Re: [PATCH 3/6] xfs: merge xfs_buf_find() and xfs_buf_get_map()

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jul 08, 2022 at 09:52:56AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Now that we factored xfs_buf_find(), we can start separating into
> distinct fast and slow paths from xfs_buf_get_map(). We start by
> moving the lookup map and perag setup to _get_map(), and then move
> all the specifics of the fast path lookup into xfs_buf_lookup()
> and call it directly from _get_map(). We the move all the slow path
> code to xfs_buf_find_insert(), which is now also called directly
> from _get_map(). As such, xfs_buf_find() now goes away.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Makes sense to /me...
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_buf.c | 207 ++++++++++++++++++++++-------------------------
>  1 file changed, 95 insertions(+), 112 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 91dc691f40a8..81ca951b451a 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -531,18 +531,16 @@ xfs_buf_map_verify(
>  
>  static int
>  xfs_buf_find_lock(
> -	struct xfs_buftarg	*btp,
>  	struct xfs_buf          *bp,
>  	xfs_buf_flags_t		flags)
>  {
>  	if (!xfs_buf_trylock(bp)) {
>  		if (flags & XBF_TRYLOCK) {
> -			xfs_buf_rele(bp);
> -			XFS_STATS_INC(btp->bt_mount, xb_busy_locked);
> +			XFS_STATS_INC(bp->b_mount, xb_busy_locked);
>  			return -EAGAIN;
>  		}
>  		xfs_buf_lock(bp);
> -		XFS_STATS_INC(btp->bt_mount, xb_get_locked_waited);
> +		XFS_STATS_INC(bp->b_mount, xb_get_locked_waited);
>  	}
>  
>  	/*
> @@ -558,113 +556,97 @@ xfs_buf_find_lock(
>  	return 0;
>  }
>  
> -static inline struct xfs_buf *
> +static inline int
>  xfs_buf_lookup(
>  	struct xfs_perag	*pag,
> -	struct xfs_buf_map	*map)
> +	struct xfs_buf_map	*map,
> +	xfs_buf_flags_t		flags,
> +	struct xfs_buf		**bpp)
>  {
>  	struct xfs_buf          *bp;
> +	int			error;
>  
> +	spin_lock(&pag->pag_buf_lock);
>  	bp = rhashtable_lookup(&pag->pag_buf_hash, map, xfs_buf_hash_params);
> -	if (!bp)
> -		return NULL;
> +	if (!bp) {
> +		spin_unlock(&pag->pag_buf_lock);
> +		return -ENOENT;
> +	}
>  	atomic_inc(&bp->b_hold);
> -	return bp;
> -}
> +	spin_unlock(&pag->pag_buf_lock);
>  
> -/*
> - * Insert the new_bp into the hash table. This consumes the perag reference
> - * taken for the lookup.
> - */
> -static int
> -xfs_buf_find_insert(
> -	struct xfs_buftarg	*btp,
> -	struct xfs_perag	*pag,
> -	struct xfs_buf		*new_bp)
> -{
> -	/* No match found */
> -	if (!new_bp) {
> -		xfs_perag_put(pag);
> -		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
> -		return -ENOENT;
> +	error = xfs_buf_find_lock(bp, flags);
> +	if (error) {
> +		xfs_buf_rele(bp);
> +		return error;
>  	}
>  
> -	/* the buffer keeps the perag reference until it is freed */
> -	new_bp->b_pag = pag;
> -	rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
> -			       xfs_buf_hash_params);
> +	trace_xfs_buf_find(bp, flags, _RET_IP_);
> +	*bpp = bp;
>  	return 0;
>  }
>  
>  /*
> - * Look up a buffer in the buffer cache and return it referenced and locked
> - * in @found_bp.
> - *
> - * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
> - * cache.
> - *
> - * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
> - * -EAGAIN if we fail to lock it.
> - *
> - * Return values are:
> - *	-EFSCORRUPTED if have been supplied with an invalid address
> - *	-EAGAIN on trylock failure
> - *	-ENOENT if we fail to find a match and @new_bp was NULL
> - *	0, with @found_bp:
> - *		- @new_bp if we inserted it into the cache
> - *		- the buffer we found and locked.
> + * Insert the new_bp into the hash table. This consumes the perag reference
> + * taken for the lookup regardless of the result of the insert.
>   */
>  static int
> -xfs_buf_find(
> +xfs_buf_find_insert(
>  	struct xfs_buftarg	*btp,
> +	struct xfs_perag	*pag,
> +	struct xfs_buf_map	*cmap,
>  	struct xfs_buf_map	*map,
>  	int			nmaps,
>  	xfs_buf_flags_t		flags,
> -	struct xfs_buf		*new_bp,
> -	struct xfs_buf		**found_bp)
> +	struct xfs_buf		**bpp)
>  {
> -	struct xfs_perag	*pag;
> +	struct xfs_buf		*new_bp;
>  	struct xfs_buf		*bp;
> -	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
>  	int			error;
> -	int			i;
> -
> -	*found_bp = NULL;
> -
> -	for (i = 0; i < nmaps; i++)
> -		cmap.bm_len += map[i].bm_len;
>  
> -	error = xfs_buf_map_verify(btp, &cmap);
> +	error = _xfs_buf_alloc(btp, map, nmaps, flags, &new_bp);
>  	if (error)
> -		return error;
> +		goto out_drop_pag;
>  
> -	pag = xfs_perag_get(btp->bt_mount,
> -			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
> +	/*
> +	 * For buffers that fit entirely within a single page, first attempt to
> +	 * allocate the memory from the heap to minimise memory usage. If we
> +	 * can't get heap memory for these small buffers, we fall back to using
> +	 * the page allocator.
> +	 */
> +	if (BBTOB(new_bp->b_length) >= PAGE_SIZE ||
> +	    xfs_buf_alloc_kmem(new_bp, flags) < 0) {
> +		error = xfs_buf_alloc_pages(new_bp, flags);
> +		if (error)
> +			goto out_free_buf;
> +	}
>  
>  	spin_lock(&pag->pag_buf_lock);
> -	bp = xfs_buf_lookup(pag, &cmap);
> -	if (bp)
> -		goto found;
> +	bp = rhashtable_lookup(&pag->pag_buf_hash, cmap, xfs_buf_hash_params);
> +	if (bp) {
> +		atomic_inc(&bp->b_hold);
> +		spin_unlock(&pag->pag_buf_lock);
> +		error = xfs_buf_find_lock(bp, flags);
> +		if (error)
> +			xfs_buf_rele(bp);
> +		else
> +			*bpp = bp;
> +		goto out_free_buf;
> +	}
>  
> -	error = xfs_buf_find_insert(btp, pag, new_bp);
> +	/* The buffer keeps the perag reference until it is freed. */
> +	new_bp->b_pag = pag;
> +	rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
> +			       xfs_buf_hash_params);
>  	spin_unlock(&pag->pag_buf_lock);
> -	if (error)
> -		return error;
> -	*found_bp = new_bp;
> +	*bpp = new_bp;
>  	return 0;
>  
> -found:
> -	spin_unlock(&pag->pag_buf_lock);
> +out_free_buf:
> +	xfs_buf_free(new_bp);
> +out_drop_pag:
>  	xfs_perag_put(pag);
> -
> -	error = xfs_buf_find_lock(btp, bp, flags);
> -	if (error)
> -		return error;
> -
> -	trace_xfs_buf_find(bp, flags, _RET_IP_);
> -	XFS_STATS_INC(btp->bt_mount, xb_get_locked);
> -	*found_bp = bp;
> -	return 0;
> +	return error;
>  }
>  
>  /*
> @@ -674,54 +656,54 @@ xfs_buf_find(
>   */
>  int
>  xfs_buf_get_map(
> -	struct xfs_buftarg	*target,
> +	struct xfs_buftarg	*btp,
>  	struct xfs_buf_map	*map,
>  	int			nmaps,
>  	xfs_buf_flags_t		flags,
>  	struct xfs_buf		**bpp)
>  {
> -	struct xfs_buf		*bp;
> -	struct xfs_buf		*new_bp;
> +	struct xfs_perag	*pag;
> +	struct xfs_buf		*bp = NULL;
> +	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
>  	int			error;
> +	int			i;
>  
> -	*bpp = NULL;
> -	error = xfs_buf_find(target, map, nmaps, flags, NULL, &bp);
> -	if (!error)
> -		goto found;
> -	if (error != -ENOENT)
> -		return error;
> -	if (flags & XBF_INCORE)
> -		return -ENOENT;
> +	for (i = 0; i < nmaps; i++)
> +		cmap.bm_len += map[i].bm_len;
>  
> -	error = _xfs_buf_alloc(target, map, nmaps, flags, &new_bp);
> +	error = xfs_buf_map_verify(btp, &cmap);
>  	if (error)
>  		return error;
>  
> -	/*
> -	 * For buffers that fit entirely within a single page, first attempt to
> -	 * allocate the memory from the heap to minimise memory usage. If we
> -	 * can't get heap memory for these small buffers, we fall back to using
> -	 * the page allocator.
> -	 */
> -	if (BBTOB(new_bp->b_length) >= PAGE_SIZE ||
> -	    xfs_buf_alloc_kmem(new_bp, flags) < 0) {
> -		error = xfs_buf_alloc_pages(new_bp, flags);
> -		if (error)
> -			goto out_free_buf;
> -	}
> +	pag = xfs_perag_get(btp->bt_mount,
> +			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
>  
> -	error = xfs_buf_find(target, map, nmaps, flags, new_bp, &bp);
> -	if (error)
> -		goto out_free_buf;
> +	error = xfs_buf_lookup(pag, &cmap, flags, &bp);
> +	if (error && error != -ENOENT)
> +		goto out_put_perag;
> +
> +	/* cache hits always outnumber misses by at least 10:1 */
> +	if (unlikely(!bp)) {
> +		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
>  
> -	if (bp != new_bp)
> -		xfs_buf_free(new_bp);
> +		if (flags & XBF_INCORE)
> +			goto out_put_perag;
>  
> -found:
> +		/* xfs_buf_find_insert() consumes the perag reference. */
> +		error = xfs_buf_find_insert(btp, pag, &cmap, map, nmaps,
> +				flags, &bp);
> +		if (error)
> +			return error;
> +	} else {
> +		XFS_STATS_INC(btp->bt_mount, xb_get_locked);
> +		xfs_perag_put(pag);
> +	}
> +
> +	/* We do not hold a perag reference anymore. */
>  	if (!bp->b_addr) {
>  		error = _xfs_buf_map_pages(bp, flags);
>  		if (unlikely(error)) {
> -			xfs_warn_ratelimited(target->bt_mount,
> +			xfs_warn_ratelimited(btp->bt_mount,
>  				"%s: failed to map %u pages", __func__,
>  				bp->b_page_count);
>  			xfs_buf_relse(bp);
> @@ -736,12 +718,13 @@ xfs_buf_get_map(
>  	if (!(flags & XBF_READ))
>  		xfs_buf_ioerror(bp, 0);
>  
> -	XFS_STATS_INC(target->bt_mount, xb_get);
> +	XFS_STATS_INC(btp->bt_mount, xb_get);
>  	trace_xfs_buf_get(bp, flags, _RET_IP_);
>  	*bpp = bp;
>  	return 0;
> -out_free_buf:
> -	xfs_buf_free(new_bp);
> +
> +out_put_perag:
> +	xfs_perag_put(pag);
>  	return error;
>  }
>  
> -- 
> 2.36.1
>