[PATCH 0/1] erofs: fix unused pcluster_pools for higher page sizes

[PATCH 0/1] erofs: fix unused pcluster_pools for higher page sizes

[Ojaswin Mujoo]

When inserting the erofs module on an older kernel with a 64K page size
machine, we were running into the following dmesg followed by a stack
dump (Needs, CONFIG_DEBUG_VM=y):

[    4.616501] kmem_cache of name 'erofs_pcluster-16' already exists

The root cause is that on 64k page size:

  Z_EROFS_PCLUSTER_MAX_PAGES = 1M / 16k = 16

and, on that kernel, our logic to create the pcluster pool is as:

  static struct z_erofs_pcluster_slab pcluster_pool[] __read_mostly = {
  	_PCLP(1), _PCLP(4), _PCLP(16), _PCLP(64), _PCLP(128),
  	_PCLP(Z_EROFS_PCLUSTER_MAX_PAGES)
  };

so we were ending up creating kmemcaches for nr_pages= 1, 4, 16, 64,
128, and Z_EROFS_PCLUSTER_MAX_PAGES (=16, again). This caused the 
kmem_cache sanity check to emit the warning.

When comparing with the upstream code, I noticed that this issue has
been fixed as a side effect of the patch:

  commit 7361d1e3763baaf7b9349c576137851458ad38d1
  Author: Gao Xiang <xiang@kernel.org>
  Date:   Mon Mar 10 17:54:59 2025 +0800
  
      erofs: support unaligned encoded data

which changes 

   static struct z_erofs_pcluster_slab pcluster_pool[] __read_mostly = {
          _PCLP(1), _PCLP(4), _PCLP(16), _PCLP(64), _PCLP(128),
  -       _PCLP(Z_EROFS_PCLUSTER_MAX_PAGES)
  +       _PCLP(Z_EROFS_PCLUSTER_MAX_PAGES + 1)
   };

and hence the final cache has nr_pages = 17 and doesn't clash with the
nr_pages=16 cache anymore. However, this is still incorrect as on higher
page sizes like 64k we will never use nr_pages > 16 and the rest of the
caches are wasted.

So this patch intends to fix the issue on upstream first and if the
approach looks okay, I'll send a backport for other the older affected
kernels.

Regards,
ojaswin

Ojaswin Mujoo (1):
  erofs: fix unused pcluster_pools for higher page sizes

 fs/erofs/zdata.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

-- 
2.55.0

[PATCH 1/1] erofs: fix unused pcluster_pools for higher page sizes

[Ojaswin Mujoo]

pcluster_pool[] hardcodes {1,4,16,64,128,Z_EROFS_PCLUSTER_MAX_PAGES+1},
but the assumption of Z_EROFS_PCLUSTER_MAX_PAGES == 256 is only right
for 4k page sizes. For higher page sizes like 16k or 64k, This results
in us ending up with clusters bigger than what we will ever use, since
we only support upto 1MB of compressed data. For example, on 64k page
size we will only ever use clusters with nrpages= 1, 4 and 17.

This patch fixes the allocation for such higher pages sizes by adding
some compile time checks.

Below are the clusters created right after boot on a 64KB page size
machine

$cat /proc/slabinfo | grep pcluster | cut -d" " -f1:

Before the patch:

erofs_pcluster-1
erofs_pcluster-4
erofs_pcluster-16
erofs_pcluster-17
erofs_pcluster-64
erofs_pcluster-128


After the patch:

erofs_pcluster-1
erofs_pcluster-4
erofs_pcluster-17

Fixes: 9f6cc76e6ff0 ("erofs: introduce physical cluster slab pools")
Reported-by: Shirisha G <shirisha@linux.ibm.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
---
 fs/erofs/zdata.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index 602ba8b7cc79..5c2c22ed64d0 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -128,7 +128,17 @@ struct z_erofs_pcluster_slab {
 #define _PCLP(n) { .maxpages = n }
 
 static struct z_erofs_pcluster_slab pcluster_pool[] __read_mostly = {
-	_PCLP(1), _PCLP(4), _PCLP(16), _PCLP(64), _PCLP(128),
+	_PCLP(1),
+	_PCLP(4),
+#if Z_EROFS_PCLUSTER_MAX_PAGES > 16
+	_PCLP(16),
+#endif
+#if Z_EROFS_PCLUSTER_MAX_PAGES > 64
+	_PCLP(64),
+#endif
+#if Z_EROFS_PCLUSTER_MAX_PAGES > 128
+	_PCLP(128),
+#endif
 	_PCLP(Z_EROFS_PCLUSTER_MAX_PAGES + 1)
 };
 
-- 
2.55.0

Re: [PATCH 1/1] erofs: fix unused pcluster_pools for higher page sizes

[Gao Xiang]

Hi Ojaswin,

On Sun, Aug 16, 2026 at 06:37:09PM +0530, Ojaswin Mujoo wrote:
> pcluster_pool[] hardcodes {1,4,16,64,128,Z_EROFS_PCLUSTER_MAX_PAGES+1},
> but the assumption of Z_EROFS_PCLUSTER_MAX_PAGES == 256 is only right
> for 4k page sizes. For higher page sizes like 16k or 64k, This results
> in us ending up with clusters bigger than what we will ever use, since
> we only support upto 1MB of compressed data. For example, on 64k page
> size we will only ever use clusters with nrpages= 1, 4 and 17.
> 
> This patch fixes the allocation for such higher pages sizes by adding
> some compile time checks.
> 
> Below are the clusters created right after boot on a 64KB page size
> machine
> 
> $cat /proc/slabinfo | grep pcluster | cut -d" " -f1:
> 
> Before the patch:
> 
> erofs_pcluster-1
> erofs_pcluster-4
> erofs_pcluster-16
> erofs_pcluster-17
> erofs_pcluster-64
> erofs_pcluster-128
> 
> 
> After the patch:
> 
> erofs_pcluster-1
> erofs_pcluster-4
> erofs_pcluster-17
> 
> Fixes: 9f6cc76e6ff0 ("erofs: introduce physical cluster slab pools")
> Reported-by: Shirisha G <shirisha@linux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>

Thanks for catching this, looks fine to me:

Reviewed-by: Gao Xiang <xiang@kernel.org>

Thanks,
Gao Xiang

Re: [PATCH 1/1] erofs: fix unused pcluster_pools for higher page sizes

[Chao Yu]

On 8/16/26 21:07, Ojaswin Mujoo wrote:
> pcluster_pool[] hardcodes {1,4,16,64,128,Z_EROFS_PCLUSTER_MAX_PAGES+1},
> but the assumption of Z_EROFS_PCLUSTER_MAX_PAGES == 256 is only right
> for 4k page sizes. For higher page sizes like 16k or 64k, This results
> in us ending up with clusters bigger than what we will ever use, since
> we only support upto 1MB of compressed data. For example, on 64k page
> size we will only ever use clusters with nrpages= 1, 4 and 17.
> 
> This patch fixes the allocation for such higher pages sizes by adding
> some compile time checks.
> 
> Below are the clusters created right after boot on a 64KB page size
> machine
> 
> $cat /proc/slabinfo | grep pcluster | cut -d" " -f1:
> 
> Before the patch:
> 
> erofs_pcluster-1
> erofs_pcluster-4
> erofs_pcluster-16
> erofs_pcluster-17
> erofs_pcluster-64
> erofs_pcluster-128
> 
> 
> After the patch:
> 
> erofs_pcluster-1
> erofs_pcluster-4
> erofs_pcluster-17
> 
> Fixes: 9f6cc76e6ff0 ("erofs: introduce physical cluster slab pools")
> Reported-by: Shirisha G <shirisha@linux.ibm.com>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>

Reviewed-by: Chao Yu <chao@kernel.org>

Thanks,