#include "xfs_log.h"
#include "xfs_ag.h"
+/*
+ * Notes on an efficient, low latency fstrim algorithm
+ *
+ * We need to walk the filesystem free space and issue discards on the free
+ * space that meet the search criteria (size and location). We cannot issue
+ * discards on extents that might be in use, or are so recently in use they are
+ * still marked as busy. To serialise against extent state changes whilst we are
+ * gathering extents to trim, we must hold the AGF lock to lock out other
+ * allocations and extent free operations that might change extent state.
+ *
+ * However, we cannot just hold the AGF for the entire AG free space walk whilst
+ * we issue discards on each free space that is found. Storage devices can have
+ * extremely slow discard implementations (e.g. ceph RBD) and so walking a
+ * couple of million free extents and issuing synchronous discards on each
+ * extent can take a *long* time. Whilst we are doing this walk, nothing else
+ * can access the AGF, and we can stall transactions and hence the log whilst
+ * modifications wait for the AGF lock to be released. This can lead hung tasks
+ * kicking the hung task timer and rebooting the system. This is bad.
+ *
+ * Hence we need to take a leaf from the bulkstat playbook. It takes the AGI
+ * lock, gathers a range of inode cluster buffers that are allocated, drops the
+ * AGI lock and then reads all the inode cluster buffers and processes them. It
+ * loops doing this, using a cursor to keep track of where it is up to in the AG
+ * for each iteration to restart the INOBT lookup from.
+ *
+ * We can't do this exactly with free space - once we drop the AGF lock, the
+ * state of the free extent is out of our control and we cannot run a discard
+ * safely on it in this situation. Unless, of course, we've marked the free
+ * extent as busy and undergoing a discard operation whilst we held the AGF
+ * locked.
+ *
+ * This is exactly how online discard works - free extents are marked busy when
+ * they are freed, and once the extent free has been committed to the journal,
+ * the busy extent record is marked as "undergoing discard" and the discard is
+ * then issued on the free extent. Once the discard completes, the busy extent
+ * record is removed and the extent is able to be allocated again.
+ *
+ * In the context of fstrim, if we find a free extent we need to discard, we
+ * don't have to discard it immediately. All we need to do it record that free
+ * extent as being busy and under discard, and all the allocation routines will
+ * now avoid trying to allocate it. Hence if we mark the extent as busy under
+ * the AGF lock, we can safely discard it without holding the AGF lock because
+ * nothing will attempt to allocate that free space until the discard completes.
+ *
+ * This also allows us to issue discards asynchronously like we do with online
+ * discard, and so for fast devices fstrim will run much faster as we can have
+ * multiple discard operations in flight at once, as well as pipeline the free
+ * extent search so that it overlaps in flight discard IO.
+ */
+
struct workqueue_struct *xfs_discard_wq;
static void
}
-STATIC int
-xfs_trim_extents(
+static int
+xfs_trim_gather_extents(
struct xfs_perag *pag,
xfs_daddr_t start,
xfs_daddr_t end,
xfs_daddr_t minlen,
+ struct xfs_alloc_rec_incore *tcur,
+ struct xfs_busy_extents *extents,
uint64_t *blocks_trimmed)
{
struct xfs_mount *mp = pag->pag_mount;
- struct block_device *bdev = mp->m_ddev_targp->bt_bdev;
struct xfs_btree_cur *cur;
struct xfs_buf *agbp;
- struct xfs_agf *agf;
int error;
int i;
+ int batch = 100;
/*
* Force out the log. This means any transactions that might have freed
error = xfs_alloc_read_agf(pag, NULL, 0, &agbp);
if (error)
return error;
- agf = agbp->b_addr;
cur = xfs_allocbt_init_cursor(mp, NULL, agbp, pag, XFS_BTNUM_CNT);
/*
- * Look up the longest btree in the AGF and start with it.
+ * Look up the extent length requested in the AGF and start with it.
*/
- error = xfs_alloc_lookup_ge(cur, 0, be32_to_cpu(agf->agf_longest), &i);
+ if (tcur->ar_startblock == NULLAGBLOCK)
+ error = xfs_alloc_lookup_ge(cur, 0, tcur->ar_blockcount, &i);
+ else
+ error = xfs_alloc_lookup_le(cur, tcur->ar_startblock,
+ tcur->ar_blockcount, &i);
if (error)
goto out_del_cursor;
+ if (i == 0) {
+ /* nothing of that length left in the AG, we are done */
+ tcur->ar_blockcount = 0;
+ goto out_del_cursor;
+ }
/*
* Loop until we are done with all extents that are large
- * enough to be worth discarding.
+ * enough to be worth discarding or we hit batch limits.
*/
while (i) {
xfs_agblock_t fbno;
error = -EFSCORRUPTED;
break;
}
- ASSERT(flen <= be32_to_cpu(agf->agf_longest));
+
+ if (--batch <= 0) {
+ /*
+ * Update the cursor to point at this extent so we
+ * restart the next batch from this extent.
+ */
+ tcur->ar_startblock = fbno;
+ tcur->ar_blockcount = flen;
+ break;
+ }
/*
* use daddr format for all range/len calculations as that is
*/
if (dlen < minlen) {
trace_xfs_discard_toosmall(mp, pag->pag_agno, fbno, flen);
+ tcur->ar_blockcount = 0;
break;
}
goto next_extent;
}
- trace_xfs_discard_extent(mp, pag->pag_agno, fbno, flen);
- error = blkdev_issue_discard(bdev, dbno, dlen, GFP_NOFS);
- if (error)
- break;
+ xfs_extent_busy_insert_discard(pag, fbno, flen,
+ &extents->extent_list);
*blocks_trimmed += flen;
-
next_extent:
error = xfs_btree_decrement(cur, 0, &i);
if (error)
break;
- if (fatal_signal_pending(current)) {
- error = -ERESTARTSYS;
- break;
- }
+ /*
+ * If there's no more records in the tree, we are done. Set the
+ * cursor block count to 0 to indicate to the caller that there
+ * is no more extents to search.
+ */
+ if (i == 0)
+ tcur->ar_blockcount = 0;
}
+ /*
+ * If there was an error, release all the gathered busy extents because
+ * we aren't going to issue a discard on them any more.
+ */
+ if (error)
+ xfs_extent_busy_clear(mp, &extents->extent_list, false);
out_del_cursor:
xfs_btree_del_cursor(cur, error);
xfs_buf_relse(agbp);
return error;
}
+/*
+ * Iterate the free list gathering extents and discarding them. We need a cursor
+ * for the repeated iteration of gather/discard loop, so use the longest extent
+ * we found in the last batch as the key to start the next.
+ */
+static int
+xfs_trim_extents(
+ struct xfs_perag *pag,
+ xfs_daddr_t start,
+ xfs_daddr_t end,
+ xfs_daddr_t minlen,
+ uint64_t *blocks_trimmed)
+{
+ struct xfs_alloc_rec_incore tcur = {
+ .ar_blockcount = pag->pagf_longest,
+ .ar_startblock = NULLAGBLOCK,
+ };
+ int error = 0;
+
+ do {
+ struct xfs_busy_extents *extents;
+
+ extents = kzalloc(sizeof(*extents), GFP_KERNEL);
+ if (!extents) {
+ error = -ENOMEM;
+ break;
+ }
+
+ extents->mount = pag->pag_mount;
+ extents->owner = extents;
+ INIT_LIST_HEAD(&extents->extent_list);
+
+ error = xfs_trim_gather_extents(pag, start, end, minlen,
+ &tcur, extents, blocks_trimmed);
+ if (error) {
+ kfree(extents);
+ break;
+ }
+
+ /*
+ * We hand the extent list to the discard function here so the
+ * discarded extents can be removed from the busy extent list.
+ * This allows the discards to run asynchronously with gathering
+ * the next round of extents to discard.
+ *
+ * However, we must ensure that we do not reference the extent
+ * list after this function call, as it may have been freed by
+ * the time control returns to us.
+ */
+ error = xfs_discard_extents(pag->pag_mount, extents);
+ if (error)
+ break;
+
+ if (fatal_signal_pending(current)) {
+ error = -ERESTARTSYS;
+ break;
+ }
+ } while (tcur.ar_blockcount != 0);
+
+ return error;
+
+}
+
/*
* trim a range of the filesystem.
*
projects / users / hch / dma-mapping.git / commitdiff