Re: [PATCH 033/119] xfs: support overlapping intervals in the rmap btree

Brian Foster
2016-07-09T13:25:56Z

On Fri, Jul 08, 2016 at 05:14:28PM -0700, Darrick J. Wong wrote:
> On Fri, Jul 08, 2016 at 02:33:55PM -0400, Brian Foster wrote:
> > On Thu, Jun 16, 2016 at 06:21:24PM -0700, Darrick J. Wong wrote:
> > > Now that the generic btree code supports overlapping intervals, plug
> > > in the rmap btree to this functionality. We will need it to find
> > > potential left neighbors in xfs_rmap_{alloc,free} later in the patch
> > > set.
> > >
> > > v2: Fix bit manipulation bug when generating high key offset.
> > > v3: Move unwritten bit to rm_offset.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > > fs/xfs/libxfs/xfs_rmap_btree.c | 59 +++++++++++++++++++++++++++++++++++++++-
> > > fs/xfs/libxfs/xfs_rmap_btree.h | 10 +++++--
> > > 2 files changed, 66 insertions(+), 3 deletions(-)
> > >
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_rmap_btree.c b/fs/xfs/libxfs/xfs_rmap_btree.c
> > > index c50c725..9adb930 100644
> > > --- a/fs/xfs/libxfs/xfs_rmap_btree.c
> > > +++ b/fs/xfs/libxfs/xfs_rmap_btree.c
> > > @@ -181,6 +181,28 @@ xfs_rmapbt_init_key_from_rec(
> > > }
> > >
> > > STATIC void
> > > +xfs_rmapbt_init_high_key_from_rec(
> > > + union xfs_btree_key *key,
> > > + union xfs_btree_rec *rec)
> > > +{
> > > + __uint64_t off;
> > > + int adj;
> > > +
> > > + adj = be32_to_cpu(rec->rmap.rm_blockcount) - 1;
> > > +
> >
> > Comments please. I had to stare at this for too long than I care to
> > admit to grok why it is modifying values. :) One liners along the lines
> > of "shift the startblock/offset to the highest value to form the high
> > key" or "don't convert offset for non-inode owners because ..." go a
> > long way for those not familiar with the code.
>
> Fair enough.
>
> /*
> * The high key for a reverse mapping record can be computed by shifting
> * the startblock and offset to the highest value that would still map
> * to that record. In practice this means that we add blockcount-1 to
> * the startblock for all records, and if the record is for a data/attr
> * fork mapping, we add blockcount-1 to the offset too.
> */
>

Sounds good. To be clear, that's even more than what I was asking for.
Just something that calls out a potentially unexpected record
transformation in this context is sufficient. E.g.,

/*
* caller is asking for high key, transform on-disk start block and
* offset using blockcount
*/

... (but the above is fine too :).

> > With regard to rm_offset, could we just copy it unconditionally here
> > (should it not be 0)?
>
> No, because one of the rmap operations (once we get to reflink) is to
> find any potential left-mappings that we could extend in order to map
> in an extent (pblk, owner, lblk) by searching for (pblk-1, owner,
> lblk-1).
>
> If the extent we're trying to map is, say, (15, 128, 5) and there's an
> existing mapping (10, 128, 0, len=5), we have to be able to compute
> the high key of that existing mapping as (14, 128, 4). We can't
> decrement the cursor here because the next record to the left might
> be (12, 150, 2, len=1).
>
> (Making that one search reasonably quick is the reason behind the entire
> overlapping btree thing.)
>

Ok. Can't say I grok this at the moment, but I'll worry about it when I
have more context on the reflink bits. :)

> > > + key->rmap.rm_startblock = rec->rmap.rm_startblock;
> > > + be32_add_cpu(&key->rmap.rm_startblock, adj);
> > > + key->rmap.rm_owner = rec->rmap.rm_owner;
> > > + key->rmap.rm_offset = rec->rmap.rm_offset;
> > > + if (XFS_RMAP_NON_INODE_OWNER(be64_to_cpu(rec->rmap.rm_owner)) ||
> > > + XFS_RMAP_IS_BMBT_BLOCK(be64_to_cpu(rec->rmap.rm_offset)))
> > > + return;
> > > + off = be64_to_cpu(key->rmap.rm_offset);
> > > + off = (XFS_RMAP_OFF(off) + adj) | (off & ~XFS_RMAP_OFF_MASK);
> > > + key->rmap.rm_offset = cpu_to_be64(off);
> > > +}
> > > +
> > > +STATIC void
> > > xfs_rmapbt_init_rec_from_cur(
> > > struct xfs_btree_cur *cur,
> > > union xfs_btree_rec *rec)
> > > @@ -235,6 +257,38 @@ xfs_rmapbt_key_diff(
> > > return 0;
> > > }
> > >
> > > +STATIC __int64_t
> > > +xfs_rmapbt_diff_two_keys(
> > > + struct xfs_btree_cur *cur,
> > > + union xfs_btree_key *k1,
> > > + union xfs_btree_key *k2)
> > > +{
> > > + struct xfs_rmap_key *kp1 = &k1->rmap;
> > > + struct xfs_rmap_key *kp2 = &k2->rmap;
> > > + __int64_t d;
> > > + __u64 x, y;
> > > +
> > > + d = (__int64_t)be32_to_cpu(kp2->rm_startblock) -
> > > + be32_to_cpu(kp1->rm_startblock);
> > > + if (d)
> > > + return d;
> > > +
> > > + x = be64_to_cpu(kp2->rm_owner);
> > > + y = be64_to_cpu(kp1->rm_owner);
> > > + if (x > y)
> > > + return 1;
> > > + else if (y > x)
> > > + return -1;
> > > +
> > > + x = XFS_RMAP_OFF(be64_to_cpu(kp2->rm_offset));
> > > + y = XFS_RMAP_OFF(be64_to_cpu(kp1->rm_offset));
> > > + if (x > y)
> > > + return 1;
> > > + else if (y > x)
> > > + return -1;
> > > + return 0;
> > > +}
> > > +
> > > static bool
> > > xfs_rmapbt_verify(
> > > struct xfs_buf *bp)
> > > @@ -350,6 +404,7 @@ xfs_rmapbt_recs_inorder(
> > > static const struct xfs_btree_ops xfs_rmapbt_ops = {
> > > .rec_len = sizeof(struct xfs_rmap_rec),
> > > .key_len = sizeof(struct xfs_rmap_key),
> > > + .flags = XFS_BTREE_OPS_OVERLAPPING,
> > >
> > > .dup_cursor = xfs_rmapbt_dup_cursor,
> > > .set_root = xfs_rmapbt_set_root,
> > > @@ -358,10 +413,12 @@ static const struct xfs_btree_ops xfs_rmapbt_ops = {
> > > .get_minrecs = xfs_rmapbt_get_minrecs,
> > > .get_maxrecs = xfs_rmapbt_get_maxrecs,
> > > .init_key_from_rec = xfs_rmapbt_init_key_from_rec,
> > > + .init_high_key_from_rec = xfs_rmapbt_init_high_key_from_rec,
> > > .init_rec_from_cur = xfs_rmapbt_init_rec_from_cur,
> > > .init_ptr_from_cur = xfs_rmapbt_init_ptr_from_cur,
> > > .key_diff = xfs_rmapbt_key_diff,
> > > .buf_ops = &xfs_rmapbt_buf_ops,
> > > + .diff_two_keys = xfs_rmapbt_diff_two_keys,
> > > #if defined(DEBUG) || defined(XFS_WARN)
> > > .keys_inorder = xfs_rmapbt_keys_inorder,
> > > .recs_inorder = xfs_rmapbt_recs_inorder,
> > > @@ -410,7 +467,7 @@ xfs_rmapbt_maxrecs(
> > > if (leaf)
> > > return blocklen / sizeof(struct xfs_rmap_rec);
> > > return blocklen /
> > > - (sizeof(struct xfs_rmap_key) + sizeof(xfs_rmap_ptr_t));
> > > + (2 * sizeof(struct xfs_rmap_key) + sizeof(xfs_rmap_ptr_t));
> >
> > Same here.. one-liner comment that reminds why we have the 2x please.
>
> /*
> * Each btree pointer has two keys representing the lowest and highest
> * keys of all records in the subtree.
> */
>

I suggest to correlate it to XFS_BTREE_OPS_OVERLAPPING support:

/* double the key size for overlapping trees (2 keys per pointer) */

Thanks!

Brian

> > > }
> > >
> > > /* Compute the maximum height of an rmap btree. */
> > > diff --git a/fs/xfs/libxfs/xfs_rmap_btree.h b/fs/xfs/libxfs/xfs_rmap_btree.h
> > > index 17fa383..796071c 100644
> > > --- a/fs/xfs/libxfs/xfs_rmap_btree.h
> > > +++ b/fs/xfs/libxfs/xfs_rmap_btree.h
> > > @@ -38,12 +38,18 @@ struct xfs_mount;
> > > #define XFS_RMAP_KEY_ADDR(block, index) \
> > > ((struct xfs_rmap_key *) \
> > > ((char *)(block) + XFS_RMAP_BLOCK_LEN + \
> > > - ((index) - 1) * sizeof(struct xfs_rmap_key)))
> > > + ((index) - 1) * 2 * sizeof(struct xfs_rmap_key)))
> > > +
> > > +#define XFS_RMAP_HIGH_KEY_ADDR(block, index) \
> > > + ((struct xfs_rmap_key *) \
> > > + ((char *)(block) + XFS_RMAP_BLOCK_LEN + \
> > > + sizeof(struct xfs_rmap_key) + \
> > > + ((index) - 1) * 2 * sizeof(struct xfs_rmap_key)))
> > >
> >
> > Could this just be 'XFS_RMAP_KEY_ADDR(block, index) + sizeof(struct
> > xfs_rmap_key)'?
>
> Yes.
>
> --D
>
> >
> > Brian
> >
> > > #define XFS_RMAP_PTR_ADDR(block, index, maxrecs) \
> > > ((xfs_rmap_ptr_t *) \
> > > ((char *)(block) + XFS_RMAP_BLOCK_LEN + \
> > > - (maxrecs) * sizeof(struct xfs_rmap_key) + \
> > > + (maxrecs) * 2 * sizeof(struct xfs_rmap_key) + \
> > > ((index) - 1) * sizeof(xfs_rmap_ptr_t)))
> > >
> > > struct xfs_btree_cur *xfs_rmapbt_init_cursor(struct xfs_mount *mp,
> > >
> > > _______________________________________________
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> > > xfs@oss.sgi.com
> > > http://oss.sgi.com/mailman/listinfo/xfs