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

[Brian Foster <bfoster@redhat.com>]

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

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__ */
>