On Wed, 2021-07-28 at 15:10 +0200, David Sterba wrote:

On Mon, Jul 26, 2021 at 11:03:17AM -0300, Marcos Paulo de Souza
wrote:

quoted

It's a common practice to start a search using offset (u64)-1,
which is
the u64 maximum value, meaning that we want the search_slot
function to
be set in the last item with the same objectid and type.

Once we are in this position, it's a matter to start a search
backwards
by calling btrfs_previous_item, which will check if we'll need to
go to
a previous leaf and other necessary checks, only to be sure that we
are
in last offset of the same object and type. If the item is found,
convert the ondisk structure to the current endianness of the
machine
running the code.

The new btrfs_search_backwards function does the all these
procedures when
necessary, and can be used to avoid code duplication.

No functional changes.

Signed-off-by: Marcos Paulo de Souza <redacted>
---
 fs/btrfs/ctree.c   | 23 +++++++++++++++++++++++
 fs/btrfs/ctree.h   |  6 ++++++
 fs/btrfs/ioctl.c   | 30 ++++++++----------------------
 fs/btrfs/super.c   | 26 ++++++--------------------
 fs/btrfs/volumes.c |  7 +------
 5 files changed, 44 insertions(+), 48 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 394fec1d3fd9..2991ee845813 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c

@@ -2100,6 +2100,29 @@ int btrfs_search_slot_for_read(struct

btrfs_root *root,
 	return 0;
 }
 
+/*
+ * Execute search and call btrfs_previous_item to traverse
backwards if the item
+ * was not found. If found, convert the stored item to the correct
endianness.
+ *
+ * Return 0 if found, 1 if not found and < 0 if error.
+ */
+int btrfs_search_backwards(struct btrfs_root *root,
+				struct btrfs_key *key,
+				struct btrfs_key *found_key,

Is it necessary to have 2 keys? All calls pass the same one, so
either
this should be just one or you have other patches that make use of
two
distinct keys?

Yes, in these cases yes, but I can see sometimes other places in the
btrfs codebase where the key used convert to cpu is different from the
key used in the search path, so I wanted to make it flexible to future
users.

I believe that we can extent this in the future if needed, so
I'll send a v2 using only one key argument, and use the same argument
to convert to the correct endianness.

Thanks,
  Marcos

quoted

-		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+		ret = btrfs_search_backwards(root, &key, &key, path);

						   &key, &key