linux-kernel - Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.0807151210400.31336@devserv.devel.redhat.com>
Date:	Tue, 15 Jul 2008 12:20:59 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, davem@...emloft.net,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments
 about VMERGE

On Tue, 15 Jul 2008, James Bottomley wrote:

> On Tue, 2008-07-15 at 11:58 -0400, Mikulas Patocka wrote:
>> You are mixing two ideas here:
>>
>> (1) virtual merging --- IOMMU maps discontinuous segments into continuous
>> area that it presents to the device.
>>
>> (2) virtual merge accounting --- block layer tries to guess how many
>> segments will be created by (1) and merges small requests into big ones.
>> The resulting requests are as big that they can't be processed by the
>> device if (1) weren't in effect.
>
> No ... I'm not ... the virtual merge implementation requires the block
> layer to get this accounting right, otherwise the iommu code can end up
> doing the wrong thing.

The virtual merge (1) can work even without accounting (2). IOMMU can 
always create less sg entries then the block layer expects.

> You're proposing to eliminate the difference between max_phys_segments
> and max_hw_segments without actually removing them.

Yes. Only for alpha and pa-risc, there is difference between these values. 
And both of these architectures are being discontinued.

>> That's why I'm proposing to remove virtual merge accounting (2), but leave
>> virtual merging (1) itself. The accounting doesn't reduce number of sg
>> slots.
>
> Yes, but it's gains very little ... architectures that don't want it can
> already turn it off, and it's useful for those, like parisc, who do.
>
> James

It increases maintainability of the code, reduces bloat and bugs.

Mikulas
--
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/