linux-kernel - Re: [PATCH] Introduce a method to catch mmap_region (was: Recent kernel "mount" slow)

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Date:	Wed, 28 Nov 2012 20:20:14 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Jens Axboe <axboe@...nel.dk>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>
Subject: Re: [PATCH] Introduce a method to catch mmap_region (was: Recent
 kernel "mount" slow)



On Wed, 28 Nov 2012, Linus Torvalds wrote:

> A bigger issue is for things that emulate what blkdev.c does, and
> doesn't do the locking. I see code in md/bitmap.c that seems a bit
> suspicious, for example. That said, it's not *new* breakage, and the
> "lock at mmap/read/write() time" approach doesn't fix it either (since
> the mapping will be different for the underlying MD device). So I do
> think that we should take a look at all the users of
> "alloc_page_buffers()" and "create_empty_buffers()" to see what *they*
> do to protect the block-size, but I think that's an independent issue
> from the raw device access case in fs/block_dev.c..
> 
> I guess I have to actually test my patch. I don't have very
> interesting test-cases, though.
> 
>                        Linus

Yes, it md looks suspicious. It disables write access in 
deny_bitmap_write_access (that functions looks buggy on its own - what 
happens if i_writecount == 0 or if it is already negative on entry?).

So we could disallow changing block size if i_writecount < 0, that should 
do the trick.

Mikulas
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