On Wed, May 16, 2018 at 10:34:36AM -0700, Allison Henderson wrote:
> Ok, with the points Dave made:
> Reviewed by: Allison Henderson <allison.henderson@xxxxxxxxxx>
>
> On 05/16/2018 12:56 AM, Dave Chinner wrote:
> > On Tue, May 15, 2018 at 03:33:58PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > >
> > > Add some helpers to assemble a list of fs block extents. Generally,
> > > repair functions will iterate the rmapbt to make a list (1) of all
> > > extents owned by the nominal owner of the metadata structure; then they
> > > will iterate all other structures with the same rmap owner to make a
> > > list (2) of active blocks; and finally we have a subtraction function to
> > > subtract all the blocks in (2) from (1), with the result that (1) is now
> > > a list of blocks that were owned by the old btree and must be disposed.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > > ---
> > > fs/xfs/scrub/repair.c | 207 +++++++++++++++++++++++++++++++++++++++++++++++++
> > > fs/xfs/scrub/repair.h | 31 +++++++
> > > 2 files changed, 238 insertions(+)
> > >
> > >
> > > diff --git a/fs/xfs/scrub/repair.c b/fs/xfs/scrub/repair.c
> > > index 72f04a717150..8e8ecddd7537 100644
> > > --- a/fs/xfs/scrub/repair.c
> > > +++ b/fs/xfs/scrub/repair.c
> > > @@ -361,3 +361,210 @@ xfs_repair_init_btblock(
> > > return 0;
> > > }
> > > +
> > > +/* Collect a dead btree extent for later disposal. */
> > > +int
> > > +xfs_repair_collect_btree_extent(
> > > + struct xfs_scrub_context *sc,
> > > + struct xfs_repair_extent_list *exlist,
> > > + xfs_fsblock_t fsbno,
> > > + xfs_extlen_t len)
> > > +{
> > > + struct xfs_repair_extent *rex;
> > > +
> > > + trace_xfs_repair_collect_btree_extent(sc->mp,
> > > + XFS_FSB_TO_AGNO(sc->mp, fsbno),
> > > + XFS_FSB_TO_AGBNO(sc->mp, fsbno), len);
> > > +
> > > + rex = kmem_alloc(sizeof(struct xfs_repair_extent), KM_MAYFAIL);
> > > + if (!rex)
> > > + return -ENOMEM;
> > Is this in transaction context? Regardless, I think we need to run
> > the entire of scrub/repair in a memalloc_nofs_save() context so we
> > don't have memory reclaim recursion issues...
> >
> > [...]
> >
> > > +/* Compare two btree extents. */
> > > +static int
> > > +xfs_repair_btree_extent_cmp(
> > > + void *priv,
> > > + struct list_head *a,
> > > + struct list_head *b)
> > > +{
> > > + struct xfs_repair_extent *ap;
> > > + struct xfs_repair_extent *bp;
> > > +
> > > + ap = container_of(a, struct xfs_repair_extent, list);
> > > + bp = container_of(b, struct xfs_repair_extent, list);
> > > +
> > > + if (ap->fsbno > bp->fsbno)
> > > + return 1;
> > > + else if (ap->fsbno < bp->fsbno)
> > > + return -1;
> > No need for the else there.
> Well, I think he meant to return 0 in the case of ap->fsbno == bp->fsbno?
> Am i reading that right? caller expects 1 for greater than, -1 for less
> than and 0 on equivalence?
Correct. I think Dave was pointing out that else-after-return is
unnecessary, since the following behaves equivalently:
if (a > b)
return 1;
if (a < b)
return -1;
return 0;
Note that gcc (7.3, anyway) generates the same asm for either version so
I assume this is mostly stylistic cleanup to make the comparisons line
up? I don't have a preference either way. :)
--D
> > > + return 0;
> > > +}
> > > +
> > > +/*
> > > + * Remove all the blocks mentioned in sublist from the extents in exlist.
> > > + *
> > > + * The intent is that callers will iterate the rmapbt for all of its records
> > > + * for a given owner to generate exlist; and iterate all the blocks of the
> > generate @exlist
> >
> > > + * metadata structures that are not being rebuilt and have the same rmapbt
> > > + * owner to generate sublist. This routine subtracts all the extents
> > generate @sublist.
> >
> > > + * mentioned in sublist from all the extents linked in exlist, which leaves
> > > + * exlist as the list of blocks that are not accounted for, which we assume
> > > + * are the dead blocks of the old metadata structure. The blocks mentioned in
> > > + * exlist can be reaped.
> > > + */
> > > +#define XFS_REPAIR_EXT_LEFT_CONTIG (1 << 0)
> > > +#define XFS_REPAIR_EXT_RIGHT_CONTIG (1 << 1)
> > > +int
> > > +xfs_repair_subtract_extents(
> > > + struct xfs_scrub_context *sc,
> > > + struct xfs_repair_extent_list *exlist,
> > > + struct xfs_repair_extent_list *sublist)
> > > +{
> > > + struct list_head *lp;
> > > + struct xfs_repair_extent *ex;
> > > + struct xfs_repair_extent *newex;
> > > + struct xfs_repair_extent *subex;
> > > + xfs_fsblock_t sub_fsb;
> > > + xfs_extlen_t sub_len;
> > > + int state;
> > > + int error = 0;
> > > +
> > > + if (list_empty(&exlist->list) || list_empty(&sublist->list))
> > > + return 0;
> > > + ASSERT(!list_empty(&sublist->list));
> > > +
> > > + list_sort(NULL, &exlist->list, xfs_repair_btree_extent_cmp);
> > > + list_sort(NULL, &sublist->list, xfs_repair_btree_extent_cmp);
> > > +
> > > + subex = list_first_entry(&sublist->list, struct xfs_repair_extent,
> > > + list);
> > > + lp = exlist->list.next;
> > > + while (lp != &exlist->list) {
> > > + ex = list_entry(lp, struct xfs_repair_extent, list);
> > > +
> > > + /*
> > > + * Advance subex and/or ex until we find a pair that
> > > + * intersect or we run out of extents.
> > > + */
> > > + while (subex->fsbno + subex->len <= ex->fsbno) {
> > > + if (list_is_last(&subex->list, &sublist->list))
> > > + goto out;
> > > + subex = list_next_entry(subex, list);
> > > + }
> > So this is a O(n^2) algorithm, right? How does it scale with large
> > extent lists? Given that these extents are dynamically allocated,
> > and we're already burning 16 bytes for a list head on each extent,
> > would it be better to use a smarter structure better suited for
> > exact lookups? e.g. an rbtree only takes an extra 8 bytes per
> > extent, and we get O(log N) searches on the inner loop here...
> >
> > I guess this isn't necessary to fix right now, but I think it's
> > going to be an issue for maybe mark this down as "needing to be
> > fixed before removing EXPERIMENTAL tags"?
> >
> > > + if (subex->fsbno >= ex->fsbno + ex->len) {
> > > + lp = lp->next;
> > > + continue;
> > > + }
> > > +
> > > + /* trim subex to fit the extent we have */
> > > + sub_fsb = subex->fsbno;
> > > + sub_len = subex->len;
> > > + if (subex->fsbno < ex->fsbno) {
> > > + sub_len -= ex->fsbno - subex->fsbno;
> > > + sub_fsb = ex->fsbno;
> > > + }
> > > + if (sub_len > ex->len)
> > > + sub_len = ex->len;
> > > +
> > > + state = 0;
> > > + if (sub_fsb == ex->fsbno)
> > > + state |= XFS_REPAIR_EXT_LEFT_CONTIG;
> > > + if (sub_fsb + sub_len == ex->fsbno + ex->len)
> > > + state |= XFS_REPAIR_EXT_RIGHT_CONTIG;
> > Ok, I think "CONTIG" is not the right word to use here. In the BMAP
> > btrees, the merge state flags were to tell us whether the edge of
> > the new extent is contiguous with the left and right extents in
> > the tree, not whether the new extents overlapped to the left/right
> > edges.
> >
> > i.e. we're checking whether extent start/end overlaps are aligned
> > here, not whether they are contiguous with some other extent. So in
> > this case, I'd just name the variables "LEFT_ALIGNED" and
> > "RIGHT_ALIGNED" and drop all the namespace bits from them.
> >
> > > + switch (state) {
> > > + case XFS_REPAIR_EXT_LEFT_CONTIG:
> > > + /* Coincides with only the left. */
> > And by calling them aligned, the comments become redundant:
> >
> > case LEFT_ALIGNED:
> > > + ex->fsbno += sub_len;
> > > + ex->len -= sub_len;
> > > + break;
> > > + case XFS_REPAIR_EXT_RIGHT_CONTIG:
> > > + /* Coincides with only the right. */
> > > + ex->len -= sub_len;
> > > + lp = lp->next;
> > > + break;
> > > + case XFS_REPAIR_EXT_LEFT_CONTIG | XFS_REPAIR_EXT_RIGHT_CONTIG:
> > > + /* Total overlap, just delete ex. */
> > > + lp = lp->next;
> > > + list_del(&ex->list);
> > > + kmem_free(ex);
> > > + break;
> > > + case 0:
> > > + /*
> > > + * Deleting from the middle: add the new right extent
> > > + * and then shrink the left extent.
> > > + */
> > > + newex = kmem_alloc(sizeof(struct xfs_repair_extent),
> > > + KM_MAYFAIL);
> > > + if (!newex) {
> > > + error = -ENOMEM;
> > > + goto out;
> > > + }
> > > + INIT_LIST_HEAD(&newex->list);
> > > + newex->fsbno = sub_fsb + sub_len;
> > > + newex->len = ex->len - (sub_fsb - ex->fsbno) - sub_len;
> > so: new len = old len - (length of first extent) - length of overlap.
> >
> > I think this is more obvious as "new len = old end - new start", i.e.:
> >
> > newex->len = ex->fsbno + ex->len - newex->fsbno;
> >
> > Cheers,
> >
> > Dave.
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
