Re: [PATCH] mm: slab: Only move management objects off-slab for sizes larger than KMALLOC_MIN_SIZE

From: Andrew Morton
Date: Thu Nov 05 2015 - 08:28:24 EST

On Thu, 5 Nov 2015 11:50:35 +0000 Catalin Marinas <catalin.marinas@xxxxxxx> wrote:

> Commit 8fc9cf420b36 ("slab: make more slab management structure off the
> slab") enables off-slab management objects for sizes starting with
> PAGE_SIZE >> 5. This means 128 bytes for a 4KB page configuration.
> However, on systems with a KMALLOC_MIN_SIZE of 128 (arm64 in 4.4), such
> optimisation does not make sense since the slab management allocation
> would take 128 bytes anyway (even though freelist_size is 32) with the
> additional overhead of another allocation.
> This patch introduces an OFF_SLAB_MIN_SIZE macro which takes
> KMALLOC_MIN_SIZE into account. It also solves a slab bug on arm64 where
> the first kmalloc_cache to be initialised after slab_early_init = 0,
> "kmalloc-128", fails to allocate off-slab management objects from the
> same "kmalloc-128" cache.

That all seems to be quite minor stuff.

> Fixes: 8fc9cf420b36 ("slab: make more slab management structure off the slab")
> Cc: <stable@xxxxxxxxxxxxxxx> # 3.15+

Yet you believe the fix should be backported.

So, the usual refrain: when fixing a bug, please describe the end-user
visible effects of that bug.

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