linux-kernel - Re: [PATCH] fs: ufs: Remove switch statement from ufs_set_de

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Message-ID: <20181021095710.GA3482@pathfinder>
Date:   Sun, 21 Oct 2018 10:57:10 +0100
From:   Phillip Potter <phil@...lpotter.co.uk>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     willy@...radead.org, dushistov@...l.ru, viro@...iv.linux.org.uk,
        David.Laight@...lab.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: ufs: Remove switch statement from ufs_set_de_type
 function

On Sun, Oct 21, 2018 at 08:30:35AM +0300, Amir Goldstein wrote:
> On Sun, Oct 21, 2018 at 1:27 AM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Sat, Oct 20, 2018 at 11:09:57PM +0100, Phillip Potter wrote:
> > > Remove switch statement from ufs_set_de_type function in fs/ufs/util.h
> > > header and replace with simple assignment. For each case, S_IFx >> 12
> > > is equal to DT_x, so in valid cases (mode & S_IFMT) >> 12 should give
> > > us the correct file type. It is expected that for *nix compatibility
> > > reasons, the relation between S_IFx and DT_x will not change. For
> > > cases where the mode is invalid, upper layer validation catches this
> > > anyway, so this improves readability and arguably performance by
> > > assigning (mode & S_IFMT) >> 12 directly without any jump table
> > > or conditional logic.
> >
> > Shouldn't we also do this for other filesystems?  A quick scan suggests
> > someone should at least look at xfs, ubifs, squashfs, romfs, ocfs2,
> > nilfs2, hfsplus, f2fs, ext4, ext2, exofs, coda, cifs & btrfs.
> >
> 
> I've tried to do that 2 years ago, not even for readability, but to
> fix a lurking
> bug in conversion table implementation that was copy&pasted from ext2 to
> many other fs:
> https://marc.info/?l=linux-fsdevel&m=148217829301701&w=2
> https://marc.info/?l=linux-fsdevel&w=2&r=1&s=amir73il+file+type+conversion&q=b
> 
> The push back from ext4/xfs maintainers was that while all fs use common
> values to encode d_type in on disk format, those on-disk format values
> are private to the fs.
> 
> Eventually, the fix that got merged (only to xfs) did the opposite, it converted
> the conversion table lookup to a switch statement:
> https://marc.info/?l=linux-fsdevel&m=148243866204444&w=2
> 
> Some fs maintainers were happy about the initial version, but I did not
> pursue pushing that cleanup:
> https://marc.info/?l=linux-fsdevel&m=148243866204444&w=2
> 
> Phillip, you are welcome to re-spin those patches if you like.
> Note that the generic conversion helpers do not rely on upper layer
> to catch invalid mode values.
> 
> Cheers,
> Amir.

I only changed this code in the first place because of the comment in the code -
I was looking for things to do basically, and this caught my eye because I play
with FreeBSD now and then which of course uses UFS. I didn't consider any use
to the other filesystems - by re-spin do you just mean fix up and make sure
the kernel still builds etc?

Regards,
Phil