6.1-stable review patch. If anyone has any objections, please let me know.
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From: Zygo Blaxell ce3g8jdj@umail.furryterror.org
commit 8a540e990d7da36813cb71a4a422712bfba448a4 upstream.
Commit f6fca3917b4d "btrfs: store chunk size in space-info struct" broke data chunk allocations on non-zoned multi-device filesystems when using default chunk_size. Commit 5da431b71d4b "btrfs: fix the max chunk size and stripe length calculation" partially fixed that, and this patch completes the fix for that case.
After commit f6fca3917b4d and 5da431b71d4b, the sequence of events for a data chunk allocation on a non-zoned filesystem is:
1. btrfs_create_chunk calls init_alloc_chunk_ctl, which copies space_info->chunk_size (default 10 GiB) to ctl->max_stripe_len unmodified. Before f6fca3917b4d, ctl->max_stripe_len value was 1 GiB for non-zoned data chunks and not configurable.
2. btrfs_create_chunk calls gather_device_info which consumes and produces more fields of chunk_ctl.
3. gather_device_info multiplies ctl->max_stripe_len by ctl->dev_stripes (which is 1 in all cases except dup) and calls find_free_dev_extent with that number as num_bytes.
4. find_free_dev_extent locates the first dev_extent hole on a device which is at least as large as num_bytes. With default max_chunk_size from f6fca3917b4d, it finds the first hole which is longer than 10 GiB, or the largest hole if that hole is shorter than 10 GiB. This is different from the pre-f6fca3917b4d behavior, where num_bytes is 1 GiB, and find_free_dev_extent may choose a different hole.
5. gather_device_info repeats step 4 with all devices to find the first or largest dev_extent hole that can be allocated on each device.
6. gather_device_info sorts the device list by the hole size on each device, using total unallocated space on each device to break ties, then returns to btrfs_create_chunk with the list.
7. btrfs_create_chunk calls decide_stripe_size_regular.
8. decide_stripe_size_regular finds the largest stripe_len that fits across the first nr_devs device dev_extent holes that were found by gather_device_info (and satisfies other constraints on stripe_len that are not relevant here).
9. decide_stripe_size_regular caps the length of the stripe it computed at 1 GiB. This cap appeared in 5da431b71d4b to correct one of the other regressions introduced in f6fca3917b4d.
10. btrfs_create_chunk creates a new chunk with the above computed size and number of devices.
At step 4, gather_device_info() has found a location where stripe up to 10 GiB in length could be allocated on several devices, and selected which devices should have a dev_extent allocated on them, but at step 9, only 1 GiB of the space that was found on each device can be used. This mismatch causes new suboptimal chunk allocation cases that did not occur in pre-f6fca3917b4d kernels.
Consider a filesystem using raid1 profile with 3 devices. After some balances, device 1 has 10x 1 GiB unallocated space, while devices 2 and 3 have 1x 10 GiB unallocated space, i.e. the same total amount of space, but distributed across different numbers of dev_extent holes. For visualization, let's ignore all the chunks that were allocated before this point, and focus on the remaining holes:
Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10x 1 GiB unallocated) Device 2: [__________] (10 GiB contig unallocated) Device 3: [__________] (10 GiB contig unallocated)
Before f6fca3917b4d, the allocator would fill these optimally by allocating chunks with dev_extents on devices 1 and 2 ([12]), 1 and 3 ([13]), or 2 and 3 ([23]):
[after 0 chunk allocations] Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10 GiB) Device 2: [__________] (10 GiB) Device 3: [__________] (10 GiB)
[after 1 chunk allocation] Device 1: [12] [_] [_] [_] [_] [_] [_] [_] [_] [_] Device 2: [12] [_________] (9 GiB) Device 3: [__________] (10 GiB)
[after 2 chunk allocations] Device 1: [12] [13] [_] [_] [_] [_] [_] [_] [_] [_] (8 GiB) Device 2: [12] [_________] (9 GiB) Device 3: [13] [_________] (9 GiB)
[after 3 chunk allocations] Device 1: [12] [13] [12] [_] [_] [_] [_] [_] [_] [_] (7 GiB) Device 2: [12] [12] [________] (8 GiB) Device 3: [13] [_________] (9 GiB)
[...]
[after 12 chunk allocations] Device 1: [12] [13] [12] [13] [12] [13] [12] [13] [_] [_] (2 GiB) Device 2: [12] [12] [23] [23] [12] [12] [23] [23] [__] (2 GiB) Device 3: [13] [13] [23] [23] [13] [23] [13] [23] [__] (2 GiB)
[after 13 chunk allocations] Device 1: [12] [13] [12] [13] [12] [13] [12] [13] [12] [_] (1 GiB) Device 2: [12] [12] [23] [23] [12] [12] [23] [23] [12] [_] (1 GiB) Device 3: [13] [13] [23] [23] [13] [23] [13] [23] [__] (2 GiB)
[after 14 chunk allocations] Device 1: [12] [13] [12] [13] [12] [13] [12] [13] [12] [13] (full) Device 2: [12] [12] [23] [23] [12] [12] [23] [23] [12] [_] (1 GiB) Device 3: [13] [13] [23] [23] [13] [23] [13] [23] [13] [_] (1 GiB)
[after 15 chunk allocations] Device 1: [12] [13] [12] [13] [12] [13] [12] [13] [12] [13] (full) Device 2: [12] [12] [23] [23] [12] [12] [23] [23] [12] [23] (full) Device 3: [13] [13] [23] [23] [13] [23] [13] [23] [13] [23] (full)
This allocates all of the space with no waste. The sorting function used by gather_device_info considers free space holes above 1 GiB in length to be equal to 1 GiB, so once find_free_dev_extent locates a sufficiently long hole on each device, all the holes appear equal in the sort, and the comparison falls back to sorting devices by total free space. This keeps usable space on each device equal so they can all be filled completely.
After f6fca3917b4d, the allocator prefers the devices with larger holes over the devices with more free space, so it makes bad allocation choices:
[after 1 chunk allocation] Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10 GiB) Device 2: [23] [_________] (9 GiB) Device 3: [23] [_________] (9 GiB)
[after 2 chunk allocations] Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10 GiB) Device 2: [23] [23] [________] (8 GiB) Device 3: [23] [23] [________] (8 GiB)
[after 3 chunk allocations] Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10 GiB) Device 2: [23] [23] [23] [_______] (7 GiB) Device 3: [23] [23] [23] [_______] (7 GiB)
[...]
[after 9 chunk allocations] Device 1: [_] [_] [_] [_] [_] [_] [_] [_] [_] [_] (10 GiB) Device 2: [23] [23] [23] [23] [23] [23] [23] [23] [23] [_] (1 GiB) Device 3: [23] [23] [23] [23] [23] [23] [23] [23] [23] [_] (1 GiB)
[after 10 chunk allocations] Device 1: [12] [_] [_] [_] [_] [_] [_] [_] [_] [_] (9 GiB) Device 2: [23] [23] [23] [23] [23] [23] [23] [23] [12] (full) Device 3: [23] [23] [23] [23] [23] [23] [23] [23] [_] (1 GiB)
[after 11 chunk allocations] Device 1: [12] [13] [_] [_] [_] [_] [_] [_] [_] [_] (8 GiB) Device 2: [23] [23] [23] [23] [23] [23] [23] [23] [12] (full) Device 3: [23] [23] [23] [23] [23] [23] [23] [23] [13] (full)
No further allocations are possible, with 8 GiB wasted (4 GiB of data space). The sort in gather_device_info now considers free space in holes longer than 1 GiB to be distinct, so it will prefer devices 2 and 3 over device 1 until all but 1 GiB is allocated on devices 2 and 3. At that point, with only 1 GiB unallocated on every device, the largest hole length on each device is equal at 1 GiB, so the sort finally moves to ordering the devices with the most free space, but by this time it is too late to make use of the free space on device 1.
Note that it's possible to contrive a case where the pre-f6fca3917b4d allocator fails the same way, but these cases generally have extensive dev_extent fragmentation as a precondition (e.g. many holes of 768M in length on one device, and few holes 1 GiB in length on the others). With the regression in f6fca3917b4d, bad chunk allocation can occur even under optimal conditions, when all dev_extent holes are exact multiples of stripe_len in length, as in the example above.
Also note that post-f6fca3917b4d kernels do treat dev_extent holes larger than 10 GiB as equal, so the bad behavior won't show up on a freshly formatted filesystem; however, as the filesystem ages and fills up, and holes ranging from 1 GiB to 10 GiB in size appear, the problem can show up as a failure to balance after adding or removing devices, or an unexpected shortfall in available space due to unequal allocation.
To fix the regression and make data chunk allocation work again, set ctl->max_stripe_len back to the original SZ_1G, or space_info->chunk_size if that's smaller (the latter can happen if the user set space_info->chunk_size to less than 1 GiB via sysfs, or it's a 32 MiB system chunk with a hardcoded chunk_size and stripe_len).
While researching the background of the earlier commits, I found that an identical fix was already proposed at:
https://lore.kernel.org/linux-btrfs/de83ac46-a4a3-88d3-85ce-255b7abc5249@gmx...
The previous review missed one detail: ctl->max_stripe_len is used before decide_stripe_size_regular() is called, when it is too late for the changes in that function to have any effect. ctl->max_stripe_len is not used directly by decide_stripe_size_regular(), but the parameter does heavily influence the per-device free space data presented to the function.
Fixes: f6fca3917b4d ("btrfs: store chunk size in space-info struct") CC: stable@vger.kernel.org # 6.1+ Link: https://lore.kernel.org/linux-btrfs/20231007051421.19657-1-ce3g8jdj@umail.fu... Reviewed-by: Qu Wenruo wqu@suse.com Signed-off-by: Zygo Blaxell ce3g8jdj@umail.furryterror.org Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/volumes.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -5139,7 +5139,7 @@ static void init_alloc_chunk_ctl_policy_ ASSERT(space_info);
ctl->max_chunk_size = READ_ONCE(space_info->chunk_size); - ctl->max_stripe_size = ctl->max_chunk_size; + ctl->max_stripe_size = min_t(u64, ctl->max_chunk_size, SZ_1G);
if (ctl->type & BTRFS_BLOCK_GROUP_SYSTEM) ctl->devs_max = min_t(int, ctl->devs_max, BTRFS_MAX_DEVS_SYS_CHUNK);