gfs2: allow fallocate to max out quotas/fs efficiently

We can quickly get an estimate of how many blocks are available
for allocation restricted by quota and fs size respectively, using
the ap->allowed field in the gfs2_alloc_parms structure.
gfs2_quota_check() and gfs2_inplace_reserve() provide these values.

Once we have the total number of blocks available to us, we can
compute how many bytes of data can be written using those blocks
instead of guessing inefficiently.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/file.c

@@ -765,22 +765,30 @@ out:
 	brelse(dibh);
 	return error;
 }
-
-static void calc_max_reserv(struct gfs2_inode *ip, loff_t max, loff_t *len,
-			    unsigned int *data_blocks, unsigned int *ind_blocks)
+/**
+ * calc_max_reserv() - Reverse of write_calc_reserv. Given a number of
+ *                     blocks, determine how many bytes can be written.
+ * @ip:          The inode in question.
+ * @len:         Max cap of bytes. What we return in *len must be <= this.
+ * @data_blocks: Compute and return the number of data blocks needed
+ * @ind_blocks:  Compute and return the number of indirect blocks needed
+ * @max_blocks:  The total blocks available to work with.
+ *
+ * Returns: void, but @len, @data_blocks and @ind_blocks are filled in.
+ */
+static void calc_max_reserv(struct gfs2_inode *ip, loff_t *len,
+			    unsigned int *data_blocks, unsigned int *ind_blocks,
+			    unsigned int max_blocks)
 {
+	loff_t max = *len;
 	const struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
-	unsigned int max_blocks = ip->i_rgd->rd_free_clone;
 	unsigned int tmp, max_data = max_blocks - 3 * (sdp->sd_max_height - 1);
 
 	for (tmp = max_data; tmp > sdp->sd_diptrs;) {
 		tmp = DIV_ROUND_UP(tmp, sdp->sd_inptrs);
 		max_data -= tmp;
 	}
-	/* This calculation isn't the exact reverse of gfs2_write_calc_reserve,
-	   so it might end up with fewer data blocks */
-	if (max_data <= *data_blocks)
-		return;
+
 	*data_blocks = max_data;
 	*ind_blocks = max_blocks - max_data;
 	*len = ((loff_t)max_data - 3) << sdp->sd_sb.sb_bsize_shift;
@@ -797,7 +805,7 @@ static long __gfs2_fallocate(struct file *file, int mode, loff_t offset, loff_t
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_alloc_parms ap = { .aflags = 0, };
 	unsigned int data_blocks = 0, ind_blocks = 0, rblocks;
-	loff_t bytes, max_bytes;
+	loff_t bytes, max_bytes, max_blks = UINT_MAX;
 	int error;
 	const loff_t pos = offset;
 	const loff_t count = len;
@@ -819,6 +827,9 @@ static long __gfs2_fallocate(struct file *file, int mode, loff_t offset, loff_t
 
 	gfs2_size_hint(file, offset, len);
 
+	gfs2_write_calc_reserv(ip, PAGE_SIZE, &data_blocks, &ind_blocks);
+	ap.min_target = data_blocks + ind_blocks;
+
 	while (len > 0) {
 		if (len < bytes)
 			bytes = len;
@@ -827,28 +838,41 @@ static long __gfs2_fallocate(struct file *file, int mode, loff_t offset, loff_t
 			offset += bytes;
 			continue;
 		}
-retry:
+
+		/* We need to determine how many bytes we can actually
+		 * fallocate without exceeding quota or going over the
+		 * end of the fs. We start off optimistically by assuming
+		 * we can write max_bytes */
+		max_bytes = (len > max_chunk_size) ? max_chunk_size : len;
+
+		/* Since max_bytes is most likely a theoretical max, we
+		 * calculate a more realistic 'bytes' to serve as a good
+		 * starting point for the number of bytes we may be able
+		 * to write */
 		gfs2_write_calc_reserv(ip, bytes, &data_blocks, &ind_blocks);
 		ap.target = data_blocks + ind_blocks;
 
 		error = gfs2_quota_lock_check(ip, &ap);
 		if (error)
 			return error;
+		/* ap.allowed tells us how many blocks quota will allow
+		 * us to write. Check if this reduces max_blks */
+		if (ap.allowed && ap.allowed < max_blks)
+			max_blks = ap.allowed;
+
 		error = gfs2_inplace_reserve(ip, &ap);
-		if (error) {
-			if (error == -ENOSPC && bytes > sdp->sd_sb.sb_bsize) {
-				bytes >>= 1;
-				bytes &= bsize_mask;
-				if (bytes == 0)
-					bytes = sdp->sd_sb.sb_bsize;
-				gfs2_quota_unlock(ip);
-				goto retry;
-			}
+		if (error)
 			goto out_qunlock;
-		}
-		max_bytes = bytes;
-		calc_max_reserv(ip, (len > max_chunk_size)? max_chunk_size: len,
-				&max_bytes, &data_blocks, &ind_blocks);
+
+		/* check if the selected rgrp limits our max_blks further */
+		if (ap.allowed && ap.allowed < max_blks)
+			max_blks = ap.allowed;
+
+		/* Almost done. Calculate bytes that can be written using
+		 * max_blks. We also recompute max_bytes, data_blocks and
+		 * ind_blocks */
+		calc_max_reserv(ip, &max_bytes, &data_blocks,
+				&ind_blocks, max_blks);
 
 		rblocks = RES_DINODE + ind_blocks + RES_STATFS + RES_QUOTA +
 			  RES_RG_HDR + gfs2_rg_blocks(ip, data_blocks + ind_blocks);