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Re: [PATCH 21/21] xfs: add CRC checks to the superblock

To: Chandra Seetharaman <sekharan@xxxxxxxxxx>
Subject: Re: [PATCH 21/21] xfs: add CRC checks to the superblock
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 27 Mar 2013 12:06:25 +1100
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1364331527.32345.135.camel@xxxxxxxxxxxxxxxxxx>
References: <1363091454-8852-1-git-send-email-david@xxxxxxxxxxxxx> <1363091454-8852-22-git-send-email-david@xxxxxxxxxxxxx> <1364331527.32345.135.camel@xxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Mar 26, 2013 at 03:58:47PM -0500, Chandra Seetharaman wrote:
> On Tue, 2013-03-12 at 23:30 +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > With the addition of CRCs, there is such a wide and varied change to
> > the on disk format that it makes sense to bump the superblock
> > version number rather than try to use feature bits for all the new
> > functionality.
> > 
> > This commit introduces all the new superblock fields needed for all
> > the new functionality: feature masks similar to ext4, separate
> > project quota inodes, a LSN field for recovery and the CRC field.
> > 
> > This commit does not bump the superblock version number, however.
> > That will be done as a separate commit at the end of the series
> > after all the new functionality is present so we switch it all on in
> > one commit. This means that we can slowly introduce the changes
> > without them being active and hence maintain bisectability of the
> > tree.
> > 
> > This patch is based on a patch originally written by myself back
> > from SGI days, which was subsequently modified by Christoph Hellwig.
> > There is relatively little of that patch remaining, but the history
> > of the patch still should be acknowledged here.
> > 
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > ---
> >  fs/xfs/xfs_buf_item.h    |    4 +-
> >  fs/xfs/xfs_log_recover.c |    8 ++++
> >  fs/xfs/xfs_mount.c       |   97 
> > ++++++++++++++++++++++++++++++++++++--------
> >  fs/xfs/xfs_mount.h       |    1 +
> >  fs/xfs/xfs_sb.h          |  100 
> > ++++++++++++++++++++++++++++++++--------------
> >  5 files changed, 164 insertions(+), 46 deletions(-)
> 
> <snip>
.....
> > @@ -276,6 +303,11 @@ typedef enum {
> >  #define XFS_SB_FDBLOCKS            XFS_SB_MVAL(FDBLOCKS)
> >  #define XFS_SB_FEATURES2   XFS_SB_MVAL(FEATURES2)
> >  #define XFS_SB_BAD_FEATURES2       XFS_SB_MVAL(BAD_FEATURES2)
> > +#define XFS_SB_FEATURES_COMPAT     XFS_SB_MVAL(FEATURES_COMPAT)
> > +#define XFS_SB_FEATURES_RO_COMPAT XFS_SB_MVAL(FEATURES_RO_COMPAT)
> > +#define XFS_SB_FEATURES_INCOMPAT XFS_SB_MVAL(FEATURES_INCOMPAT)
> > +#define XFS_SB_CRC         XFS_SB_MVAL(CRC)
> > +#define XFS_SB_PQUOTINO            XFS_SB_MVAL(PQUOTINO)
> 
> missing define for XFS_SB_LSN ?!

No, not at all. We only have bits for variables that get logged. The
LSn of the object is never logged as it is stamped into the object
when it is being written back. i.e. it is an indication of when the
object was last modified, not a field that is recorded during
modifications.

> > @@ -325,6 +358,8 @@ static inline int xfs_sb_good_version(xfs_sb_t *sbp)
> > 
> >             return 1;
> >     }
> > +   if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5)
> > +           return 1;
> > 
> >     return 0;
> >  }
> > @@ -365,7 +400,7 @@ static inline int xfs_sb_version_hasattr(xfs_sb_t *sbp)
> >  {
> >     return sbp->sb_versionnum == XFS_SB_VERSION_2 ||
> >             sbp->sb_versionnum == XFS_SB_VERSION_3 ||
> > -           (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > +           (XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4 &&
> >              (sbp->sb_versionnum & XFS_SB_VERSION_ATTRBIT));
> >  }
> 
> Do you expect version 5 and later have this bit to be exclusively set to
> use attribues ? (i.e can't we just assume version 5 and later would have
> attributes ?)

This just indicates that there are inodes with attributes in the
filesystem. It's not an "enabling" feature at all. If it's not
present when the first attribute is created, then the kernel code
will set it. Hence all this does it maintain the existing
behaviour....

> Since we are changing the version, can't we get rid of some on these
> bits (i.e slowly deprecate them) ? 

No, because we still support version 4 superblocks and hence all
these bit for the indefinite future (i.e. forever).

> > @@ -373,7 +408,7 @@ static inline void xfs_sb_version_addattr(xfs_sb_t *sbp)
> >  {
> >     if (sbp->sb_versionnum == XFS_SB_VERSION_1)
> >             sbp->sb_versionnum = XFS_SB_VERSION_2;
> > -   else if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4)
> > +   else if (XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4)
> >             sbp->sb_versionnum |= XFS_SB_VERSION_ATTRBIT;
> >     else
> >             sbp->sb_versionnum = XFS_SB_VERSION_4 | XFS_SB_VERSION_ATTRBIT;
> > @@ -382,7 +417,7 @@ static inline void xfs_sb_version_addattr(xfs_sb_t *sbp)
> >  static inline int xfs_sb_version_hasnlink(xfs_sb_t *sbp)
> >  {
> >     return sbp->sb_versionnum == XFS_SB_VERSION_3 ||
> > -            (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > +            (XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4 &&
> >               (sbp->sb_versionnum & XFS_SB_VERSION_NLINKBIT));
> 
> Same here

This is for version 1/2 inodes and which formats are present in the
filesystem. It has been set by default in mkfs for a long time.
Version 5 superblocks don't change this. This bit can also be set on
demand by the kernel if it is not present and an inode requires a
link count of > 2^16.

Further, CRCs require v3 inodes, so this is only relevant for
non-crc enabled filesystems.

> > @@ -396,13 +431,13 @@ static inline void xfs_sb_version_addnlink(xfs_sb_t 
> > *sbp)
> > 
> >  static inline int xfs_sb_version_hasquota(xfs_sb_t *sbp)
> >  {
> > -   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > +   return XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4 &&
> >             (sbp->sb_versionnum & XFS_SB_VERSION_QUOTABIT);
> 
> same here

Set when quotas are turned on by the kernel. Behaves the same for
v4/v5 superblocks.

> >  }
> > 
> >  static inline void xfs_sb_version_addquota(xfs_sb_t *sbp)
> >  {
> > -   if (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4)
> > +   if (XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4)
> >             sbp->sb_versionnum |= XFS_SB_VERSION_QUOTABIT;
> >     else
> >             sbp->sb_versionnum = xfs_sb_version_tonew(sbp->sb_versionnum) |
> > @@ -411,13 +446,14 @@ static inline void xfs_sb_version_addquota(xfs_sb_t 
> > *sbp)
> > 
> >  static inline int xfs_sb_version_hasalign(xfs_sb_t *sbp)
> >  {
> > -   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > -           (sbp->sb_versionnum & XFS_SB_VERSION_ALIGNBIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> > +          (XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4 &&
> > +           (sbp->sb_versionnum & XFS_SB_VERSION_ALIGNBIT));
> >  }
> 
> same here

mkfs already sets this bit unconditionally for v4 superblocks, so V5
makes it mandatory.

> >  static inline int xfs_sb_version_hasdalign(xfs_sb_t *sbp)
> >  {
> > -   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > +   return XFS_SB_VERSION_NUM(sbp) >= XFS_SB_VERSION_4 &&
> >             (sbp->sb_versionnum & XFS_SB_VERSION_DALIGNBIT);
> >  }
> 
> same here

Necessary to indicate sunit/swidth are set at mkfs time, indicating
alignment restrictions are present on filesystem layout. Behaves the
same for v4/v5 superblocks.

> > @@ -429,38 +465,42 @@ static inline int xfs_sb_version_hasshared(xfs_sb_t 
> > *sbp)
> > 
> >  static inline int xfs_sb_version_hasdirv2(xfs_sb_t *sbp)
> >  {
> > -   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > -           (sbp->sb_versionnum & XFS_SB_VERSION_DIRV2BIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> 
> Why some places we have version == 5 and others (version == 5 && (XXX))

Because some things are simply set as supported on v5 superblocks,
while other are still optional or indicative of on-disk formats in
use.

> >  static inline int xfs_sb_version_hasmorebits(xfs_sb_t *sbp)
> >  {
> > -   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > -           (sbp->sb_versionnum & XFS_SB_VERSION_MOREBITSBIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> > +          (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_4 &&
> > +           (sbp->sb_versionnum & XFS_SB_VERSION_MOREBITSBIT));
> 
> Gets more confusing here. version 5 means it has more bits ? (even if
> the MOREBITSBIT is not set ?
> 
> May be we shouldn't add version 5 check here ?!

The MOREBITSBIT bit means that the sb_features2 field exists in the
on disk structure. That is clearly true for all v5 superblocks, and
so we don't need to care if the MOREBITSBIT is set or not.

> > @@ -475,14 +515,16 @@ static inline int xfs_sb_version_hasmorebits(xfs_sb_t 
> > *sbp)
> > 
> >  static inline int xfs_sb_version_haslazysbcount(xfs_sb_t *sbp)
> >  {
> > -   return xfs_sb_version_hasmorebits(sbp) &&
> > -           (sbp->sb_features2 & XFS_SB_VERSION2_LAZYSBCOUNTBIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> > +          (xfs_sb_version_hasmorebits(sbp) &&
> > +           (sbp->sb_features2 & XFS_SB_VERSION2_LAZYSBCOUNTBIT));
> >  }
> 
> If we remove the version 5 check in xfs_sb_hasmorebits(), then this
> change is correct. Otherwise, this change is not needed.

No, this change means lazy superblock counters are a fixed feature
of v5 superblocks - it is not optional like it currently is. The
check inside xfs_sb_version_hasmorebits() is to determine if the
features2 field is present, not that the lazycount feature is
supported....

> >  static inline int xfs_sb_version_hasattr2(xfs_sb_t *sbp)
> >  {
> > -   return xfs_sb_version_hasmorebits(sbp) &&
> > -           (sbp->sb_features2 & XFS_SB_VERSION2_ATTR2BIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> > +          (xfs_sb_version_hasmorebits(sbp) &&
> > +           (sbp->sb_features2 & XFS_SB_VERSION2_ATTR2BIT));
> >  }
> 
> If we remove the version 5 check in xfs_sb_hasmorebits(), then this
> change is correct. Otherwise, this change is not needed.

Same again - v5 implies attr2 behaviour is the only attr fork
behaviour supported.

> >  static inline void xfs_sb_version_addattr2(xfs_sb_t *sbp)
> > @@ -500,14 +542,14 @@ static inline void 
> > xfs_sb_version_removeattr2(xfs_sb_t *sbp)
> > 
> >  static inline int xfs_sb_version_hasprojid32bit(xfs_sb_t *sbp)
> >  {
> > -   return xfs_sb_version_hasmorebits(sbp) &&
> > -           (sbp->sb_features2 & XFS_SB_VERSION2_PROJID32BIT);
> > +   return (XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5) ||
> > +          (xfs_sb_version_hasmorebits(sbp) &&
> > +           (sbp->sb_features2 & XFS_SB_VERSION2_PROJID32BIT));
> >  }
> If we remove the version 5 check in xfs_sb_hasmorebits(), then this
> change is correct. Otherwise, this change is not needed.

Same again - v5 superblocks *always* support 32 bit project ids.

> >  static inline int xfs_sb_version_hascrc(xfs_sb_t *sbp)
> >  {
> > -   return (xfs_sb_version_hasmorebits(sbp) &&
> > -           (sbp->sb_features2 & XFS_SB_VERSION2_CRCBIT));
> > +   return XFS_SB_VERSION_NUM(sbp) == XFS_SB_VERSION_5;
> 
> what happens to the superblocks that already have version 4 and this bit
> set ?

It makes the XFS_SB_VERSION2_CRCBIT an invalid feature bit - it has
never, ever been used by anything - so any filesystem with that bit
set will fail to mount.

> Also don't xfs_sb_version_toold() and xfs_sb_version_tonew() need
> changes ?

No. the code that calls xfs_sb_version_tonew() only does so for
superblocks whose version is <= 3. i.e. filesystems that came from
Irix 6.2 or earlier....

> I see that xfs_sb_version_toold() is not used anywhere, can we remove
> it ?

Used by userspace, and seeing as this file is common between kernel
and userspace, it hasn't been removed as that simply imposes higher
maintenance cost on us.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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