linux-kernel - Re: [PATCH] slub: Fix sysfs duplicate filename creation when slub

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Date:   Wed, 8 Nov 2017 09:05:12 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Miles Chen <miles.chen@...iatek.com>
cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        wsd_upstream@...iatek.com, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH] slub: Fix sysfs duplicate filename creation when
 slub_debug=O

On Wed, 8 Nov 2017, Miles Chen wrote:

> > Ok then the aliasing failed for some reason. The creation of the unique id
> > and the alias detection needs to be in sync otherwise duplicate filenames
> > are created. What is the difference there?
>
> The aliasing failed because find_mergeable() returns if (flags &
> SLAB_NEVER_MERGE) is true. So we do not go to search for alias caches.
>
> __kmem_cache_alias()
>   find_mergeable()
>     kmem_cache_flags()  --> setup flag by the slub_debug
>     if (flags & SLAB_NEVER_MERGE) return NULL;
>     ...
>     search alias logic...
>
>
> The flags maybe changed if disable_higher_order_debug=1. So the
> unmergeable cache becomes mergeable later.

Ok so make sure taht the aliasing logic also clears those flags before
checking for SLAB_NEVER_MERGE.

> > The clearing of the DEBUG_METADATA_FLAGS looks ok to me. kmem_cache_alias
> > should do the same right?
> >
> Yes, I think clearing DEBUG_METADATA flags in kmem_cache_alias is
> another solution for this issue.
>
> We will need to do calculate_sizes() by using original flags and compare
> the order of s->size and s->object_size when
> disable_higher_order_debug=1.

Hmmm... Or move the aliasing check to a point where we know the size of
the slab objects?