Message-ID: <CAJD7tkZh6ufHQef5HjXf_F5b5LC1EATexgseD=4WvrO+a6Ni6w@mail.gmail.com>
Date: Mon, 30 Sep 2024 16:09:12 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	nphamcs@...il.com, chengming.zhou@...ux.dev, usamaarif642@...il.com, 
	shakeel.butt@...ux.dev, ryan.roberts@....com, ying.huang@...el.com, 
	21cnbao@...il.com, akpm@...ux-foundation.org, willy@...radead.org, 
	nanhai.zou@...el.com, wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v9 6/7] mm: zswap: Support large folios in zswap_store().

On Mon, Sep 30, 2024 at 3:12 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> zswap_store() will store large folios by compressing them page by page.
>
> This patch provides a sequential implementation of storing a large folio
> in zswap_store() by iterating through each page in the folio to compress
> and store it in the zswap zpool.
>
> zswap_store() calls the newly added zswap_store_page() function for each
> page in the folio. zswap_store_page() handles compressing and storing each
> page.
>
> We check the global and per-cgroup limits once at the beginning of
> zswap_store(), and only check that the limit is not reached yet. This is
> racy and inaccurate, but it should be sufficient for now. We also obtain
> initial references to the relevant objcg and pool to guarantee that
> subsequent references can be acquired by zswap_store_page(). A new function
> zswap_pool_get() is added to facilitate this.
>
> If these one-time checks pass, we compress the pages of the folio, while
> maintaining a running count of compressed bytes for all the folio's pages.
> If all pages are successfully compressed and stored, we do the cgroup
> zswap charging with the total compressed bytes, and batch update the
> zswap_stored_pages atomic/zswpout event stats with folio_nr_pages() once,
> before returning from zswap_store().
>
> If an error is encountered during the store of any page in the folio,
> all pages in that folio currently stored in zswap will be invalidated.
> Thus, a folio is either entirely stored in zswap, or entirely not stored
> in zswap.
>
> The most important value provided by this patch is it enables swapping out
> large folios to zswap without splitting them. Furthermore, it batches some
> operations while doing so (cgroup charging, stats updates).
>
> This patch also forms the basis for building compress batching of pages in
> a large folio in zswap_store() by compressing up to say, 8 pages of the
> folio in parallel in hardware using the Intel In-Memory Analytics
> Accelerator (Intel IAA).
>
> 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
>
> Also, addressed some of the RFC comments from the discussion in [1].
>
> Co-developed-by: Ryan Roberts
> Signed-off-by:

We haven't been able to get a signoff from Ryan. Andrew, what's the policy here?

> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  mm/zswap.c | 220 +++++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 153 insertions(+), 67 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 2b8da50f6322..b74c8de99646 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -411,6 +411,12 @@ static int __must_check zswap_pool_tryget(struct zswap_pool *pool)
>         return percpu_ref_tryget(&pool->ref);
>  }
>
> +/* The caller must already have a reference. */
> +static void zswap_pool_get(struct zswap_pool *pool)
> +{
> +       percpu_ref_get(&pool->ref);
> +}
> +
>  static void zswap_pool_put(struct zswap_pool *pool)
>  {
>         percpu_ref_put(&pool->ref);
> @@ -1402,68 +1408,52 @@ static void shrink_worker(struct work_struct *w)
>  /*********************************
>  * main API
>  **********************************/
> -bool zswap_store(struct folio *folio)
> +
> +/*
> + * Stores the page at specified "index" in a folio.
> + *
> + * @page:  The page to store in zswap.
> + * @objcg: The folio's objcg. Caller has a reference.
> + * @pool:  The zswap_pool to store the compressed data for the page.
> + *         The caller should have obtained a reference to a valid
> + *         zswap_pool by calling zswap_pool_tryget(), to pass as this
> + *         argument.
> + * @tree:  The xarray for the @page's folio's swap.
> + * @compressed_bytes: The compressed entry->length value is added
> + *                    to this, so that the caller can get the total
> + *                    compressed lengths of all sub-pages in a folio.
> + */
> +static bool zswap_store_page(struct page *page,
> +                            struct obj_cgroup *objcg,
> +                            struct zswap_pool *pool,
> +                            struct xarray *tree,
> +                            size_t *compressed_bytes)
>  {
> -       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;
> -
> -       VM_WARN_ON_ONCE(!folio_test_locked(folio));
> -       VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> -
> -       /* Large folios aren't supported */
> -       if (folio_test_large(folio))
> -               return false;
> -
> -       if (!zswap_enabled)
> -               goto check_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);
> -       }
> -
> -       if (zswap_check_limits())
> -               goto reject;
>
>         /* allocate entry */
> -       entry = zswap_entry_cache_alloc(GFP_KERNEL, folio_nid(folio));
> +       entry = zswap_entry_cache_alloc(GFP_KERNEL, folio_nid(page_folio(page)));

Can we just use page_to_nid() here? I think the node info exists even
for tail pages, right?

>         if (!entry) {
>                 zswap_reject_kmemcache_fail++;
>                 goto reject;
>         }
>
> -       /* if entry is successfully added, it keeps the reference */
> -       entry->pool = zswap_pool_current_get();
> -       if (!entry->pool)
> -               goto freepage;
> +       /* zswap_store() already holds a ref on 'objcg' and 'pool' */
> +       if (objcg)
> +               obj_cgroup_get(objcg);
> +       zswap_pool_get(pool);
>
> -       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);
> -       }
> +       /* if entry is successfully added, it keeps the reference */
> +       entry->pool = pool;
>
> -       if (!zswap_compress(&folio->page, entry))
> -               goto put_pool;
> +       if (!zswap_compress(page, entry))
> +               goto put_pool_objcg;
>
> -       entry->swpentry = swp;
> +       entry->swpentry = page_swap_entry(page);
>         entry->objcg = objcg;
>         entry->referenced = true;
>
> -       old = xa_store(tree, offset, entry, GFP_KERNEL);
> +       old = xa_store(tree, swp_offset(entry->swpentry), entry, GFP_KERNEL);
>         if (xa_is_err(old)) {
>                 int err = xa_err(old);
>
> @@ -1480,11 +1470,6 @@ bool zswap_store(struct folio *folio)
>         if (old)
>                 zswap_entry_free(old);
>
> -       if (objcg) {
> -               obj_cgroup_charge_zswap(objcg, entry->length);
> -               count_objcg_events(objcg, ZSWPOUT, 1);
> -       }
> -
>         /*
>          * We finish initializing the entry while it's already in xarray.
>          * This is safe because:
> @@ -1496,36 +1481,137 @@ bool zswap_store(struct folio *folio)
>          *    an incoherent entry.
>          */
>         if (entry->length) {
> +               *compressed_bytes += entry->length;
>                 INIT_LIST_HEAD(&entry->lru);
>                 zswap_lru_add(&zswap_list_lru, entry);
>         }
>
> -       /* update stats */
> -       atomic_long_inc(&zswap_stored_pages);
> -       count_vm_event(ZSWPOUT);
> -
> +       /*
> +        * We shouldn't have any possibility of failure after the entry is
> +        * added in the xarray. The pool/objcg refs obtained here will only
> +        * be dropped if/when zswap_entry_free() gets called.
> +        */
>         return true;
>
>  store_failed:
>         zpool_free(entry->pool->zpool, entry->handle);
> -put_pool:
> -       zswap_pool_put(entry->pool);
> -freepage:
> +put_pool_objcg:
> +       zswap_pool_put(pool);
> +       obj_cgroup_put(objcg);

I think if we reorder the function we can drop these calls, make the
comments positioned a bit better, and centralize the entry
initializations. I am also not a fan of passing a semi-initialized
entry to zswap_compress() to get the pool pointer.

Does the following diff improve things or did I miss something?

diff --git a/mm/zswap.c b/mm/zswap.c
index b74c8de996468..eac1f846886a6 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -881,7 +881,8 @@ static int zswap_cpu_comp_dead(unsigned int cpu,
struct hlist_node *node)
        return 0;
 }

-static bool zswap_compress(struct page *page, struct zswap_entry *entry)
+static bool zswap_compress(struct page *page, struct zswap_entry *entry,
+                          struct zswap_pool *pool)
 {
        struct crypto_acomp_ctx *acomp_ctx;
        struct scatterlist input, output;
@@ -893,7 +894,7 @@ static bool zswap_compress(struct page *page,
struct zswap_entry *entry)
        gfp_t gfp;
        u8 *dst;

-       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
+       acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);

        mutex_lock(&acomp_ctx->mutex);

@@ -926,7 +927,7 @@ static bool zswap_compress(struct page *page,
struct zswap_entry *entry)
        if (comp_ret)
                goto unlock;

-       zpool = entry->pool->zpool;
+       zpool = pool->zpool;
        gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
        if (zpool_malloc_support_movable(zpool))
                gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;
@@ -1435,23 +1436,11 @@ static bool zswap_store_page(struct page *page,
        entry = zswap_entry_cache_alloc(GFP_KERNEL,
folio_nid(page_folio(page)));
        if (!entry) {
                zswap_reject_kmemcache_fail++;
-               goto reject;
+               return false;
        }

-       /* zswap_store() already holds a ref on 'objcg' and 'pool' */
-       if (objcg)
-               obj_cgroup_get(objcg);
-       zswap_pool_get(pool);
-
-       /* if entry is successfully added, it keeps the reference */
-       entry->pool = pool;
-
-       if (!zswap_compress(page, entry))
-               goto put_pool_objcg;
-
-       entry->swpentry = page_swap_entry(page);
-       entry->objcg = objcg;
-       entry->referenced = true;
+       if (!zswap_compress(page, entry, pool))
+               goto compress_failed;

        old = xa_store(tree, swp_offset(entry->swpentry), entry, GFP_KERNEL);
        if (xa_is_err(old)) {
@@ -1470,6 +1459,16 @@ static bool zswap_store_page(struct page *page,
        if (old)
                zswap_entry_free(old);

+       /*
+        * The entry is successfully compressed and stored in the tree, there is
+        * no further possibility of failure. Grab refs to the pool and objcg.
+        * These refs will be dropped by zswap_entry_free() when the entry is
+        * removed from the tree.
+        */
+       zswap_pool_get(pool);
+       if (objcg)
+               obj_cgroup_get(objcg);
+
        /*
         * We finish initializing the entry while it's already in xarray.
         * This is safe because:
@@ -1480,26 +1479,22 @@ static bool zswap_store_page(struct page *page,
         *    The publishing order matters to prevent writeback from seeing
         *    an incoherent entry.
         */
+       entry->pool = pool;
+       entry->swpentry = page_swap_entry(page);
+       entry->objcg = objcg;
+       entry->referenced = true;
        if (entry->length) {
                *compressed_bytes += entry->length;
                INIT_LIST_HEAD(&entry->lru);
                zswap_lru_add(&zswap_list_lru, entry);
        }

-       /*
-        * We shouldn't have any possibility of failure after the entry is
-        * added in the xarray. The pool/objcg refs obtained here will only
-        * be dropped if/when zswap_entry_free() gets called.
-        */
        return true;

 store_failed:
-       zpool_free(entry->pool->zpool, entry->handle);
-put_pool_objcg:
-       zswap_pool_put(pool);
-       obj_cgroup_put(objcg);
+       zpool_free(pool->zpool, entry->handle);
+compress_failed:
        zswap_entry_cache_free(entry);
-reject:
        return false;
 }


>         zswap_entry_cache_free(entry);
>  reject:
> +       return false;
> +}
> +
> +bool zswap_store(struct folio *folio)
> +{
> +       long nr_pages = folio_nr_pages(folio);
> +       swp_entry_t swp = folio->swap;
> +       struct xarray *tree = swap_zswap_tree(swp);
> +       struct obj_cgroup *objcg = NULL;
> +       struct mem_cgroup *memcg = NULL;
> +       struct zswap_pool *pool;
> +       size_t compressed_bytes = 0;
> +       bool ret = false;
> +       long index;
> +
> +       VM_WARN_ON_ONCE(!folio_test_locked(folio));
> +       VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> +
> +       if (!zswap_enabled)
> +               goto check_old;
> +
> +       /*
> +        * Check cgroup zswap limits:
> +        *
> +        * The cgroup zswap limit check is done once at the beginning of
> +        * zswap_store(). The cgroup charging is done once, at the end
> +        * of a successful folio store. What this means is, if the cgroup
> +        * was within the zswap_max limit at the beginning of a large folio
> +        * store, it could go over the limit by at most (HPAGE_PMD_NR - 1)
> +        * pages due to this store.
> +        */
> +       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);
> +       }
> +
> +       /*
> +        * Check zpool utilization against zswap limits:
> +        *
> +        * The zswap zpool utilization is also checked against the limits
> +        * just once, at the start of zswap_store(). If the check passes,
> +        * any breaches of the limits set by zswap_max_pages() or
> +        * zswap_accept_thr_pages() that may happen while storing this
> +        * folio, will only be detected during the next call to
> +        * zswap_store() by any process.
> +        */

This is essentially a rephrased repetition of the last comment, just
refer to it instead. Something like:

         /*
           * Check zpool utilization against zswap limits. The possibility of
           * going overlimit is the same as the cgroup limit check.
           */

> +       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 deleting all the pages for this folio
> +        * currently in zswap.
> +        */
> +       for (index = 0; index < nr_pages; ++index) {
> +               if (!zswap_store_page(folio_page(folio, index), objcg, pool, tree, &compressed_bytes))
> +                       goto put_pool;
> +       }
> +
> +       if (objcg) {
> +               obj_cgroup_charge_zswap(objcg, compressed_bytes);
> +               count_objcg_events(objcg, ZSWPOUT, nr_pages);
> +       }
> +
> +       atomic_long_add(nr_pages, &zswap_stored_pages);
> +       count_vm_events(ZSWPOUT, nr_pages);
> +
> +       ret = true;
> +
> +put_pool:
> +       zswap_pool_put(pool);
> +put_objcg:
>         obj_cgroup_put(objcg);
> -       if (zswap_pool_reached_full)
> +       if (!ret && zswap_pool_reached_full)
>                 queue_work(shrink_wq, &zswap_shrink_work);
>  check_old:
>         /*
> -        * 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.
> +        * 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.
>          */
> -       entry = xa_erase(tree, offset);
> -       if (entry)
> -               zswap_entry_free(entry);
> -       return false;
> +       if (!ret) {
> +               pgoff_t offset = swp_offset(swp);
> +               struct zswap_entry *entry;
> +
> +               for (index = 0; index < nr_pages; ++index) {
> +                       entry = xa_erase(tree, offset + index);
> +                       if (entry)
> +                               zswap_entry_free(entry);
> +               }
> +       }
> +
> +       return ret;
>  }
>
>  bool zswap_load(struct folio *folio)
> --
> 2.27.0
>

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