linux-kernel - Re: [PATCH] block: replace __getblk_slow misfix by grow_dev

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Message-ID: <x49obm1fil0.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 23 Aug 2012 16:40:11 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	"Richard W.M. Jones" <rjones@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Torsten Hilbrich <torsten.hilbrich@...unet.com>,
	Josh Boyer <jwboyer@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: replace __getblk_slow misfix by grow_dev_page fix

Hugh Dickins <hughd@...gle.com> writes:

> [PATCH] block: replace __getblk_slow misfix by grow_dev_page fix
>
> Commit 91f68c89d8f3 ("block: fix infinite loop in __getblk_slow")
> is not good: a successful call to grow_buffers() cannot guarantee
> that the page won't be reclaimed before the immediate next call to
> __find_get_block(), which is why there was always a loop there.
[snip]
> Revert 91f68c89d8f3, restoring __getblk_slow() to how it was (plus
> a checkpatch nitfix).  Simplify the interface between grow_buffers()
> and grow_dev_page(), and avoid the infinite loop beyond end of device
> by instead checking init_page_buffers()'s end_block there (I presume
> that's more efficient than a repeated call to blkdev_max_block()),
> returning -ENXIO to __getblk_slow() in that case.
>
> And remove akpm's ten-year-old "__getblk() cannot fail ... weird"
> comment, but that is worrying: are all users of __getblk() really
> now prepared for a NULL bh beyond end of device, or will some oops??

Hi, Hugh,

Thanks for digging into this.  I had wondered whether that loop was
required for other purposes, and you've verified that it was.  I mostly
like your fix.  Returning end_block from init_page_buffers is a little
strange, but I understand not wanting to redo the call to
blkdev_max_block.

I went ahead to re-tested with the reproducer that I had, and your patch
works fine.  Thanks again for tracking this down, and sorry I wasn't
more diligent to begin with.

Reviewed-and-Tested-by: Jeff Moyer <jmoyer@...hat.com>
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