linux-kernel - Re: [PATCH] fix bmap-vs-truncate race

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Date:	Tue, 31 Mar 2009 18:42:34 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	viro@...iv.linux.org.uk,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix bmap-vs-truncate race

On Tue, 31 Mar 2009, Christoph Hellwig wrote:

> On Mon, Mar 30, 2009 at 03:20:24PM -0400, Mikulas Patocka wrote:
> > Hi
> > 
> > I'm submitting this patch for 2.6.30 merge window.
> 
> Please not.  i_alloc_sem is really a horrible hack needed for a couple
> filesystems only and we should not leak it into more generic code but
> rather move the few instances into the filesystem.

Could you please document locking rules for get_block(), truncate, bmap & 
direct i/o in Documentation/filesystems/Locking ?

There is a lot of text about directories, but nothing about locking of 
block mappings.

I was living under an impression that get_block() cannot be called on a 
block that is being truncated. That's what read/write/direct-io vs 
truncate seems to guarante --- truncate will first lower i_size 
(preventing any new pages past i_size from being created), then destroy 
any existing pages past i_size (that includes waiting for pagelock until 
all get_blocks on that page end) and finally truncate the metadata on the 
filesystem.

So there should be no situation when you truncate block and call get_block 
on it simultaneously. If get_block can race with truncate, document it.

There are filesystems that don't do any locking on get_block() (for 
example UFS, HPFS; FAT does it only for bmap and doesn't do it for general 
accesses) and other filesystems verify indirect block chains obsessively 
if they were truncated under get_block (why? because of bmap? or some 
other possibility?) --- so the rules should really be documented.

Mikulas
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