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***** SUSPECTED SPAM ***** Re: [PATCH 48/49] xfs: Add read-only support

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: ***** SUSPECTED SPAM ***** Re: [PATCH 48/49] xfs: Add read-only support for dirent filetype field
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 12 Aug 2013 10:59:05 +1000
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
Importance: Low
In-reply-to: <51F80FA8.4060304@xxxxxxx>
References: <1374215120-7271-1-git-send-email-david@xxxxxxxxxxxxx> <1374215120-7271-49-git-send-email-david@xxxxxxxxxxxxx> <51F80FA8.4060304@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Jul 30, 2013 at 02:10:32PM -0500, Mark Tinguely wrote:
> On 07/19/13 01:25, Dave Chinner wrote:
> >From: Dave Chinner<dchinner@xxxxxxxxxx>
> >
> >Add support for the file type field in directory entries so that
> >readdir can return the type of the inode the dirent points to to
> >userspace without first having to read the inode off disk.
> >
> >The encoding of the type field is a single byte that is added to the
> >end of the directory entry name length. For all intents and
> >purposes, it appends a "hidden" byte to the name field which
> >contains the type information. As the directory entry is already of
> >dynamic size, helpers are already required to access and decode the
> >direct entry structures.
> >
> >Hence the relevent extraction and iteration helpers are updated to
> >understand the hidden byte.  Helpers for reading and writing the
> >filetype field from the directory entries are also added. Only the
> >read helpers are used by this patch.  It also adds all the code
> >necessary to read the type information out of the dirents on disk.
> >
> >Further we add the superblock feature bit and helpers to indicate
> >that we understand the on-disk format change. This is not a
> >compatible change - existing kernels cannot read the new format
> >successfully - so an incompatible feature flag is added. We don't
> >yet allow filesystems to mount with this flag yet - that will be
> >added once write support is added.
> >
> >Finally, the code to take the type from the VFS, convert it to an
> >XFS on-disk type and put it into the xfs_name structures passed
> >around is added, but the directory code does not use this field yet.
> >That will be in the next patch.
> >
> >Signed-off-by: Dave Chinner<dchinner@xxxxxxxxxx>
> >---
> >
> >+static inline int xfs_sb_version_hasftype(struct xfs_sb *sbp)
> >+{
> >+    return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5&&
> >+            xfs_sb_has_incompat_feature(sbp, XFS_SB_FEAT_INCOMPAT_FTYPE);
> >  }
> >
> This feature should support inode version 2 and 3.

Has nothing to do with the inode version number - it has to do with
the directory structure being modified.

We're changing the directory structure for CRCs, and this builds on
top of that. It is essentially part of the V3 directory format, and
should be treated as such. Suggesting that we retrofit and support a
modified v2 directory format is close to insane - instead of only
having to test v2 vs v3 directory formats, you're suggesting we
support v2 vs v2+dtype vs v3 vs v3+dtype. We simply do not have the
resources to adequately test and support such an explosion of
filesystem configurations.

We've had this discussion before - new on-disk features go into the
v5 superblock format - the v4 superblock format from this point
onwards is essentially legacy support from an upstream development
perspective.  Upstream development is about moving forward as
quickly and cleanly as possible - quadrupling the test matrix for
every minor on-disk change is simply not something we can sustain,
and that's why I'm pushing back *hard* on any suggestion that we
shoul dbe supporting new on-disk format changes on both v4 and v5
superblocks formats. v5 is the future and so all new features need
to target v5 filesystem formats and ignore the restrictions that v4
filesystem formats might place on them.

That said, there's nothing to stop anyone from backporting such a
feature to an older kernel and maintaining it themselves - it's open
source software. But the idea that development should be constrained
by having to support both old and new formats is wrong - the old v4
format should be considered stable and we need think very hard about
changing it at all now, especially as much of the development focus
is now shifting to taking advantage of the additions to the v5


Dave Chinner

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