linux-kernel - Re: [PATCH 1/1] slob: Only use list functions when safe to do so

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Date:   Mon, 1 Apr 2019 21:41:28 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     LKP <lkp@...org>, Roman Gushchin <guro@...com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/1] slob: Only use list functions when safe to do so

On Tue,  2 Apr 2019 14:29:57 +1100 "Tobin C. Harding" <tobin@...nel.org> wrote:

> Currently we call (indirectly) list_del() then we manually try to combat
> the fact that the list may be in an undefined state by getting 'prev'
> and 'next' pointers in a somewhat contrived manner.  It is hard to
> verify that this works for all initial states of the list.  Clearly the
> author (me) got it wrong the first time because the 0day kernel testing
> robot managed to crash the kernel thanks to this code.
> 
> All this is done in order to do an optimisation aimed at preventing
> fragmentation at the start of a slab.  We can just skip this
> optimisation any time the list is put into an undefined state since this
> only occurs when an allocation completely fills the slab and in this
> case the optimisation is unnecessary since we have not fragmented the slab
> by this allocation.
> 
> Change the page pointer passed to slob_alloc_page() to be a double
> pointer so that we can set it to NULL to indicate that the page was
> removed from the list.  Skip the optimisation if the page was removed.
> 
> Found thanks to the kernel test robot, email subject:
> 
> 	340d3d6178 ("mm/slob.c: respect list_head abstraction layer"):  kernel BUG at lib/list_debug.c:31!
> 

It's regrettable that this fixes
slob-respect-list_head-abstraction-layer.patch but doesn't apply to
that patch - slob-use-slab_list-instead-of-lru.patch gets in the way. 
So we end up with a patch series which introduces a bug and later
fixes it.

I guess we can live with that but if the need comes to respin this
series, please do simply fix
slob-respect-list_head-abstraction-layer.patch so we get a clean
series.