linux-ext4 - Re: [PATCH] ext4: add ext4_sb_bread() to disambiguate ENOMEM cases

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Date:   Thu, 22 Nov 2018 16:12:33 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: add ext4_sb_bread() to disambiguate ENOMEM cases

On Thu, Nov 22, 2018 at 01:21:55PM +0100, Jan Kara wrote:
> 
> Is there a reason for REQ_PRIO? I'm not sure all REQ_META reads are really
> a priority ones...

Hmm, good question.  With the exception of readahead, most reads will
be blocking some process.  We are currently using REQ_PRIO for all
directory block reads.  The ext4_sb_read() function gets used for
resizing, indirect block map to extent tree migration, and extended
attribute reads.  The last is the most common, and arguably the most
justifiable to be REQ_PRIO.  (Of course my understanding is that the
block layer is ignoring REQ_PRIO, so this is mostly academic...)

So I think what I'll do is this.  I'll add a parameter to
ext4_sb_read() so each caller can use use REQ_PRIO.  REQ_PRIO will be
used from xattr.c, but not from fs/ext4/migrate.c and
fs/ext4/resize.c.

						- Ted