linux-kernel - Re: [patch] rewrite rd

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Message-ID: <20080114172114.GR6258@kernel.dk>
Date:	Mon, 14 Jan 2008 18:21:15 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>, rob@...dley.net
Subject: Re: [patch] rewrite rd

On Mon, Jan 14 2008, Matthew Wilcox wrote:
> On Tue, Dec 04, 2007 at 05:26:28AM +0100, Nick Piggin wrote:
> > +static void copy_to_brd(struct brd_device *brd, const void *src,
> > +			sector_t sector, size_t n)
> > +{
> > +	struct page *page;
> > +	void *dst;
> > +	unsigned int offset = (sector & (PAGE_SECTORS-1)) << SECTOR_SHIFT;
> > +	size_t copy;
> > +
> > +	copy = min((unsigned long)n, PAGE_SIZE - offset);
> > +	page = brd_lookup_page(brd, sector);
> > +	BUG_ON(!page);
> > +
> > +	dst = kmap_atomic(page, KM_USER1);
> > +	memcpy(dst + offset, src, copy);
> > +	kunmap_atomic(dst, KM_USER1);
> 
> You're using kmap_atomic, but I see no reason you can't be preempted.
> Don't you need to at least disable preemption while you have stuff
> atomically kmapped?

kmap_atomic() disables preemption through pagefault_disable().

-- 
Jens Axboe

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