netdev - Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging

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Message-ID: <87r652i69e.fsf@basil.nowhere.org>
Date:	Sun, 23 Nov 2008 20:36:45 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging

Joerg Roedel <joerg.roedel@....com> writes:
> +/* Hash list to save the allocated dma addresses */
> +static struct list_head dma_entry_hash[HASH_SIZE];

Hash tables should use hlists.

> +static int hash_fn(struct dma_debug_entry *entry)
> +{
> +	/*
> +	 * Hash function is based on the dma address.
> +	 * We use bits 20-27 here as the index into the hash
> +	 */
> +	BUG_ON(entry->dev_addr == bad_dma_address);
> +
> +	return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK;

It would be probably safer to use a stronger hash like FNV
There are a couple to reuse in include/

> +}
> +
> +static struct dma_debug_entry *dma_entry_alloc(void)
> +{
> +	gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
> +
> +	if (in_atomic())
> +		gfp |= GFP_ATOMIC;
> +
> +	return kmem_cache_alloc(dma_entry_cache, gfp);
> +}

While the basic idea is reasonable this function is unfortunately
broken. It's not always safe to allocate memory (e.g. in the block
write out path which uses map_sg). You would need to use
a mempool or something.

Besides the other problem of using GFP_ATOMIC is that it can 
fail under high load and you don't handle this case very well
(would report a bug incorrectly). And stress tests tend to 
trigger that, reporting false positives in such a case is a very very
bad thing, it leads to QA people putting these messages
on their blacklists.

-Andi

-- 
ak@...ux.intel.com
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