25  * We need to walk the filesystem free space and issue discards on the free
26  * space that meet the search criteria (size and location). We cannot issue
28  * still marked as busy. To serialise against extent state changes whilst we are
29  * gathering extents to trim, we must hold the AGF lock to lock out other
32  * However, we cannot just hold the AGF for the entire AG free space walk whilst
33  * we issue discards on each free space that is found. Storage devices can have
36  * extent can take a *long* time. Whilst we are doing this walk, nothing else
37  * can access the AGF, and we can stall transactions and hence the log whilst
41  * Hence we need to take a leaf from the bulkstat playbook. It takes the AGI
47  * We can't do this exactly with free space - once we drop the AGF lock, the
48  * state of the free extent is out of our control and we cannot run a discard
49  * safely on it in this situation. Unless, of course, we've marked the free
50  * extent as busy and undergoing a discard operation whilst we held the AGF
59  * In the context of fstrim, if we find a free extent we need to discard, we
60  * don't have to discard it immediately. All we need to do it record that free
62  * now avoid trying to allocate it. Hence if we mark the extent as busy under
63  * the AGF lock, we can safely discard it without holding the AGF lock because
66  * This also allows us to issue discards asynchronously like we do with online
67  * discard, and so for fast devices fstrim will run much faster as we can have
102  * list. We plug and chain the bios so that we only need a single completion
166 	 * space before we take the AGF buffer lock are now on disk, and the  in xfs_trim_gather_extents()
188 		/* nothing of that length left in the AG, we are done */  in xfs_trim_gather_extents()
194 	 * Loop until we are done with all extents that are large  in xfs_trim_gather_extents()
195 	 * enough to be worth discarding or we hit batch limits.  in xfs_trim_gather_extents()
213 			 * Update the cursor to point at this extent so we  in xfs_trim_gather_extents()
239 		 * If the extent is entirely outside of the range we are  in xfs_trim_gather_extents()
266 		 * If there's no more records in the tree, we are done. Set the  in xfs_trim_gather_extents()
276 	 * we aren't going to issue a discard on them any more.  in xfs_trim_gather_extents()
293  * Iterate the free list gathering extents and discarding them. We need a cursor
295  * we found in the last batch as the key to start the next.
332 		 * We hand the extent list to the discard function here so the  in xfs_trim_extents()
337 		 * However, we must ensure that we do not reference the extent  in xfs_trim_extents()
358  * filesystem which does not match to the format we use for filesystem block
360  * is a linear address range. Hence we need to use DADDR based conversions and
383 	 * We haven't recovered the log, so we cannot use our bnobt-guided  in xfs_ioc_trim()
396 	 * BBTOB would mean we trivially get overflows for values  in xfs_ioc_trim()