linux-kernel - Re: [patch] ext2: xip check fix

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Date:	Thu, 6 Dec 2007 09:52:23 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	carsteno@...ibm.com
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>, rob@...dley.net,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [patch] ext2: xip check fix

On Thu, Dec 06, 2007 at 09:43:27AM +0100, Carsten Otte wrote:
> Nick Piggin wrote:
> >>Xip does only work, if both do match PAGE_SIZE because it 
> >>does'nt support multiple calls to direct_access in the get_xip_page 
> >>address space operation. Thus we check both here, actually this was 
> >>changed from how it looks after your patch as a bugfix where our 
> >>tester tried a 4k filesystem on a 2k blockdev.
> >>Did I miss something?
> >
> >However, the bdev block size may be changed with sb_set_blocksize. It
> >doesn't actually have to match the hardware sector size -- if this
> >does matter for XIP, then I think you need some other check here.
> Hmmmmhh. For a bdev with PAGE_SIZE hardsect size, there is no other 
> valid value then PAGE_SIZE that one could set it to. Or can it indeed 
> be changed to a value greater then PAGE_SIZE or smaller then hardsect 
> size?

It can't be made smaller (or larger, in current kernels). But you
already get all that checking done for you -- both by checking that
the filesystem blocksize == PAGE_SIZE, and by the error checking in
sb_set_blocksize.

After my patch, we can do XIP in a hardsect size < PAGE_SIZE block
device -- this seems to be a fine thing to do at least for the
ramdisk code. Would this situation be problematic for existing drivers,
and if so, in what way?

Thanks,
Nick

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