linux-kernel - Re: [PATCH] mm, slab, slub: clear the slab

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Message-ID: <56e5c740-2eaa-daa5-3df9-369f4d8be34a@google.com>
Date:   Thu, 10 Dec 2020 15:46:55 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm, slab, slub: clear the slab_cache field when freeing
 page

On Thu, 10 Dec 2020, Vlastimil Babka wrote:

> The page allocator expects that page->mapping is NULL for a page being freed.
> SLAB and SLUB use the slab_cache field which is in union with mapping, but
> before freeing the page, the field is referenced with the "mapping" name when
> set to NULL.
> 
> It's IMHO more correct (albeit functionally the same) to use the slab_cache
> name as that's the field we use in SL*B, and document why we clear it in a
> comment (we don't clear fields such as s_mem or freelist, as page allocator
> doesn't care about those). While using the 'mapping' name would automagically
> keep the code correct if the unions in struct page changed, such changes should
> be done consciously and needed changes evaluated - the comment should help with
> that.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: David Rientjes <rientjes@...gle.com>