Re: [PATCH] common/populate: Ensure that S_IFDIR.FMT_BTREE is in btree format

Gao Xiang
2022-12-07T02:12:07Z

Hi Dave,

On Wed, Dec 07, 2022 at 10:34:17AM +1100, Dave Chinner wrote:
> On Fri, Dec 02, 2022 at 10:23:07AM +0800, Gao Xiang wrote:
> > Hi Darrick,
> >
> > On Thu, Dec 01, 2022 at 07:52:44AM -0800, Darrick J. Wong wrote:
> > > On Thu, Dec 01, 2022 at 04:12:08PM +0800, Gao Xiang wrote:
> > > > Sometimes "$((128 * dblksz / 40))" dirents cannot make sure that
> > > > S_IFDIR.FMT_BTREE could become btree format for its DATA fork.
> > > >
> > > > Actually we just observed it can fail after apply our inode
> > > > extent-to-btree workaround. The root cause is that the kernel may be
> > > > too good at allocating consecutive blocks so that the data fork is
> > > > still in extents format.
> > > >
> > > > Therefore instead of using a fixed number, let's make sure the number
> > > > of extents is large enough than (inode size - inode core size) /
> > > > sizeof(xfs_bmbt_rec_t).
> > > >
> > > > Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
> > > > Cc: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
> > > > Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> > > > ---
> > > > common/populate | 22 +++++++++++++++++++++-
> > > > 1 file changed, 21 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/common/populate b/common/populate
> > > > index 6e004997..e179a300 100644
> > > > --- a/common/populate
> > > > +++ b/common/populate
> > > > @@ -71,6 +71,25 @@ __populate_create_dir() {
> > > > done
> > > > }
> > > >
> > > > +# Create a large directory and ensure that it's a btree format
> > > > +__populate_create_btree_dir() {
> > >
> > > Since this encodes behavior specific to xfs, this ought to be called
> > >
> > > __populate_xfs_create_btree_dir
> > >
> > > or something like that.
> > >
> > > > + name="$1"
> > > > + isize="$2"
> > >
> > > These ought to be local variables, e.g.
> > >
> > > local name="$1"
> > > local isize="$2"
> > >
> > > So that they don't pollute the global name scope. Yay bash.
> > >
> > > > +
> > > > + mkdir -p "${name}"
> > > > + d=0
> > > > + while true; do
> > > > + creat=mkdir
> > > > + test "$((d % 20))" -eq 0 && creat=touch
> > > > + $creat "${name}/$(printf "%.08d" "$d")"
> > > > + if [ "$((d % 40))" -eq 0 ]; then
> > > > + nexts="$($XFS_IO_PROG -c "stat" $name | grep 'fsxattr.nextents' | sed -e 's/^.*nextents = //g' -e 's/\([0-9]*\).*$/\1/g')"
> > >
> > > _xfs_get_fsxattr...
>
> The grep/sed expression is also overly complex - it can easily be
> replaced with just this:
>
> nexts=`$XFS_IO_PROG -c "stat" $name | sed -ne 's/^fsxattr.nextents = //p'
>
> The "-n" option to sed suppresses the printing of the stream
> (pattern space) to the output as it processes the input, which gets
> rid of the need for using grep to suppress non-matching input. The "p"
> suffix to the search string forces matched patterns to be printed to
> the output.
>
> This results in only matched, substituted pattern spaces to be
> printed, avoiding the need for grep and multiple sed regexes to be
> matched to strip the text down to just the integer string.

I just copied it from another reference at that time as a copy-and-paste
engineer.. Also note that Ziyang's new patch already use
_xfs_get_fsxattr to get this field.

>
> > > > + [ "$nexts" -gt "$(((isize - 176) / 16))" ] && break
> > >
> > > Only need to calculate this once if you declare this at the top:
> > >
> > > # We need enough extents to guarantee that the data fork is in
> > > # btree format. Cycling the mount to use xfs_db is too slow, so
> > > # watch for when the extent count exceeds the space after the
> > > # inode core.
> > > local max_nextents="$(((isize - 176) / 16))"
> > >
> > > and then you can do:
> > >
> > > [[ $nexts -gt $max_nextents ]] && break
> > >
> > > Also not a fan of hardcoding 176 around fstests, but I don't know how
> > > we'd detect that at all.
> > >
> > > # Number of bytes reserved for only the inode record, excluding the
> > > # immediate fork areas.
> > > _xfs_inode_core_bytes()
> > > {
> > > echo 176
> > > }
> > >
> > > I guess? Or extract it from tests/xfs/122.out?
> >
> > Thanks for your comments.
> >
> > I guess hard-coded 176 in _xfs_inode_core_bytes() is fine for now
> > (It seems a bit weird to extract a number from a test expected result..)
>
> Which is wrong when testing a v4 filesystem - in that case the inode
> core size is 96 bytes and so max extents may be larger on v4
> filesystems than v5 filesystems....

Do we really care v4 fs for now since it's deprecated?... Darrick once also
suggested using (isize / 16) but it seems it could take unnecessary time to
prepare.. Or we could just use (isize - 96) / 16 to keep v4 work.

Thanks,
Gao Xiang

>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com