linux-ext4 - Re: Incorrect block count during stat of a file if fallocate follows a buffered write.

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Date:   Tue, 2 Jan 2018 11:39:22 -0600
From:   Jayashree Mohan <jayashree2912@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     "Theodore Ts'o" <tytso@....edu>,
        Vijaychidambaram Velayudhan Pillai <vijay@...utexas.edu>
Subject: Re: Incorrect block count during stat of a file if fallocate follows
 a buffered write.

Hi Ted,
I wasn't sure if you missed this mail in your inbox. So I wanted to
send a gentle reminder about the same.

On Tue, Dec 26, 2017 at 4:07 PM, Jayashree Mohan
<jayashree2912@...il.com> wrote:
> Hi,
> I am Jayashree Mohan, a PhD student at the University of Texas at
> Austin, working with Prof. Vijay Chidambaram. I've been working on the
> Crashmonkey project, which is a test harness for file system crash
> consistency[1].
>
> While trying to reproduce some of the reported bugs on ext4 using
> CrashMonkey we stumbled upon the following issue:
> Whenever a fallocate follows a buffered write(delayed allocated), with
> fallocate overlapping with a certain range of the write, the block
> count reported by stat of the file is incorrect until the write is
> resolved.
>
> Consider the following workload(The number within the braces indicate
> the start and end offsets)
> 1. Write (79581 - 136029)
> 2. Fallocate (46385 - 92886)
>
> If we now stat the file, the result is as follows :
>
> 1. Stat after Write (79581 - 136029)
>   File: '/mnt/5gbdisk/only-fallocate.txt'
>   Size: 136029        Blocks: 120        IO Block: 4096   regular file
> Device: 810h/2064d    Inode: 14          Links: 1
>
> 2. Stat after fallocate (46385 - 92886)
>   File: '/mnt/5gbdisk/only-fallocate.txt'
>   Size: 136029        Blocks: 216        IO Block: 4096   regular file
> Device: 810h/2064d    Inode: 14          Links: 1
>
> 3. Stat 30 seconds later, after the checkpoint
>   File: '/mnt/5gbdisk/only-fallocate.txt'
>   Size: 136029        Blocks: 184        IO Block: 4096   regular file
> Device: 810h/2064d    Inode: 14          Links: 1
>
> So we see here that the actual block count should be 184, as there is
> an overlapping region between the buffered write(120 blocks) and
> fallocate(96 blocks). Note that there are 32 overlapping blocks in the
> above workload(Blocks are in units of 512B). However until the write
> is resolved, the block count is shown to be 216, which is just the sum
> of the blocks reserved for writes and fallocate(120+96=216), without
> consideration for the overlap(120+96-32 = 184).
>
> So, if any program stats the file before the write is resolved (the 30
> sec checkpoint interval), the block count returned seems to be
> incorrect.
>
> Let me know if I am missing some detail.
>
> [1] https://github.com/utsaslab/crashmonkey
>
> Thanks,
> Jayashree