linux-kernel - Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

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Date:   Wed, 05 Apr 2017 08:50:35 +1000
From:   NeilBrown <neilb@...e.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, akpm@...ux-foundation.org,
        tytso@....edu, jack@...e.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

On Tue, Apr 04 2017, Matthew Wilcox wrote:

> On Tue, Apr 04, 2017 at 12:25:46PM -0400, Jeff Layton wrote:
>> That said, I think giving more specific errors where we can is useful.
>> When your program is erroring out and writing 'I/O error' to the logs,
>> then how much time will your admins burn before they figure out that it
>> really failed because the filesystem was full?
>
> df is one of the first things I check ... a few years ago, I also learned
> to check df -i ... ;-)
>
> Anyway, given the decision to simply report the last error lets us do this
> implementation:
>
> void filemap_set_wb_error(struct address_space *mapping, int err)
> {
> 	struct inode *inode = mapping->host;
> 	unsigned int wb_err;
>
> 	if (!err)
> 		return;
> 	/*
> 	 * This should be called with the error code that we want to return
> 	 * on fsync. Thus, it should always be <= 0.
> 	 */
> 	WARN_ON(err > 0 || err < -MAX_ERRNO);
>
> 	spin_lock(&inode->i_lock);
> 	wb_err = ((mapping->wb_err & ~MAX_ERRNO) + (1 << 12)) | -err;

Seriously?  You are missing "MAX_ERRNO" (4095) together with "1 << 12"
(4096) in the one expression without a big comment saying why?

Surely:
>       BUILD_BUG_ON_NOT_POWER_OF_2(MAX_ERRNO+1);
> 	wb_err = ((mapping->wb_err & ~MAX_ERRNO) + (MAX_ERRNO+1)) | -err;

NeilBrown

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