From 501b1d7983ecc9373ed4a98ac381d86a6ea8899d Mon Sep 17 00:00:00 2001
From: Andrea Mazzoleni <amadvance@gmail.com>
Date: Mon, 25 Jul 2016 21:46:15 +0200
Subject: [PATCH] Avoid extra allocation in parity file

After growing the parity file, call an extra ftruncate()
to ensure to cut the file to the exact size, removing any extra
allocation that could result by failed attempts to grow it.
Note that how fallocate() behaves on failures is not really
specified, so it's possible that extra space is allocated.
---
 cmdline/parity.c | 48 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 13 deletions(-)

diff --git a/cmdline/parity.c b/cmdline/parity.c
index 52ed225e..3cac083e 100644
--- a/cmdline/parity.c
+++ b/cmdline/parity.c
@@ -322,6 +322,11 @@ static int parity_handle_grow(struct snapraid_split_handle* split, data_off_t si
 	return ret;
 }
 
+static int parity_handle_shrink(struct snapraid_split_handle* split, data_off_t size)
+{
+	return ftruncate(split->f, size);
+}
+
 /**
  * Get the highest bit set.
  */
@@ -360,29 +365,33 @@ static int parity_handle_fill(struct snapraid_split_handle* split, data_off_t si
 		int ret;
 		data_off_t run = hbit_u64(delta);
 
-		ret = parity_handle_grow(split, base + run, skip_fallocate);
+		/* mask out the bit we process */
+		delta &= ~run;
 
+		ret = parity_handle_grow(split, base + run, skip_fallocate);
 		if (ret != 0) {
-			/* we cannot grow, fallback to the smaller bits */
+			/* we cannot grow, fallback enabling all the smaller bits */
 			delta = run - 1;
 
 			/* mask out the block size */
 			delta &= ~block_mask;
 		} else {
-			/* mask out this bit */
-			delta &= ~run;
+			/* increase the effective size */
 			base += run;
 		}
 	}
 
-	/* reallocate the expected size */
-	/* if this fail, is there a problem */
-	return parity_handle_grow(split, base, skip_fallocate);
-}
+	/* ensure that the resulting size if block aligned */
+	if ((base & block_mask) != 0) {
+		/* LCOV_EXCL_START */
+		log_fatal("Internal inconsistency in requested parity size %llu with block %u\n", base, block_size);
+		os_abort();
+		/* LCOV_EXCL_STOP */
+	}
 
-static int parity_handle_shrink(struct snapraid_split_handle* split, data_off_t size)
-{
-	return ftruncate(split->f, size);
+	/* shrink to the expected size to ensure to throw away any extra */
+	/* data allocated when the grow operation fails */
+	return parity_handle_shrink(split, base);
 }
 
 static int parity_handle_chsize(struct snapraid_split_handle* split, data_off_t size, uint32_t block_size, int skip_fallocate)
@@ -454,6 +463,10 @@ int parity_chsize(struct snapraid_parity_handle* handle, struct snapraid_parity*
 {
 	int ret;
 	unsigned s;
+	data_off_t block_mask;
+
+	/* mask of bits used by the block size */
+	block_mask = ((data_off_t)block_size) - 1;
 
 	if (size < 0)
 		return -1;
@@ -489,8 +502,17 @@ int parity_chsize(struct snapraid_parity_handle* handle, struct snapraid_parity*
 			return -1;
 			/* LCOV_EXCL_STOP */
 		} else {
-			size -= split->st.st_size;
-			split->size = split->st.st_size;
+			run = split->st.st_size;
+
+			if ((run & block_mask) != 0) {
+				/* LCOV_EXCL_START */
+				log_fatal("Internal inconsistency in final parity size %llu with block size %u\n", run, block_size);
+				os_abort();
+				/* LCOV_EXCL_STOP */
+			}
+
+			size -= run;
+			split->size = run;
 		}
 	}