@@ -1,4 +1,39 @@
// SPDX-License-Identifier: GPL-2.0
+/*
+ * This contains the logic to handle async discard.
+ *
+ * Async discard manages trimming of free space outside of transaction commit.
+ * Discarding is done by managing the block_groups on a LRU list based on free
+ * space recency. Two passes are used to first prioritize discarding extents
+ * and then allow for trimming in the bitmap the best opportunity to coalesce.
+ * The block_groups are maintained on multiple lists to allow for multiple
+ * passes with different discard filter requirements. A delayed work item is
+ * used to manage discarding with timeout determined by a max of the delay
+ * incurred by the iops rate limit, byte rate limit, and the timeout of max
+ * delay of BTRFS_DISCARD_MAX_DELAY.
+ *
+ * The first list is special to manage discarding of fully free block groups.
+ * This is necessary because we issue a final trim for a full free block group
+ * after forgetting it. When a block group becomes unused, instead of directly
+ * being added to the unused_bgs list, we add it to this first list. Then
+ * from there, if it becomes fully discarded, we place it onto the unused_bgs
+ * list.
+ *
+ * The in-memory free space cache serves as the backing state for discard.
+ * Consequently this means there is no persistence. We opt to load all the
+ * block groups in as not discarded, so the mount case degenerates to the
+ * crashing case.
+ *
+ * As the free space cache uses bitmaps, there exists a tradeoff between
+ * ease/efficiency for find_free_extent() and the accuracy of discard state.
+ * Here we opt to let untrimmed regions merge with everything while only letting
+ * trimmed regions merge with other trimmed regions. This can cause
+ * overtrimming, but the coalescing benefit seems to be worth it. Additionally,
+ * bitmap state is tracked as a whole. If we're able to fully trim a bitmap,
+ * the trimmed flag is set on the bitmap. Otherwise, if an allocation comes in,
+ * this resets the state and we will retry trimming the whole bitmap. This is a
+ * tradeoff between discard state accuracy and the cost of accounting.
+ */
#include <linux/jiffies.h>
#include <linux/kernel.h>