[PATCH v5 0/2] mm: zswap: fixes for global shrinker

[PATCH v5 0/2] mm: zswap: fixes for global shrinker

[Takero Funaki]

This series addresses issues in the zswap global shrinker that could not
shrink stored pages. With this series, the shrinker continues to shrink
pages until it reaches the accept threshold more reliably, gives much
higher writeback when the zswap pool limit is hit.

v5 is additional cleanup on comments I missed in the last thread, and
adds Acked-by and Reviewed-by tags into the patch 1.
No behavioral changes have been made since v4:
https://lore.kernel.org/lkml/20240727230635.3170-1-flintglass@gmail.com/

Chnaged in v5:
- Cleaned up more comments (Yosry)

Changes in v4:
- Updated comments and commit logs to clarify expected behaviors (Yosry,
  Nhat)
- Merged duplicated spin_unlock() in if branches (Nhat)
- Renamed writeback attempts counter (Nhat, Chengming)

Changes in v3:
- Extract fixes for shrinker as a separate patch series.
- Fix comments and commit messages. (Chengming, Yosry)
- Drop logic to detect rare doubly advancing cursor. (Yosry)

Changes in v2:
mm: zswap: fix global shrinker memcg iteration:
- Change the loop style (Yosry, Nhat, Shakeel)
mm: zswap: fix global shrinker error handling logic:
- Change error code for no-writeback memcg. (Yosry)
- Use nr_scanned to check if lru is empty. (Yosry)

Changes in v1:
mm: zswap: fix global shrinker memcg iteration:
- Drop and reacquire spinlock before skipping a memcg.
- Add some comment to clarify the locking mechanism.

---

Takero Funaki (2):
  mm: zswap: fix global shrinker memcg iteration
  mm: zswap: fix global shrinker error handling logic

 mm/zswap.c | 106 ++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 76 insertions(+), 30 deletions(-)

-- 
2.43.0

[PATCH v5 1/2] mm: zswap: fix global shrinker memcg iteration

[Takero Funaki]

This patch fixes an issue where the zswap global shrinker stopped
iterating through the memcg tree.

The problem was that shrink_worker() would restart iterating memcg tree
from the tree root, considering an offline memcg as a failure, and abort
shrinking after encountering the same offline memcg 16 times even if
there is only one offline memcg. After this change, an offline memcg in
the tree is no longer considered a failure. This allows the shrinker to
continue shrinking the other online memcgs regardless of whether an
offline memcg exists, gives higher zswap writeback activity.

To avoid holding refcount of offline memcg encountered during the memcg
tree walking, shrink_worker() must continue iterating to release the
offline memcg to ensure the next memcg stored in the cursor is online.

The offline memcg cleaner has also been changed to avoid the same issue.
When the next memcg of the offlined memcg is also offline, the refcount
stored in the iteration cursor was held until the next shrink_worker()
run. The cleaner must release the offline memcg recursively.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Acked-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Takero Funaki <flintglass@gmail.com>
---
 mm/zswap.c | 68 +++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 44 insertions(+), 24 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index adeaf9c97fde..3c16a1192252 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -765,12 +765,25 @@ void zswap_folio_swapin(struct folio *folio)
 	}
 }
 
+/*
+ * This function should be called when a memcg is being offlined.
+ *
+ * Since the global shrinker shrink_worker() may hold a reference
+ * of the memcg, we must check and release the reference in
+ * zswap_next_shrink.
+ *
+ * shrink_worker() must handle the case where this function releases
+ * the reference of memcg being shrunk.
+ */
 void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
 {
 	/* lock out zswap shrinker walking memcg tree */
 	spin_lock(&zswap_shrink_lock);
-	if (zswap_next_shrink == memcg)
-		zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
+	if (zswap_next_shrink == memcg) {
+		do {
+			zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
+		} while (zswap_next_shrink && !mem_cgroup_online(zswap_next_shrink));
+	}
 	spin_unlock(&zswap_shrink_lock);
 }
 
@@ -1304,43 +1317,50 @@ static void shrink_worker(struct work_struct *w)
 	/* Reclaim down to the accept threshold */
 	thr = zswap_accept_thr_pages();
 
-	/* global reclaim will select cgroup in a round-robin fashion. */
+	/*
+	 * Global reclaim will select cgroup in a round-robin fashion.
+	 *
+	 * We save iteration cursor memcg into zswap_next_shrink,
+	 * which can be modified by the offline memcg cleaner
+	 * zswap_memcg_offline_cleanup().
+	 *
+	 * Since the offline cleaner is called only once, we cannot leave an
+	 * offline memcg reference in zswap_next_shrink.
+	 * We can rely on the cleaner only if we get online memcg under lock.
+	 *
+	 * If we get an offline memcg, we cannot determine if the cleaner has
+	 * already been called or will be called later. We must put back the
+	 * reference before returning from this function. Otherwise, the
+	 * offline memcg left in zswap_next_shrink will hold the reference
+	 * until the next run of shrink_worker().
+	 */
 	do {
 		spin_lock(&zswap_shrink_lock);
-		zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
-		memcg = zswap_next_shrink;
 
 		/*
-		 * We need to retry if we have gone through a full round trip, or if we
-		 * got an offline memcg (or else we risk undoing the effect of the
-		 * zswap memcg offlining cleanup callback). This is not catastrophic
-		 * per se, but it will keep the now offlined memcg hostage for a while.
-		 *
+		 * Start shrinking from the next memcg after zswap_next_shrink.
+		 * When the offline cleaner has already advanced the cursor,
+		 * advancing the cursor here overlooks one memcg, but this
+		 * should be negligibly rare.
+		 */
+		do {
+			memcg = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
+			zswap_next_shrink = memcg;
+		} while (memcg && !mem_cgroup_tryget_online(memcg));
+		/*
 		 * Note that if we got an online memcg, we will keep the extra
 		 * reference in case the original reference obtained by mem_cgroup_iter
 		 * is dropped by the zswap memcg offlining callback, ensuring that the
 		 * memcg is not killed when we are reclaiming.
 		 */
-		if (!memcg) {
-			spin_unlock(&zswap_shrink_lock);
-			if (++failures == MAX_RECLAIM_RETRIES)
-				break;
-
-			goto resched;
-		}
-
-		if (!mem_cgroup_tryget_online(memcg)) {
-			/* drop the reference from mem_cgroup_iter() */
-			mem_cgroup_iter_break(NULL, memcg);
-			zswap_next_shrink = NULL;
-			spin_unlock(&zswap_shrink_lock);
+		spin_unlock(&zswap_shrink_lock);
 
+		if (!memcg) {
 			if (++failures == MAX_RECLAIM_RETRIES)
 				break;
 
 			goto resched;
 		}
-		spin_unlock(&zswap_shrink_lock);
 
 		ret = shrink_memcg(memcg);
 		/* drop the extra reference */
-- 
2.43.0

Re: [PATCH v5 1/2] mm: zswap: fix global shrinker memcg iteration

[Yosry Ahmed]

On Tue, Jul 30, 2024 at 5:49 PM Takero Funaki <flintglass@gmail.com> wrote:
>
> This patch fixes an issue where the zswap global shrinker stopped
> iterating through the memcg tree.
>
> The problem was that shrink_worker() would restart iterating memcg tree
> from the tree root, considering an offline memcg as a failure, and abort
> shrinking after encountering the same offline memcg 16 times even if
> there is only one offline memcg. After this change, an offline memcg in
> the tree is no longer considered a failure. This allows the shrinker to
> continue shrinking the other online memcgs regardless of whether an
> offline memcg exists, gives higher zswap writeback activity.
>
> To avoid holding refcount of offline memcg encountered during the memcg
> tree walking, shrink_worker() must continue iterating to release the
> offline memcg to ensure the next memcg stored in the cursor is online.
>
> The offline memcg cleaner has also been changed to avoid the same issue.
> When the next memcg of the offlined memcg is also offline, the refcount
> stored in the iteration cursor was held until the next shrink_worker()
> run. The cleaner must release the offline memcg recursively.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Acked-by: Yosry Ahmed <yosryahmed@google.com>
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Takero Funaki <flintglass@gmail.com>
> ---
>  mm/zswap.c | 68 +++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 44 insertions(+), 24 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index adeaf9c97fde..3c16a1192252 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -765,12 +765,25 @@ void zswap_folio_swapin(struct folio *folio)
>         }
>  }
>
> +/*
> + * This function should be called when a memcg is being offlined.
> + *
> + * Since the global shrinker shrink_worker() may hold a reference
> + * of the memcg, we must check and release the reference in
> + * zswap_next_shrink.
> + *
> + * shrink_worker() must handle the case where this function releases
> + * the reference of memcg being shrunk.
> + */
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  {
>         /* lock out zswap shrinker walking memcg tree */
>         spin_lock(&zswap_shrink_lock);
> -       if (zswap_next_shrink == memcg)
> -               zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +       if (zswap_next_shrink == memcg) {
> +               do {
> +                       zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +               } while (zswap_next_shrink && !mem_cgroup_online(zswap_next_shrink));
> +       }
>         spin_unlock(&zswap_shrink_lock);
>  }
>
> @@ -1304,43 +1317,50 @@ static void shrink_worker(struct work_struct *w)
>         /* Reclaim down to the accept threshold */
>         thr = zswap_accept_thr_pages();
>
> -       /* global reclaim will select cgroup in a round-robin fashion. */
> +       /*
> +        * Global reclaim will select cgroup in a round-robin fashion.
> +        *
> +        * We save iteration cursor memcg into zswap_next_shrink,
> +        * which can be modified by the offline memcg cleaner
> +        * zswap_memcg_offline_cleanup().
> +        *
> +        * Since the offline cleaner is called only once, we cannot leave an
> +        * offline memcg reference in zswap_next_shrink.
> +        * We can rely on the cleaner only if we get online memcg under lock.
> +        *
> +        * If we get an offline memcg, we cannot determine if the cleaner has
> +        * already been called or will be called later. We must put back the
> +        * reference before returning from this function. Otherwise, the
> +        * offline memcg left in zswap_next_shrink will hold the reference
> +        * until the next run of shrink_worker().
> +        */
>         do {
>                 spin_lock(&zswap_shrink_lock);
> -               zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> -               memcg = zswap_next_shrink;
>
>                 /*
> -                * We need to retry if we have gone through a full round trip, or if we
> -                * got an offline memcg (or else we risk undoing the effect of the
> -                * zswap memcg offlining cleanup callback). This is not catastrophic
> -                * per se, but it will keep the now offlined memcg hostage for a while.
> -                *
> +                * Start shrinking from the next memcg after zswap_next_shrink.
> +                * When the offline cleaner has already advanced the cursor,
> +                * advancing the cursor here overlooks one memcg, but this
> +                * should be negligibly rare.
> +                */
> +               do {
> +                       memcg = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
> +                       zswap_next_shrink = memcg;
> +               } while (memcg && !mem_cgroup_tryget_online(memcg));

I took a look at refactoring the loop to a helper, but it's probably
not going to be any clearer because this loop has a tryget, and the
loop in zswap_memcg_offline_cleanup() only has an online check. Using
a tryget in the offline cleanup version would be wasteful and we'll
put the ref right away.

Instead, I think we should just move the spin_lock/unlock() closer to
the loop to make the critical section more obvious, and unify the
comments above and below into a single block.

Andrew, could you please fold in the following diff (unless Takero objects):

diff --git a/mm/zswap.c b/mm/zswap.c
index babf0abbcc765..df620eacd1d11 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1364,24 +1364,22 @@ static void shrink_worker(struct work_struct *w)
         * until the next run of shrink_worker().
         */
        do {
-               spin_lock(&zswap_shrink_lock);
-
                /*
                 * Start shrinking from the next memcg after zswap_next_shrink.
                 * When the offline cleaner has already advanced the cursor,
                 * advancing the cursor here overlooks one memcg, but this
                 * should be negligibly rare.
+                *
+                * If we get an online memcg, keep the extra reference in case
+                * the original one obtained by mem_cgroup_iter() is dropped by
+                * zswap_memcg_offline_cleanup() while we are shrinking the
+                * memcg.
                 */
+               spin_lock(&zswap_shrink_lock);
                do {
                        memcg = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
                        zswap_next_shrink = memcg;
                } while (memcg && !mem_cgroup_tryget_online(memcg));
-               /*
-                * Note that if we got an online memcg, we will keep the extra
-                * reference in case the original reference obtained
by mem_cgroup_iter
-                * is dropped by the zswap memcg offlining callback,
ensuring that the
-                * memcg is not killed when we are reclaiming.
-                */
                spin_unlock(&zswap_shrink_lock);

                if (!memcg) {

> +               /*
>                  * Note that if we got an online memcg, we will keep the extra
>                  * reference in case the original reference obtained by mem_cgroup_iter
>                  * is dropped by the zswap memcg offlining callback, ensuring that the
>                  * memcg is not killed when we are reclaiming.
>                  */
> -               if (!memcg) {
> -                       spin_unlock(&zswap_shrink_lock);
> -                       if (++failures == MAX_RECLAIM_RETRIES)
> -                               break;
> -
> -                       goto resched;
> -               }
> -
> -               if (!mem_cgroup_tryget_online(memcg)) {
> -                       /* drop the reference from mem_cgroup_iter() */
> -                       mem_cgroup_iter_break(NULL, memcg);
> -                       zswap_next_shrink = NULL;
> -                       spin_unlock(&zswap_shrink_lock);
> +               spin_unlock(&zswap_shrink_lock);
>
> +               if (!memcg) {
>                         if (++failures == MAX_RECLAIM_RETRIES)
>                                 break;
>
>                         goto resched;
>                 }
> -               spin_unlock(&zswap_shrink_lock);
>
>                 ret = shrink_memcg(memcg);
>                 /* drop the extra reference */
> --
> 2.43.0
>

[PATCH v5 2/2] mm: zswap: fix global shrinker error handling logic

[Takero Funaki]

This patch fixes the zswap global shrinker, which did not shrink the
zpool as expected.

The issue addressed is that shrink_worker() did not distinguish between
unexpected errors and expected errors, such as failed writeback from an
empty memcg. The shrinker would stop shrinking after iterating through
the memcg tree 16 times, even if there was only one empty memcg.

With this patch, the shrinker no longer considers encountering an empty
memcg, encountering a memcg with writeback disabled, or reaching the end
of a memcg tree walk as a failure, as long as there are memcgs that are
candidates for writeback. Systems with one or more empty memcgs will now
observe significantly higher zswap writeback activity after the zswap
pool limit is hit.

To avoid an infinite loop when there are no writeback candidates, this
patch tracks writeback attempts during memcg tree walks and limits
reties if no writeback candidates are found.

To handle the empty memcg case, the helper function shrink_memcg() is
modified to check if the memcg is empty and then return -ENOENT.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Signed-off-by: Takero Funaki <flintglass@gmail.com>
---
 mm/zswap.c | 40 +++++++++++++++++++++++++++++++++-------
 1 file changed, 33 insertions(+), 7 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 3c16a1192252..d46caa42ed4f 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1287,10 +1287,10 @@ static struct shrinker *zswap_alloc_shrinker(void)
 
 static int shrink_memcg(struct mem_cgroup *memcg)
 {
-	int nid, shrunk = 0;
+	int nid, shrunk = 0, scanned = 0;
 
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
-		return -EINVAL;
+		return -ENOENT;
 
 	/*
 	 * Skip zombies because their LRUs are reparented and we would be
@@ -1304,21 +1304,34 @@ static int shrink_memcg(struct mem_cgroup *memcg)
 
 		shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
 					    &shrink_memcg_cb, NULL, &nr_to_walk);
+		scanned += 1 - nr_to_walk;
 	}
+
+	if (!scanned)
+		return -ENOENT;
+
 	return shrunk ? 0 : -EAGAIN;
 }
 
 static void shrink_worker(struct work_struct *w)
 {
 	struct mem_cgroup *memcg;
-	int ret, failures = 0;
+	int ret, failures = 0, attempts = 0;
 	unsigned long thr;
 
 	/* Reclaim down to the accept threshold */
 	thr = zswap_accept_thr_pages();
 
 	/*
-	 * Global reclaim will select cgroup in a round-robin fashion.
+	 * Global reclaim will select cgroup in a round-robin fashion from all
+	 * online memcgs, but memcgs that have no pages in zswap and
+	 * writeback-disabled memcgs (memory.zswap.writeback=0) are not
+	 * candidates for shrinking.
+	 *
+	 * Shrinking will be aborted if we encounter the following
+	 * MAX_RECLAIM_RETRIES times:
+	 * - No writeback-candidate memcgs found in a memcg tree walk.
+	 * - Shrinking a writeback-candidate memcg failed.
 	 *
 	 * We save iteration cursor memcg into zswap_next_shrink,
 	 * which can be modified by the offline memcg cleaner
@@ -1356,9 +1369,14 @@ static void shrink_worker(struct work_struct *w)
 		spin_unlock(&zswap_shrink_lock);
 
 		if (!memcg) {
-			if (++failures == MAX_RECLAIM_RETRIES)
+			/*
+			 * Continue shrinking without incrementing failures if
+			 * we found candidate memcgs in the last tree walk.
+			 */
+			if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
 				break;
 
+			attempts = 0;
 			goto resched;
 		}
 
@@ -1366,8 +1384,16 @@ static void shrink_worker(struct work_struct *w)
 		/* drop the extra reference */
 		mem_cgroup_put(memcg);
 
-		if (ret == -EINVAL)
-			break;
+		/*
+		 * There are no writeback-candidate pages in the memcg.
+		 * This is not an issue as long as we can find another memcg
+		 * with pages in zswap. Skip this without incrementing attempts
+		 * and failures.
+		 */
+		if (ret == -ENOENT)
+			continue;
+		++attempts;
+
 		if (ret && ++failures == MAX_RECLAIM_RETRIES)
 			break;
 resched:
-- 
2.43.0

Re: [PATCH v5 2/2] mm: zswap: fix global shrinker error handling logic

[Chengming Zhou]

On 2024/7/31 08:49, Takero Funaki wrote:
> This patch fixes the zswap global shrinker, which did not shrink the
> zpool as expected.
> 
> The issue addressed is that shrink_worker() did not distinguish between
> unexpected errors and expected errors, such as failed writeback from an
> empty memcg. The shrinker would stop shrinking after iterating through
> the memcg tree 16 times, even if there was only one empty memcg.
> 
> With this patch, the shrinker no longer considers encountering an empty
> memcg, encountering a memcg with writeback disabled, or reaching the end
> of a memcg tree walk as a failure, as long as there are memcgs that are
> candidates for writeback. Systems with one or more empty memcgs will now
> observe significantly higher zswap writeback activity after the zswap
> pool limit is hit.
> 
> To avoid an infinite loop when there are no writeback candidates, this
> patch tracks writeback attempts during memcg tree walks and limits
> reties if no writeback candidates are found.
> 
> To handle the empty memcg case, the helper function shrink_memcg() is
> modified to check if the memcg is empty and then return -ENOENT.
> 
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@gmail.com>

Looks good to me:

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks.

> ---
>   mm/zswap.c | 40 +++++++++++++++++++++++++++++++++-------
>   1 file changed, 33 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 3c16a1192252..d46caa42ed4f 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1287,10 +1287,10 @@ static struct shrinker *zswap_alloc_shrinker(void)
>   
>   static int shrink_memcg(struct mem_cgroup *memcg)
>   {
> -	int nid, shrunk = 0;
> +	int nid, shrunk = 0, scanned = 0;
>   
>   	if (!mem_cgroup_zswap_writeback_enabled(memcg))
> -		return -EINVAL;
> +		return -ENOENT;
>   
>   	/*
>   	 * Skip zombies because their LRUs are reparented and we would be
> @@ -1304,21 +1304,34 @@ static int shrink_memcg(struct mem_cgroup *memcg)
>   
>   		shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
>   					    &shrink_memcg_cb, NULL, &nr_to_walk);
> +		scanned += 1 - nr_to_walk;
>   	}
> +
> +	if (!scanned)
> +		return -ENOENT;
> +
>   	return shrunk ? 0 : -EAGAIN;
>   }
>   
>   static void shrink_worker(struct work_struct *w)
>   {
>   	struct mem_cgroup *memcg;
> -	int ret, failures = 0;
> +	int ret, failures = 0, attempts = 0;
>   	unsigned long thr;
>   
>   	/* Reclaim down to the accept threshold */
>   	thr = zswap_accept_thr_pages();
>   
>   	/*
> -	 * Global reclaim will select cgroup in a round-robin fashion.
> +	 * Global reclaim will select cgroup in a round-robin fashion from all
> +	 * online memcgs, but memcgs that have no pages in zswap and
> +	 * writeback-disabled memcgs (memory.zswap.writeback=0) are not
> +	 * candidates for shrinking.
> +	 *
> +	 * Shrinking will be aborted if we encounter the following
> +	 * MAX_RECLAIM_RETRIES times:
> +	 * - No writeback-candidate memcgs found in a memcg tree walk.
> +	 * - Shrinking a writeback-candidate memcg failed.
>   	 *
>   	 * We save iteration cursor memcg into zswap_next_shrink,
>   	 * which can be modified by the offline memcg cleaner
> @@ -1356,9 +1369,14 @@ static void shrink_worker(struct work_struct *w)
>   		spin_unlock(&zswap_shrink_lock);
>   
>   		if (!memcg) {
> -			if (++failures == MAX_RECLAIM_RETRIES)
> +			/*
> +			 * Continue shrinking without incrementing failures if
> +			 * we found candidate memcgs in the last tree walk.
> +			 */
> +			if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
>   				break;
>   
> +			attempts = 0;
>   			goto resched;
>   		}
>   
> @@ -1366,8 +1384,16 @@ static void shrink_worker(struct work_struct *w)
>   		/* drop the extra reference */
>   		mem_cgroup_put(memcg);
>   
> -		if (ret == -EINVAL)
> -			break;
> +		/*
> +		 * There are no writeback-candidate pages in the memcg.
> +		 * This is not an issue as long as we can find another memcg
> +		 * with pages in zswap. Skip this without incrementing attempts
> +		 * and failures.
> +		 */
> +		if (ret == -ENOENT)
> +			continue;
> +		++attempts;
> +
>   		if (ret && ++failures == MAX_RECLAIM_RETRIES)
>   			break;
>   resched:

Re: [PATCH v5 2/2] mm: zswap: fix global shrinker error handling logic

[Nhat Pham]

On Tue, Jul 30, 2024 at 5:49 PM Takero Funaki <flintglass@gmail.com> wrote:
>
> This patch fixes the zswap global shrinker, which did not shrink the
> zpool as expected.
>
> The issue addressed is that shrink_worker() did not distinguish between
> unexpected errors and expected errors, such as failed writeback from an
> empty memcg. The shrinker would stop shrinking after iterating through
> the memcg tree 16 times, even if there was only one empty memcg.
>
> With this patch, the shrinker no longer considers encountering an empty
> memcg, encountering a memcg with writeback disabled, or reaching the end
> of a memcg tree walk as a failure, as long as there are memcgs that are
> candidates for writeback. Systems with one or more empty memcgs will now
> observe significantly higher zswap writeback activity after the zswap
> pool limit is hit.
>
> To avoid an infinite loop when there are no writeback candidates, this
> patch tracks writeback attempts during memcg tree walks and limits
> reties if no writeback candidates are found.
>
> To handle the empty memcg case, the helper function shrink_memcg() is
> modified to check if the memcg is empty and then return -ENOENT.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@gmail.com>

Nice! LGTM FWIW.
Reviewed-by: Nhat Pham <nphamcs@gmail.com>

Thank you so much for working on this, Takero! I look forward to your
other global shrinker fixes :)