed page data
> - * value - value of the same-value filled pages which have same content
> * objcg - the obj_cgroup that the compressed memory is charged to
> * lru - handle to the pool's lru used to evict pages.
> */
> @@ -876,7 +883,7 @@ static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
> return 0;
> }
>
> -static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
> +static bool zswap_compress(struct page *page, struct zswap_entry *entry)
> {
> struct crypto_acomp_ctx *acomp_ctx;
> struct scatterlist input, output;
> @@ -894,7 +901,7 @@ static bool zswap_compress(struct folio *folio, struct zswap_entry *entry)
>
> dst = acomp_ctx->buffer;
> sg_init_table(&input, 1);
> - sg_set_folio(&input, folio, PAGE_SIZE, 0);
> + sg_set_page(&input, page, PAGE_SIZE, 0);
>
> /*
> * We need PAGE_SIZE * 2 here since there maybe over-compression case,
> @@ -1404,35 +1411,82 @@ static void shrink_worker(struct work_struct *w)
> /*********************************
> * main API
> **********************************/
> -bool zswap_store(struct folio *folio)
> +
> +/*
> + * Returns true if the entry was successfully
> + * stored in the xarray, and false otherwise.
> + */
> +static bool zswap_store_entry(struct xarray *tree,
> + struct zswap_entry *entry)
> {
> - swp_entry_t swp = folio->swap;
> - pgoff_t offset = swp_offset(swp);
> - struct xarray *tree = swap_zswap_tree(swp);
> - struct zswap_entry *entry, *old;
> - struct obj_cgroup *objcg = NULL;
> - struct mem_cgroup *memcg = NULL;
> + struct zswap_entry *old;
> + pgoff_t offset = swp_offset(entry->swpentry);
>
> - VM_WARN_ON_ONCE(!folio_test_locked(folio));
> - VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> + old = xa_store(tree, offset, entry, GFP_KERNEL);
>
> - /* Large folios aren't supported */
> - if (folio_test_large(folio))
> + if (xa_is_err(old)) {
> + int err = xa_err(old);
> +
> + WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n", err);
> + zswap_reject_alloc_fail++;
> return false;
> + }
>
> - if (!zswap_enabled)
> - goto check_old;
> + /*
> + * We may have had an existing entry that became stale when
> + * the folio was redirtied and now the new version is being
> + * swapped out. Get rid of the old.
> + */
> + if (old)
> + zswap_entry_free(old);
>
> - /* Check cgroup limits */
> - objcg = get_obj_cgroup_from_folio(folio);
> - if (objcg && !obj_cgroup_may_zswap(objcg)) {
> - memcg = get_mem_cgroup_from_objcg(objcg);
> - if (shrink_memcg(memcg)) {
> - mem_cgroup_put(memcg);
> - goto reject;
> - }
> - mem_cgroup_put(memcg);
> + return true;
> +}
> +
> +/*
> + * If the zswap store fails or zswap is disabled, we must invalidate the
> + * possibly stale entries which were previously stored at the offsets
> + * corresponding to each page of the folio. Otherwise, writeback could
> + * overwrite the new data in the swapfile.
> + *
> + * This is called after the store of the i-th offset in a large folio has
> + * failed. All zswap entries in the folio must be deleted. This helps make
> + * sure that a swapped-out mTHP is either entirely stored in zswap, or
> + * entirely not stored in zswap.
> + *
> + * This is also called if zswap_store() is invoked, but zswap is not enabled.
> + * All offsets for the folio are deleted from zswap in this case.
> + */
> +static void zswap_delete_stored_offsets(struct xarray *tree,
> + pgoff_t offset,
> + long nr_pages)
> +{
> + struct zswap_entry *entry;
> + long i;
> +
> + for (i = 0; i < nr_pages; ++i) {
> + entry = xa_erase(tree, offset + i);
> + if (entry)
> + zswap_entry_free(entry);
> }
> +}
> +
> +/*
> + * Stores the page at specified "index" in a folio.
> + */
> +static bool zswap_store_page(struct folio *folio, long index,
> + struct obj_cgroup *objcg,
> + struct zswap_pool *pool)
> +{
> + swp_entry_t swp = folio->swap;
> + int type = swp_type(swp);
> + pgoff_t offset = swp_offset(swp) + index;
> + struct page *page = folio_page(folio, index);
> + struct xarray *tree = swap_zswap_tree(swp);
> + struct zswap_entry *entry;
> +
> + if (objcg)
> + obj_cgroup_get(objcg);
>
> if (zswap_check_limits())
> goto reject;
> @@ -1445,42 +1499,20 @@ bool zswap_store(struct folio *folio)
> }
>
> /* if entry is successfully added, it keeps the reference */
> - entry->pool = zswap_pool_current_get();
> - if (!entry->pool)
> + if (!zswap_pool_get(pool))
> goto freepage;
>
> - if (objcg) {
> - memcg = get_mem_cgroup_from_objcg(objcg);
> - if (memcg_list_lru_alloc(memcg, &zswap_list_lru, GFP_KERNEL)) {
> - mem_cgroup_put(memcg);
> - goto put_pool;
> - }
> - mem_cgroup_put(memcg);
> - }
> + entry->pool = pool;
>
> - if (!zswap_compress(folio, entry))
> + if (!zswap_compress(page, entry))
> goto put_pool;
>
> - entry->swpentry = swp;
> + entry->swpentry = swp_entry(type, offset);
> entry->objcg = objcg;
> entry->referenced = true;
>
> - old = xa_store(tree, offset, entry, GFP_KERNEL);
> - if (xa_is_err(old)) {
> - int err = xa_err(old);
> -
> - WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n", err);
> - zswap_reject_alloc_fail++;
> + if (!zswap_store_entry(tree, entry))
> goto store_failed;
> - }
> -
> - /*
> - * We may have had an existing entry that became stale when
> - * the folio was redirtied and now the new version is being
> - * swapped out. Get rid of the old.
> - */
> - if (old)
> - zswap_entry_free(old);
>
> if (objcg) {
> obj_cgroup_charge_zswap(objcg, entry->length);
> @@ -1511,23 +1543,112 @@ bool zswap_store(struct folio *folio)
> store_failed:
> zpool_free(entry->pool->zpool, entry->handle);
> put_pool:
> - zswap_pool_put(entry->pool);
> + zswap_pool_put(pool);
> freepage:
> zswap_entry_cache_free(entry);
> reject:
> obj_cgroup_put(objcg);
> if (zswap_pool_reached_full)
> queue_work(shrink_wq, &zswap_shrink_work);
> -check_old:
> +
> + return false;
> +}
> +
> +/*
> + * Modified to store mTHP folios. Each page in the mTHP will be compressed
> + * and stored sequentially.
> + */
> +bool zswap_store(struct folio *folio)
> +{
> + long nr_pages = folio_nr_pages(folio);
> + swp_entry_t swp = folio->swap;
> + pgoff_t offset = swp_offset(swp);
> + struct xarray *tree = swap_zswap_tree(swp);
> + struct obj_cgroup *objcg = NULL;
> + struct mem_cgroup *memcg = NULL;
> + struct zswap_pool *pool;
> + bool ret = false;
> + long index;
> +
> + VM_WARN_ON_ONCE(!folio_test_locked(folio));
> + VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> +
> + /* Storing large folios isn't enabled */
> + if (!zswap_mthp_enabled && folio_test_large(folio))
> + return false;
> +
> + if (!zswap_enabled)
> + goto reject;
> +
> /*
> - * If the zswap store fails or zswap is disabled, we must invalidate the
> - * possibly stale entry which was previously stored at this offset.
> - * Otherwise, writeback could overwrite the new data in the swapfile.
> + * Check cgroup limits:
> + *
> + * The cgroup zswap limit check is done once at the beginning of an
> + * mTHP store, and not within zswap_store_page() for each page
> + * in the mTHP. We do however check the zswap pool limits at the
> + * start of zswap_store_page(). What this means is, the cgroup
> + * could go over the limits by at most (HPAGE_PMD_NR - 1) pages.
> + * However, the per-store-page zswap pool limits check should
> + * hopefully trigger the cgroup aware and zswap LRU aware global
> + * reclaim implemented in the shrinker. If this assumption holds,
> + * the cgroup exceeding the zswap limits could potentially be
> + * resolved before the next zswap_store, and if it is not, the next
> + * zswap_store would fail the cgroup zswap limit check at the start.
> */
> - entry = xa_erase(tree, offset);
> - if (entry)
> - zswap_entry_free(entry);
> - return false;
> + objcg = get_obj_cgroup_from_folio(folio);
> + if (objcg && !obj_cgroup_may_zswap(objcg)) {
> + memcg = get_mem_cgroup_from_objcg(objcg);
> + if (shrink_memcg(memcg)) {
> + mem_cgroup_put(memcg);
> + goto put_objcg;
> + }
> + mem_cgroup_put(memcg);
> + }
> +
> + if (zswap_check_limits())
> + goto put_objcg;
> +
> + pool = zswap_pool_current_get();
> + if (!pool)
> + goto put_objcg;
> +
> + if (objcg) {
> + memcg = get_mem_cgroup_from_objcg(objcg);
> + if (memcg_list_lru_alloc(memcg, &zswap_list_lru, GFP_KERNEL)) {
> + mem_cgroup_put(memcg);
> + goto put_pool;
> + }
> + mem_cgroup_put(memcg);
> + }
> +
> + /*
> + * Store each page of the folio as a separate entry. If we fail to store
> + * a page, unwind by removing all the previous pages we stored.
> + */
> + for (index = 0; index < nr_pages; ++index) {
> + if (!zswap_store_page(folio, index, objcg, pool))
> + goto put_pool;
> + }
> +
> + ret = true;
> +
> +put_pool:
> + zswap_pool_put(pool);
> +put_objcg:
> + obj_cgroup_put(objcg);
> + if (zswap_pool_reached_full)
> + queue_work(shrink_wq, &zswap_shrink_work);
> +reject:
> + /*
> + * If the zswap store fails or zswap is disabled, we must invalidate
> + * the possibly stale entries which were previously stored at the
> + * offsets corresponding to each page of the folio. Otherwise,
> + * writeback could overwrite the new data in the swapfile.
> + */
> + if (!ret)
> + zswap_delete_stored_offsets(tree, offset, nr_pages);
> +
> + return ret;
> }
>
> bool zswap_load(struct folio *folio)
> --
> 2.27.0
>

Thanks
Barry