linux-ext4 - Re: [PATCH] ext4: make sure enough credits are reserved for dioread

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Message-ID: <20181226040713.GA4158@mit.edu>
Date:   Tue, 25 Dec 2018 23:07:13 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Liu Bo <obuil.liubo@...il.com>
CC:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        <stable@...nel.org>
Subject: Re: [PATCH] ext4: make sure enough credits are reserved for
 dioread_nolock writes

On Tue, Dec 25, 2018 at 04:10:25PM -0800, Liu Bo wrote:
> 
> I had a patch[1] to address the problem but with adding reservation
> only when we need to (in ext4_ext_try_to_merge_up).

Sorry, I had forgotten that you had that patch pending.  I believe
that merging up isn't the only case we need to worry about, though.
For example, if we write into the middle of a larger unwritten region,
we might havec to change a single region into three regions, which
might require a node split --- and in that case, it's not optional, as
would be in the try_to_merge_up situation.  If we don't have enough
credits to do a node split our only choices would be mark the file
system as corrupted, or to fail the write with ENOSPC (which would be
very confusing to the user/application).

Cheers,

					- Ted