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[v3,2/2] btrfs: account for new extents being deleted in total_bytes_pinned

Message ID 9a73d5fe6572858e859f739183c8a94f05d22bf0.1610747242.git.josef@toxicpanda.com (mailing list archive)
State New, archived
Headers show
Series ->total_bytes_pinned fixes for early ENOSPC issues | expand

Commit Message

Josef Bacik Jan. 15, 2021, 9:48 p.m. UTC
My recent patch set "A variety of lock contention fixes", found here

https://github.com/btrfs/linux/issues/86

that reduce lock contention on the extent root by running delayed refs
less often resulted in a regression in generic/371.  This test
fallocate()'s the fs until it's full, deletes all the files, and then
tries to fallocate() until full again.

Before these patches we would run all of the delayed refs during
flushing, and then would commit the transaction because we had plenty of
pinned space to recover in order to allocate.  However my patches made
it so we weren't running the delayed refs as aggressively, which meant
that we appeared to have less pinned space when we were deciding to
commit the transaction.

We use the space_info->total_bytes_pinned to approximate how much space
we have pinned.  It's approximate because if we remove a reference to an
extent we may free it, but there may be more references to it than we
know of at that point, but we account it as pinned at the creation time,
and then it's properly accounted when the delayed ref runs.

The way we account for pinned space is if the
delayed_ref_head->total_ref_mod is < 0, because that is clearly a
free'ing option.  However there is another case, and that is where
->total_ref_mod == 0 && ->must_insert_reserved == 1.

When we allocate a new extent, we have ->total_ref_mod == 1 and we have
->must_insert_reserved == 1.  This is used to indicate that it is a
brand new extent and will need to have its extent entry added before we
modify any references on the delayed ref head.  But if we subsequently
remove that extent reference, our ->total_ref_mod will be 0, and that
space will be pinned and freed.  Accounting for this case properly
allows for generic/371 to pass with my delayed refs patches applied.

It's important to note that this problem exists without the referenced
patches, it just was uncovered by them.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/delayed-ref.c |  7 +++++++
 fs/btrfs/extent-tree.c | 34 ++++++++++++++++++++--------------
 2 files changed, 27 insertions(+), 14 deletions(-)
diff mbox series

Patch

diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index 1edb0095f08d..33247f2d0c27 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -732,6 +732,9 @@  static noinline void update_existing_head_ref(struct btrfs_trans_handle *trans,
 	 * 2. We were negative and went to 0 or positive, so no longer can say
 	 *    that the space would be pinned, decrement our counter from the
 	 *    total_bytes_pinned counter.
+	 * 3. We are now at 0 and have ->must_insert_reserved set, which means
+	 *    this was a new allocation and then we dropped it, and thus must
+	 *    add our space to the total_bytes_pinned counter.
 	 */
 	if (existing->total_ref_mod < 0 && old_ref_mod >= 0)
 		btrfs_mod_total_bytes_pinned(fs_info, flags,
@@ -739,6 +742,10 @@  static noinline void update_existing_head_ref(struct btrfs_trans_handle *trans,
 	else if (existing->total_ref_mod >= 0 && old_ref_mod < 0)
 		btrfs_mod_total_bytes_pinned(fs_info, flags,
 					     -existing->num_bytes);
+	else if (existing->total_ref_mod == 0 &&
+		 existing->must_insert_reserved)
+		btrfs_mod_total_bytes_pinned(fs_info, flags,
+					     existing->num_bytes);
 
 	spin_unlock(&existing->lock);
 }
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 290d957f4603..65a88f9333e6 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -1754,23 +1754,29 @@  void btrfs_cleanup_ref_head_accounting(struct btrfs_fs_info *fs_info,
 {
 	int nr_items = 1;	/* Dropping this ref head update. */
 
-	if (head->total_ref_mod < 0) {
+	/*
+	 * We had csum deletions accounted for in our delayed refs rsv, we need
+	 * to drop the csum leaves for this update from our delayed_refs_rsv.
+	 */
+	if (head->total_ref_mod < 0 && head->is_data) {
+		spin_lock(&delayed_refs->lock);
+		delayed_refs->pending_csums -= head->num_bytes;
+		spin_unlock(&delayed_refs->lock);
+		nr_items += btrfs_csum_bytes_to_leaves(fs_info,
+						       head->num_bytes);
+	}
+
+	/*
+	 * We were dropping refs, or had a new ref and dropped it, and thus must
+	 * adjust down our total_bytes_pinned, the space may or may not have
+	 * been pinned and so is accounted for properly in the pinned space by
+	 * now.
+	 */
+	if (head->total_ref_mod < 0 ||
+	    (head->total_ref_mod == 0 && head->must_insert_reserved)) {
 		u64 flags = btrfs_ref_head_to_space_flags(head);
 		btrfs_mod_total_bytes_pinned(fs_info, flags,
 					     -head->num_bytes);
-
-		/*
-		 * We had csum deletions accounted for in our delayed refs rsv,
-		 * we need to drop the csum leaves for this update from our
-		 * delayed_refs_rsv.
-		 */
-		if (head->is_data) {
-			spin_lock(&delayed_refs->lock);
-			delayed_refs->pending_csums -= head->num_bytes;
-			spin_unlock(&delayed_refs->lock);
-			nr_items += btrfs_csum_bytes_to_leaves(fs_info,
-				head->num_bytes);
-		}
 	}
 
 	btrfs_delayed_refs_rsv_release(fs_info, nr_items);