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

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Message-ID: <4AF1A34B.80109@bitbox.co.uk>
Date:	Wed, 04 Nov 2009 15:52:43 +0000
From:	Peter Horton <phorton@...box.co.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ecashin@...aid.com, linux-kernel@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH] prevent AoE causing cache aliases

Andrew Morton wrote:
> On Wed, 04 Nov 2009 10:54:34 +0000 Peter Horton <phorton@...box.co.uk> wrote:
> 
>> 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 ...
>>
> 
> Could.  We'll need to change each arch _somehow_.  Even if it's a
> matter of adding `#define i_am_not_house_trained' to the troublesome
> ones or something, then ifdeffing existing code.
> 
> I was thinking that a general bio_flush_dcache_pages() in block core
> (or in each arch) would be a suitable way to handle this but I was
> unable to find other drivers which needed it after a brief search.
> 
> 

IDE does it at a lower level (arch/mips/include/asm/mach-generic/ide.h 
for example).

Looks like the generic PIO ops in drivers/ata/libata-sff.c could cause 
problems too.

I think the problem is often masked by the small cache sizes on the 
platforms with VI caches. The original AoE problem I see is on an ARM926 
with 32K D-cache, I don't see the problem at all on another ARM926 with 
16K D-cache.

P.
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