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Date:   Thu, 25 May 2023 13:59:20 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     David Sterba <dsterba@suse.com>, Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Roman Gushchin <roman.gushchin@linux.dev>
Subject: Re: [PATCH] mm/slab: add new flag SLAB_NO_MERGE to avoid merging per
 slab
Message-ID: <20230525115920.GG30909@suse.cz>
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References: <20230524101748.30714-1-dsterba@suse.com>
 <ZG4JYr14RpNQb8li@debian-BULLSEYE-live-builder-AMD64>
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On Wed, May 24, 2023 at 09:56:02PM +0900, Hyeonggon Yoo wrote:
> On Wed, May 24, 2023 at 12:17:48PM +0200, David Sterba wrote:
> > Add a flag that allows to disable merging per slab. This can be used for
> > more fine grained control over the caches or for debugging builds where
> > separate slabs can verify that no objects leak.
> > The slab_nomerge boot option is too coarse and would need to be enabled
> > on all testing hosts. 
> 
> There is no users nor interface to set this flag, I guess you're going
> to use it by modifying source code, when debugging?

An example usage

--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -12,6 +12,12 @@
 #include "async-thread.h"
 #include "block-rsv.h"
 
+#ifdef CONFIG_BTRFS_DEBUG
+#define SLAB_DEBUG_NO_MERGE            0
+#else
+#define SLAB_DEBUG_NO_MERGE            SLAB_NO_MERGE
+#endif
+
 #define BTRFS_MAX_EXTENT_SIZE SZ_128M
 
 #define BTRFS_OLDEST_GENERATION        0ULL
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -5049,7 +5049,7 @@ int __init btrfs_ctree_init(void)
 {
        btrfs_path_cachep = kmem_cache_create("btrfs_path",
                        sizeof(struct btrfs_path), 0,
-                       SLAB_MEM_SPREAD, NULL);
+                       SLAB_MEM_SPREAD | SLAB_DEBUG_NO_MERGE, NULL);
        if (!btrfs_path_cachep)
                return -ENOMEM;
        return 0;
---

and this will be a permanent change, not added as needed.

> Does introducing new slub_debug option (i.e. slub_debug=N,pid_namespace)
> work for your use case? (there are some boot-time slub_debug options described in
> Documentation/mm/slub.rst)

I'd like to keep boot parameters unchanged, the testing setups are
different, physical, local VM, hosted. For the same reason the config
option CONFIG_SLUB_DEBUG_ON=y is very convenient.