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

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Message-ID: <CAOQ4uxg2Vb4fKUcN1nHUtYuapBmUNFNiMZYJaPsjKf5z4EAEuA@mail.gmail.com>
Date:   Sun, 21 Oct 2018 14:02:57 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Phillip Potter <phil@...lpotter.co.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Evgeniy Dushistov <dushistov@...l.ru>,
        Al Viro <viro@...iv.linux.org.uk>, David.Laight@...lab.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <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 12:57 PM Phillip Potter <phil@...lpotter.co.uk> wrote:
>
> 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?
>

Yes. If you are looking for a cleanup task, you can
apply relevant patches from my series, starting with:
https://patchwork.kernel.org/patch/9481237/
(Leave the xfs patch [11/11] out)

But besides verifying that patches still apply and build,
you will need to address the concerns of fs maintainers.
Take for example the btrfs patch:
https://patchwork.kernel.org/patch/9480725/

It says:
+ *
+ * Values 0..7 should match common file type values in file_type.h.
  */
 #define BTRFS_FT_UNKNOWN 0
 #define BTRFS_FT_REG_FILE 1

But that is not enough.
When converting code to use the generic defines FT_*, instead of
filesystem defined we need to leave in the code build time assertions
that will catch an attempt to change fs contancts in the future, e.g.:

static inline u8 btrfs_inode_type(struct inode *inode)
 {
- return btrfs_type_by_mode[(inode->i_mode & S_IFMT) >> S_SHIFT];
+ BUILD_BUG_ON(BTRFS_FT_UNKNOWN != FT_UNKNOWN);
+ BUILD_BUG_ON(BTRFS_FT_REG_FILE != FT_REG_FILE);
...
+ return fs_umode_to_ftype(inode->i_mode);
 }

Same should be done for all relevant filesystems.
Then you need to hope that fs maintainers will like this cleanup and
want to take the patches ;-)

Cheers,
Amir.