linux-kernel - Re: [PATCH] prevent AoE causing cache aliases

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Date:	Wed, 04 Nov 2009 10:54:34 +0000
From:	Peter Horton <phorton@...box.co.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ecashin@...aid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] prevent AoE causing cache aliases

Andrew Morton wrote:
> On Thu, 22 Oct 2009 15:22:28 +0100
> phorton@...box.co.uk (Peter Horton) wrote:
> 
>> To: ecashin@...aid.com
> 
> Have you heard back from Ed on this?
>

No.

>> Cc: linux-kernel@...r.kernel.org
>> Subject: [PATCH] prevent AoE causing cache aliases
>> Date: 	Thu, 22 Oct 2009 15:22:28 +0100
>> Sender: linux-kernel-owner@...r.kernel.org
>> User-Agent: Mutt/1.5.9i
>>
>> This patch prevents the AoE block driver from creating cache aliases of
>> page cache pages on machines with virtually indexed caches.
>>
>> Building kernels on an AT91SAM9G20 board without this patch fails with
>> segmentation faults after a couple of passes.
>>
>>
>> Index: linux-2.6.31/drivers/block/aoe/aoecmd.c
>> ===================================================================
>> --- linux-2.6.31.orig/drivers/block/aoe/aoecmd.c	2009-09-09 23:13:59.000000000 +0100
>> +++ linux-2.6.31/drivers/block/aoe/aoecmd.c	2009-10-22 10:24:50.000000000 +0100
>> @@ -735,6 +735,21 @@
>>  	part_stat_unlock();
>>  }
>>  
>> +/*
>> + * Ensure we don't create aliases in VI caches
>> + */
>> +static inline void
>> +killalias(struct bio *bio)
>> +{
>> +	struct bio_vec *bv;
>> +	int i;
>> +
>> +	if (bio_data_dir(bio) == READ)
>> +		__bio_for_each_segment(bv, bio, i, 0) {
>> +			flush_dcache_page(bv->bv_page);
>> +		}
>> +}
>> +
>>  void
>>  aoecmd_ata_rsp(struct sk_buff *skb)
>>  {
>> @@ -853,8 +868,12 @@
>>  
>>  	if (buf && --buf->nframesout == 0 && buf->resid == 0) {
>>  		diskstats(d->gd, buf->bio, jiffies - buf->stime, buf->sector);
>> -		n = (buf->flags & BUFFL_FAIL) ? -EIO : 0;
>> -		bio_endio(buf->bio, n);
>> +		if (buf->flags & BUFFL_FAIL)
>> +			bio_endio(buf->bio, -EIO);
>> +		else {
>> +			killalias(buf->bio);
>> +			bio_endio(buf->bio, 0);
>> +		}
>>  		mempool_free(buf, d->bufpool);
>>  	}
> 
> Looks OK.
> 
> This bugfix will cause a pointless __bio_for_each_segment() busywait
> loop to be executed on architectures for which flush_dcache_page() is a
> no-op.
> 
> We don't have infrastructure to fix that.

Couldn't we add a flag to the bio that users could set to indicate that 
they are not house trained with respect to the D-cache (i.e non-DMA 
drivers). Architectures that needed to could then flush the relevant 
pages in the bio_endio() path somewhere. At the moment all the non-DMA 
block drivers need to be aware of the cache aliasing issue which means 
this problem keeps arising ...

P.
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