Patch "btrfs: alloc_chunk: fix DUP stripe size handling" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

btrfs: alloc_chunk: fix DUP stripe size handling

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-alloc_chunk-fix-dup-stripe-size-handling.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hans van Kranenburg <[email protected]>
Date: Mon, 5 Feb 2018 17:45:11 +0100
Subject: btrfs: alloc_chunk: fix DUP stripe size handling

From: Hans van Kranenburg <[email protected]>

commit 92e222df7b8f05c565009c7383321b593eca488b upstream.

In case of using DUP, we search for enough unallocated disk space on a device to hold two stripes.

The devices_info[ndevs-1].max_avail that holds the amount of unallocated space found is directly assigned to stripe_size, while it's actually twice the stripe size.

Later on in the code, an unconditional division of stripe_size by dev_stripes corrects the value, but in the meantime there's a check to see if the stripe_size does not exceed max_chunk_size. Since during this check stripe_size is twice the amount as intended, the check will reduce the stripe_size to max_chunk_size if the actual correct to be used stripe_size is more than half the amount of max_chunk_size.

The unconditional division later tries to correct stripe_size, but will actually make sure we can't allocate more than half the max_chunk_size.

Fix this by moving the division by dev_stripes before the max chunk size check, so it always contains the right value, instead of putting a duct tape division in further on to get it fixed again.

Since in all other cases than DUP, dev_stripes is 1, this change only affects DUP.

Other attempts in the past were made to fix this:
* 37db63a400 "Btrfs: fix max chunk size check in chunk allocator" tried to fix the same problem, but still resulted in part of the code acting on a wrongly doubled stripe_size value.
* 86db25785a "Btrfs: fix max chunk size on raid5/6" unintentionally broke this fix again.

The real problem was already introduced with the rest of the code in 73c5de0051.

The user visible result however will be that the max chunk size for DUP will suddenly double, while it's actually acting according to the limits in the code again like it was 5 years ago.

---
 fs/btrfs/volumes.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

@@ -4737,10 +4737,13 @@ static int __btrfs_alloc_chunk(struct bt
 	ndevs = min(ndevs, devs_max);
 
 	/*
-	 * the primary goal is to maximize the number of stripes, so use as many
-	 * devices as possible, even if the stripes are not maximum sized.
+	 * The primary goal is to maximize the number of stripes, so use as
+	 * many devices as possible, even if the stripes are not maximum sized.
+	 *
+	 * The DUP profile stores more than one stripe per device, the
+	 * max_avail is the total size so we have to adjust.
 	 */
-	stripe_size = devices_info[ndevs-1].max_avail;
+	stripe_size = div_u64(devices_info[ndevs - 1].max_avail, dev_stripes);
 	num_stripes = ndevs * dev_stripes;
 
 	/*

@@ -4775,8 +4778,6 @@ static int __btrfs_alloc_chunk(struct bt
 			stripe_size = devices_info[ndevs-1].max_avail;
 	}
 
-	stripe_size = div_u64(stripe_size, dev_stripes);
-
 	/* align to BTRFS_STRIPE_LEN */
 	stripe_size = round_down(stripe_size, BTRFS_STRIPE_LEN);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/btrfs-alloc_chunk-fix-dup-stripe-size-handling.patch