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Date: Wed, 02 Apr 2008 15:06:43 -0400 From: Jeff Garzik <jeff@...zik.org> To: David Miller <davem@...emloft.net> CC: peterz@...radead.org, kosaki.motohiro@...fujitsu.com, akpm@...ux-foundation.org, csnook@...hat.com, davej@...emonkey.org.uk, nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: GFP_ATOMIC page allocation failures. David Miller wrote: > From: Peter Zijlstra <peterz@...radead.org> > Date: Wed, 02 Apr 2008 13:04:59 +0200 > >> Would we not hope that most net drivers can handle {,net}dev_alloc_skb() >> failing? Otherwise we have some serious trouble. > > False presumption. > > Most can but some legacy ones really do not handle this well. Correct... In addition, I remain worried about potential edge cases in drivers that do "burst refill" style allocations, rather than the [IMO safer] as-you-go style that tg3 employs. I haven't done a serious audit, but I have nagging doubts about the behavior of burst-refill drivers when faced with a burst of failed allocations... do they stop DMA operation, or do they permit hardware to wander into part of the DMA ring where, in the previous "cycle" through the ring, valid DMA addresses were placed. But now with a string of allocation failures, you have a sequence of DMA ring descriptors that point to invalid memory. If the DMA engine is not stopped, or in some other way made to avoid those invalid DMA descriptors, then you can easily run into problems. Sometimes, it's as easy as making sure to properly police the 'OWN' bit of each descriptor, something easily and commonly done as a matter of course. But modern hardware with multi-descriptor packets make things more complex, so it is actually getting more difficult to get these details right. </ramble> Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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