linux-kernel - Re: [PATCH] Containment measures for slab objects on scatter gather lists

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Message-ID: <20070630001138.6caeb745@the-village.bc.nu>
Date:	Sat, 30 Jun 2007 00:11:38 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, clameter@....com,
	hugh@...itas.com, James.Bottomley@...eleye.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Containment measures for slab objects on scatter gather
 lists

> DMA to or from memory should be done via the DMA mapping API.  If we're
> DMAing to/from a limited range within a page, either we should be using
> dma_map_single(), or dma_map_page() with an appropriate offset and size.

If those ranges overlap a cache line then the dma mapping API will not
save your backside.

On a system with a 32 byte cache granularity what happens if you get two
dma mapping calls for x and x+16. Right now the thing that avoids this
occurring is that the allocators don't pack stuff in that hard so x+16
always belongs to the same driver and we can hope driver authors are
sensible 

> sizes, but they do happen.  We handle this on ARM by writing back
> the overlapped lines and invalidating the rest before the DMA operation
> commences, and hope that the overlapped lines aren't touched for the
> duration of the DMA.)

The combination of "hope" and "DMA" isn't a good one for stable system
design. In this situation we should be waving large red flags
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