xfs: switch (back) to a per-buftarg buffer hash

The per-AG buffer hashes were added when all buffer lookups took a
per-hash look.  Since then we've made lookups entirely lockless and
removed the need for a hash-wide lock for inserts and removals as
well.  With this there is no need to sharding the hash, so reduce the
used resources by using a per-buftarg hash for all buftargs.

Long after writing this initially, syzbot found a problem in the buffer
cache teardown order, which this happens to fix as well by doing the
entire buffer cache teardown in one places instead of splitting it
between destroying the buftarg and the perag structures.

Link: https://lore.kernel.org/linux-xfs/aLeUdemAZ5wmtZel@dread.disaster.area/
Reported-by: syzbot+0391d34e801643e2809b@syzkaller.appspotmail.com
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: syzbot+0391d34e801643e2809b@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

fs/xfs/libxfs/xfs_ag.c

@@ -110,10 +110,7 @@ xfs_perag_uninit(
 	struct xfs_group	*xg)
 {
 #ifdef __KERNEL__
-	struct xfs_perag	*pag = to_perag(xg);
-
-	cancel_delayed_work_sync(&pag->pag_blockgc_work);
-	xfs_buf_cache_destroy(&pag->pag_bcache);
+	cancel_delayed_work_sync(&to_perag(xg)->pag_blockgc_work);
 #endif
 }
 
@@ -235,10 +232,6 @@ xfs_perag_alloc(
 	INIT_RADIX_TREE(&pag->pag_ici_root, GFP_ATOMIC);
 #endif /* __KERNEL__ */
 
-	error = xfs_buf_cache_init(&pag->pag_bcache);
-	if (error)
-		goto out_free_perag;
-
 	/*
 	 * Pre-calculated geometry
 	 */
@@ -250,12 +243,10 @@ xfs_perag_alloc(
 
 	error = xfs_group_insert(mp, pag_group(pag), index, XG_TYPE_AG);
 	if (error)
-		goto out_buf_cache_destroy;
+		goto out_free_perag;
 
 	return 0;
 
-out_buf_cache_destroy:
-	xfs_buf_cache_destroy(&pag->pag_bcache);
 out_free_perag:
 	kfree(pag);
 	return error;

fs/xfs/libxfs/xfs_ag.h

@@ -85,8 +85,6 @@ struct xfs_perag {
 	int		pag_ici_reclaimable;	/* reclaimable inodes */
 	unsigned long	pag_ici_reclaim_cursor;	/* reclaim restart point */
 
-	struct xfs_buf_cache	pag_bcache;
-
 	/* background prealloc block trimming */
 	struct delayed_work	pag_blockgc_work;
 #endif /* __KERNEL__ */

fs/xfs/xfs_buf.c

@@ -363,20 +363,6 @@ static const struct rhashtable_params xfs_buf_hash_params = {
 	.obj_cmpfn		= _xfs_buf_obj_cmp,
 };
 
-int
-xfs_buf_cache_init(
-	struct xfs_buf_cache	*bch)
-{
-	return rhashtable_init(&bch->bc_hash, &xfs_buf_hash_params);
-}
-
-void
-xfs_buf_cache_destroy(
-	struct xfs_buf_cache	*bch)
-{
-	rhashtable_destroy(&bch->bc_hash);
-}
-
 static int
 xfs_buf_map_verify(
 	struct xfs_buftarg	*btp,
@@ -434,7 +420,7 @@ xfs_buf_find_lock(
 
 static inline int
 xfs_buf_lookup(
-	struct xfs_buf_cache	*bch,
+	struct xfs_buftarg	*btp,
 	struct xfs_buf_map	*map,
 	xfs_buf_flags_t		flags,
 	struct xfs_buf		**bpp)
@@ -443,7 +429,7 @@ xfs_buf_lookup(
 	int			error;
 
 	rcu_read_lock();
-	bp = rhashtable_lookup(&bch->bc_hash, map, xfs_buf_hash_params);
+	bp = rhashtable_lookup(&btp->bt_hash, map, xfs_buf_hash_params);
 	if (!bp || !lockref_get_not_dead(&bp->b_lockref)) {
 		rcu_read_unlock();
 		return -ENOENT;
@@ -468,7 +454,6 @@ xfs_buf_lookup(
 static int
 xfs_buf_find_insert(
 	struct xfs_buftarg	*btp,
-	struct xfs_buf_cache	*bch,
 	struct xfs_perag	*pag,
 	struct xfs_buf_map	*cmap,
 	struct xfs_buf_map	*map,
@@ -488,7 +473,7 @@ xfs_buf_find_insert(
 	new_bp->b_pag = pag;
 
 	rcu_read_lock();
-	bp = rhashtable_lookup_get_insert_fast(&bch->bc_hash,
+	bp = rhashtable_lookup_get_insert_fast(&btp->bt_hash,
 			&new_bp->b_rhash_head, xfs_buf_hash_params);
 	if (IS_ERR(bp)) {
 		rcu_read_unlock();
@@ -530,16 +515,6 @@ xfs_buftarg_get_pag(
 	return xfs_perag_get(mp, xfs_daddr_to_agno(mp, map->bm_bn));
 }
 
-static inline struct xfs_buf_cache *
-xfs_buftarg_buf_cache(
-	struct xfs_buftarg		*btp,
-	struct xfs_perag		*pag)
-{
-	if (pag)
-		return &pag->pag_bcache;
-	return btp->bt_cache;
-}
-
 /*
  * Assembles a buffer covering the specified range. The code is optimised for
  * cache hits, as metadata intensive workloads will see 3 orders of magnitude
@@ -553,7 +528,6 @@ xfs_buf_get_map(
 	xfs_buf_flags_t		flags,
 	struct xfs_buf		**bpp)
 {
-	struct xfs_buf_cache	*bch;
 	struct xfs_perag	*pag;
 	struct xfs_buf		*bp = NULL;
 	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
@@ -570,9 +544,8 @@ xfs_buf_get_map(
 		return error;
 
 	pag = xfs_buftarg_get_pag(btp, &cmap);
-	bch = xfs_buftarg_buf_cache(btp, pag);
 
-	error = xfs_buf_lookup(bch, &cmap, flags, &bp);
+	error = xfs_buf_lookup(btp, &cmap, flags, &bp);
 	if (error && error != -ENOENT)
 		goto out_put_perag;
 
@@ -584,7 +557,7 @@ xfs_buf_get_map(
 			goto out_put_perag;
 
 		/* xfs_buf_find_insert() consumes the perag reference. */
-		error = xfs_buf_find_insert(btp, bch, pag, &cmap, map, nmaps,
+		error = xfs_buf_find_insert(btp, pag, &cmap, map, nmaps,
 				flags, &bp);
 		if (error)
 			return error;
@@ -848,11 +821,8 @@ xfs_buf_destroy(
 	ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));
 
 	if (!xfs_buf_is_uncached(bp)) {
-		struct xfs_buf_cache	*bch =
-			xfs_buftarg_buf_cache(bp->b_target, bp->b_pag);
-
-		rhashtable_remove_fast(&bch->bc_hash, &bp->b_rhash_head,
-				xfs_buf_hash_params);
+		rhashtable_remove_fast(&bp->b_target->bt_hash,
+				&bp->b_rhash_head, xfs_buf_hash_params);
 
 		if (bp->b_pag)
 			xfs_perag_put(bp->b_pag);
@@ -1618,6 +1588,7 @@ xfs_destroy_buftarg(
 	ASSERT(percpu_counter_sum(&btp->bt_readahead_count) == 0);
 	percpu_counter_destroy(&btp->bt_readahead_count);
 	list_lru_destroy(&btp->bt_lru);
+	rhashtable_destroy(&btp->bt_hash);
 }
 
 void
@@ -1712,8 +1683,10 @@ xfs_init_buftarg(
 	ratelimit_state_init(&btp->bt_ioerror_rl, 30 * HZ,
 			     DEFAULT_RATELIMIT_BURST);
 
-	if (list_lru_init(&btp->bt_lru))
+	if (rhashtable_init(&btp->bt_hash, &xfs_buf_hash_params))
 		return -ENOMEM;
+	if (list_lru_init(&btp->bt_lru))
+		goto out_destroy_hash;
 	if (percpu_counter_init(&btp->bt_readahead_count, 0, GFP_KERNEL))
 		goto out_destroy_lru;
 
@@ -1731,6 +1704,8 @@ xfs_init_buftarg(
 	percpu_counter_destroy(&btp->bt_readahead_count);
 out_destroy_lru:
 	list_lru_destroy(&btp->bt_lru);
+out_destroy_hash:
+	rhashtable_destroy(&btp->bt_hash);
 	return -ENOMEM;
 }
 

fs/xfs/xfs_buf.h

@@ -69,13 +69,6 @@ typedef unsigned int xfs_buf_flags_t;
 	{ XBF_INCORE,		"INCORE" }, \
 	{ XBF_TRYLOCK,		"TRYLOCK" }
 
-struct xfs_buf_cache {
-	struct rhashtable	bc_hash;
-};
-
-int xfs_buf_cache_init(struct xfs_buf_cache *bch);
-void xfs_buf_cache_destroy(struct xfs_buf_cache *bch);
-
 /*
  * The xfs_buftarg contains 2 notions of "sector size" -
  *
@@ -113,8 +106,7 @@ struct xfs_buftarg {
 	unsigned int		bt_awu_min;
 	unsigned int		bt_awu_max;
 
-	/* built-in cache, if we're not using the perag one */
-	struct xfs_buf_cache	bt_cache[];
+	struct rhashtable	bt_hash;
 };
 
 struct xfs_buf_map {

fs/xfs/xfs_buf_mem.c

@@ -58,7 +58,7 @@ xmbuf_alloc(
 	struct xfs_buftarg	*btp;
 	int			error;
 
-	btp = kzalloc_flex(*btp, bt_cache, 1);
+	btp = kzalloc_obj(*btp);
 	if (!btp)
 		return -ENOMEM;
 
@@ -81,10 +81,6 @@ xmbuf_alloc(
 	/* ensure all writes are below EOF to avoid pagecache zeroing */
 	i_size_write(inode, inode->i_sb->s_maxbytes);
 
-	error = xfs_buf_cache_init(btp->bt_cache);
-	if (error)
-		goto out_file;
-
 	/* Initialize buffer target */
 	btp->bt_mount = mp;
 	btp->bt_dev = (dev_t)-1U;
@@ -95,15 +91,13 @@ xmbuf_alloc(
 
 	error = xfs_init_buftarg(btp, XMBUF_BLOCKSIZE, descr);
 	if (error)
-		goto out_bcache;
+		goto out_file;
 
 	trace_xmbuf_create(btp);
 
 	*btpp = btp;
 	return 0;
 
-out_bcache:
-	xfs_buf_cache_destroy(btp->bt_cache);
 out_file:
 	fput(file);
 out_free_btp:
@@ -122,7 +116,6 @@ xmbuf_free(
 	trace_xmbuf_free(btp);
 
 	xfs_destroy_buftarg(btp);
-	xfs_buf_cache_destroy(btp->bt_cache);
 	fput(btp->bt_file);
 	kfree(btp);
 }