linux-kernel - Re: vmalloc_32 and SLAB incompatible in 2.6.23.9

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Message-ID: <p73wsq0lw9r.fsf@crumb.suse.de>
Date:	23 Jan 2008 10:35:28 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Loïc Grenié <loic.grenie@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: vmalloc_32 and SLAB incompatible in 2.6.23.9

"Loïc Grenié" <loic.grenie@...il.com> writes:

>      Since GFP_DMA32 is not considered a valid flag, vmalloc_32 and
>   SLAB are incompatible in kernel 2.6.23.9 (and probably later ones).
>   It might be as simple as adding GFP_DMA32 everywhere where
>   GFP_DMA is used in mm/slab.c, but I don't dare do it. I personally
>   switched to SLUB.

I don't think it would really work for slub either.

If you look at /proc/slabinfo you see that there are separate DMA
caches. That is because sl[abo]b all cache objects and if the 
objects have different requirements (like GFP_DMA, GFP_DMA32, GFP_KERNEL)
they would need different caches. Otherwise you risk getting
later objects originally allocated with the wrong flags.

To make GFP_DMA32 work would require adding another set of DMA32
caches to all kmalloc caches etc or a separate one for each
custom kmem cache that you use that only gets _DMA32 objects.

Better just use __get_free_pages() directly. 

-Andi
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