linux-ext4 - Re: [PATCH v3] direct-io: fix direct write stale data exposure from concurrent buffered read

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Message-ID: <x49eg8r1bzp.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 24 May 2016 15:39:54 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Eryu Guan <guaneryu@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, viro@...IV.linux.org.uk
Subject: Re: [PATCH v3] direct-io: fix direct write stale data exposure from concurrent buffered read

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Sat, 14 May 2016 00:25:28 +0800 Eryu Guan <guaneryu@...il.com> wrote:
>
>> Currently direct writes inside i_size on a DIO_SKIP_HOLES filesystem are
>> not allowed to allocate blocks(get_more_blocks() sets 'create' to 0
>> before calling get_block() callback), if it's a sparse file, direct
>> writes fall back to buffered writes to avoid stale data exposure from
>> concurrent buffered read. But there're two cases that can result in
>> stale data exposure are not correctly detected.
>> 
>> 1. The detection for "writing inside i_size" is not sufficient, writes
>> can be treated as "extending writes" wrongly. For example, direct write
>> 1FSB to a 1FSB sparse file on ext2/3/4, starting from offset 0, in this
>> case it's writing inside i_size, but 'create' is non-zero, because
>> 'block_in_file' and '(i_size_read(inode) >> blkbits' are both zero.
>
> um, what is an "FSB"?

File System Block, as opposed to a block device block.  :)

-Jeff
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