Re: [PATCH 5/6] xfs: only allocate memory for scrubbing attributes when we need it

Darrick J. Wong
2019-07-05T16:49:38Z

On Fri, Jul 05, 2019 at 10:52:33AM -0400, Brian Foster wrote:
> On Wed, Jun 26, 2019 at 01:47:04PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> >
> > In examining a flame graph of time spent running xfs_scrub on various
> > filesystems, I noticed that we spent nearly 7% of the total runtime on
> > allocating a zeroed 65k buffer for every SCRUB_TYPE_XATTR invocation.
> > We do this even if none of the attribute values were anywhere near 64k
> > in size, even if there were no attribute blocks to check space on, and
> > even if it just turns out there are no attributes at all.
> >
> > Therefore, rearrange the xattr buffer setup code to support reallocating
> > with a bigger buffer and redistribute the callers of that function so
> > that we only allocate memory just prior to needing it, and only allocate
> > as much as we need. If we can't get memory with the ILOCK held we'll
> > bail out with EDEADLOCK which will allocate the maximum memory.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> > fs/xfs/scrub/attr.c | 67 ++++++++++++++++++++++++++++++++++++++++++++-------
> > fs/xfs/scrub/attr.h | 6 ++++-
> > 2 files changed, 63 insertions(+), 10 deletions(-)
> >
> >
> > diff --git a/fs/xfs/scrub/attr.c b/fs/xfs/scrub/attr.c
> > index c20b6da1db84..09081d8ab34b 100644
> > --- a/fs/xfs/scrub/attr.c
> > +++ b/fs/xfs/scrub/attr.c
> ...
> > @@ -47,10 +53,23 @@ xchk_setup_xattr_buf(
> > sz = 3 * sizeof(long) * BITS_TO_LONGS(sc->mp->m_attr_geo->blksize);
> > sz = max_t(size_t, sz, value_size);
> >
> > - sc->buf = kmem_zalloc_large(sz, KM_SLEEP);
> > - if (!sc->buf)
> > + /*
> > + * If there's already a buffer, figure out if we need to reallocate it
> > + * to accomdate a larger size.
>
> accommodate

Fixed. Thanks for catching that.

--D

> Otherwise looks good:
>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
>
> > + */
> > + if (ab) {
> > + if (sz <= ab->sz)
> > + return 0;
> > + kmem_free(ab);
> > + sc->buf = NULL;
> > + }
> > +
> > + ab = kmem_zalloc_large(sizeof(*ab) + sz, flags);
> > + if (!ab)
> > return -ENOMEM;
> >
> > + ab->sz = sz;
> > + sc->buf = ab;
> > return 0;
> > }
> >
> > @@ -62,9 +81,16 @@ xchk_setup_xattr(
> > {
> > int error;
> >
> > - error = xchk_setup_xattr_buf(sc, XATTR_SIZE_MAX);
> > - if (error)
> > - return error;
> > + /*
> > + * We failed to get memory while checking attrs, so this time try to
> > + * get all the memory we're ever going to need. Allocate the buffer
> > + * without the inode lock held, which means we can sleep.
> > + */
> > + if (sc->flags & XCHK_TRY_HARDER) {
> > + error = xchk_setup_xattr_buf(sc, XATTR_SIZE_MAX, KM_SLEEP);
> > + if (error)
> > + return error;
> > + }
> >
> > return xchk_setup_inode_contents(sc, ip, 0);
> > }
> > @@ -115,6 +141,19 @@ xchk_xattr_listent(
> > return;
> > }
> >
> > + /*
> > + * Try to allocate enough memory to extrat the attr value. If that
> > + * doesn't work, we overload the seen_enough variable to convey
> > + * the error message back to the main scrub function.
> > + */
> > + error = xchk_setup_xattr_buf(sx->sc, valuelen, KM_MAYFAIL);
> > + if (error == -ENOMEM)
> > + error = -EDEADLOCK;
> > + if (error) {
> > + context->seen_enough = error;
> > + return;
> > + }
> > +
> > args.flags = ATTR_KERNOTIME;
> > if (flags & XFS_ATTR_ROOT)
> > args.flags |= ATTR_ROOT;
> > @@ -128,7 +167,7 @@ xchk_xattr_listent(
> > args.hashval = xfs_da_hashname(args.name, args.namelen);
> > args.trans = context->tp;
> > args.value = xchk_xattr_valuebuf(sx->sc);
> > - args.valuelen = XATTR_SIZE_MAX;
> > + args.valuelen = valuelen;
> >
> > error = xfs_attr_get_ilocked(context->dp, &args);
> > if (error == -EEXIST)
> > @@ -281,16 +320,26 @@ xchk_xattr_block(
> > struct xfs_attr_leafblock *leaf = bp->b_addr;
> > struct xfs_attr_leaf_entry *ent;
> > struct xfs_attr_leaf_entry *entries;
> > - unsigned long *usedmap = xchk_xattr_usedmap(ds->sc);
> > + unsigned long *usedmap;
> > char *buf_end;
> > size_t off;
> > __u32 last_hashval = 0;
> > unsigned int usedbytes = 0;
> > unsigned int hdrsize;
> > int i;
> > + int error;
> >
> > if (*last_checked == blk->blkno)
> > return 0;
> > +
> > + /* Allocate memory for block usage checking. */
> > + error = xchk_setup_xattr_buf(ds->sc, 0, KM_MAYFAIL);
> > + if (error == -ENOMEM)
> > + return -EDEADLOCK;
> > + if (error)
> > + return error;
> > + usedmap = xchk_xattr_usedmap(ds->sc);
> > +
> > *last_checked = blk->blkno;
> > bitmap_zero(usedmap, mp->m_attr_geo->blksize);
> >
> > diff --git a/fs/xfs/scrub/attr.h b/fs/xfs/scrub/attr.h
> > index 27e879aeaafc..13a1d2e8424d 100644
> > --- a/fs/xfs/scrub/attr.h
> > +++ b/fs/xfs/scrub/attr.h
> > @@ -10,6 +10,9 @@
> > * Temporary storage for online scrub and repair of extended attributes.
> > */
> > struct xchk_xattr_buf {
> > + /* Size of @buf, in bytes. */
> > + size_t sz;
> > +
> > /*
> > * Memory buffer -- either used for extracting attr values while
> > * walking the attributes; or for computing attr block bitmaps when
> > @@ -62,6 +65,7 @@ xchk_xattr_dstmap(
> > BITS_TO_LONGS(sc->mp->m_attr_geo->blksize);
> > }
> >
> > -int xchk_setup_xattr_buf(struct xfs_scrub *sc, size_t value_size);
> > +int xchk_setup_xattr_buf(struct xfs_scrub *sc, size_t value_size,
> > + xfs_km_flags_t flags);
> >
> > #endif /* __XFS_SCRUB_ATTR_H__ */
> >