linux-kernel - [PATCH 0/1] JFFS2: Less locking when reading directory entries

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Date:	Mon, 18 May 2015 16:54:00 +1200
From:	Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
To:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 0/1] JFFS2: Less locking when reading directory entries

I have posted this before, but have extended the patch into a few more
functions. The intent of the code is as before -- to improve JFFS2 lookups
by not locking i_mutex for long periods when files are not in cache. For
our embedded environment, we see a *five second* improvement in boot time.

This patch is an attempt to improve the speed of JFFS2 at startup. Our
particular problem is that we have a 30MB file on NOR flash which takes
about five seconds to read and test the data CRCs. During this time access
to other files in the same directory is blocked, due to
parent->d_inode->i_mutex being locked.

This patch solves this problem by adding a 'pre-lookup' call down to JFFS2,
which can be called without this mutex held. When the actual lookup is
performed, the results are in JFFS2's cache, and the dentry can be filled
in quickly.

However, given that I do not have experience at Linux filesystem code,
I can't be sure that this is a correct solution, or that there isn't a
better way of achieving what I'm trying to do. I feel there must be a way
to do this without creating a new VFS function call.

I suspect other filesystems could benefit from this too, as a lot of them
call the same d_splice_alias() function to fill in the dentry. JFFS2
already seems to have all the lower-level locks that are needed for this to
work; I don't know if that's true in other filesystems which could be
relying on the directory's i_mutex being locked. Because JFFS2 needs to
walk the entire file, there are big gains to be made here; other filesystems
may gain little to nothing.

I'm not expecting that this patch will get applied as-is, but please let me
know if there is any merit to it, whether it should work, and what still
needs to be done to if this is to be made part of the kernel.

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