From: Andreas Dilger <adilger@dilger.ca>
Subject: Re: [PATCH] fs: make generic_block_fiemap sig-tolerant PING2...
Date: Fri, 19 Dec 2014 16:38:55 -0700
Message-ID: <6ACCC57E-556A-46A9-9758-EA42282A35E8@dilger.ca>
References: <1413552269-32203-1-git-send-email-dmonakhov@openvz.org> <871toikt7c.fsf@openvz.org> <87zjavehsx.fsf@openvz.org> <20141218151247.b6ccd73351f40db66818c07e@linux-foundation.org> <87egrwf1dd.fsf@openvz.org> <20141219133339.5c8afaa0c47760e9b141df49@linux-foundation.org>
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Cc: Dmitry Monakhov <dmonakhov@openvz.org>,
	linux-kernel@vger.kernel.org,
	ext4 development <linux-ext4@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>,
	Al Viro <viro@ZenIV.linux.org.uk>,
	Michael Kerrisk <mtk.manpages@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20141219133339.5c8afaa0c47760e9b141df49@linux-foundation.org>
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On Dec 19, 2014, at 2:33 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Fri, 19 Dec 2014 12:13:50 +0300 Dmitry Monakhov <dmonakhov@openvz.org> wrote:
> 
>> --- a/Documentation/filesystems/fiemap.txt
>> +++ b/Documentation/filesystems/fiemap.txt
>> @@ -196,7 +196,8 @@ struct fiemap_extent_info {
>> };
>> 
>> It is intended that the file system should not need to access any of this
>> -structure directly.
>> +structure directly. Filesystem handlers should be tolerant to signals and return
>> +EINTR once fatal signal received.
> 
> Thanks.  I was concerned about userspace effects and back-compatibility
> issues, because I'd misread fatal_signal_pending() as signal_pending().
> 
> Because it uses fatal_signal_pending(), the effects of this change
> should be indiscernible to userspace, yes?
> 
> I'm now wondering if the above doc update is unneeded and incorrect. 
> Is it likely that the fs handler (fiemap_fill_next_extent) will ever
> consume a large amount of time?  If not then we can leave the logic in
> __generic_block_fiemap() and not bother callees.
> 
> 
> 
> The fix only addresses filesystems which use generic_block_fiemap(). 
> Presumably ocfs2, btrfs, nilfs2, lustre and xfs remain vulnerable to
> the problem you identified?

I don't think they are - those filesystems generate the FIEMAP mapping
by walking the extent tree directly, while the "compat" code for block
based filesystems are (or were) essentially walking every possible
block offset to see if there was anything mapped at that position.

Cheers, Andreas