The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From fd0ddbe2509568b00df364156f47561e9f469f15 Mon Sep 17 00:00:00 2001

From: Filipe Manana fdmanana@suse.com Date: Wed, 30 Oct 2019 12:23:01 +0000 Subject: [PATCH] Btrfs: send, skip backreference walking for extents with many references

Backreference walking, which is used by send to figure if it can issue clone operations instead of write operations, can be very slow and use too much memory when extents have many references. This change simply skips backreference walking when an extent has more than 64 references, in which case we fallback to a write operation instead of a clone operation. This limit is conservative and in practice I observed no signicant slowdown with up to 100 references and still low memory usage up to that limit.

This is a temporary workaround until there are speedups in the backref walking code, and as such it does not attempt to add extra interfaces or knobs to tweak the threshold.

Reported-by: Atemu atemu.main@gmail.com Link: https://lore.kernel.org/linux-btrfs/CAE4GHgkvqVADtS4AzcQJxo0Q1jKQgKaW3JGp3SG... CC: stable@vger.kernel.org # 4.4+ Reviewed-by: Qu Wenruo wqu@suse.com Signed-off-by: Filipe Manana fdmanana@suse.com Signed-off-by: David Sterba dsterba@suse.com

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c index cbf4909f5cd9..ae2db5eb1549 100644 --- a/fs/btrfs/send.c +++ b/fs/btrfs/send.c @@ -24,6 +24,14 @@ #include "transaction.h" #include "compression.h"

+/* + * Maximum number of references an extent can have in order for us to attempt to + * issue clone operations instead of write operations. This currently exists to + * avoid hitting limitations of the backreference walking code (taking a lot of + * time and using too much memory for extents with large number of references). + */ +#define SEND_MAX_EXTENT_REFS 64 + /* * A fs_path is a helper to dynamically build path names with unknown size. * It reallocates the internal buffer on demand. @@ -1310,6 +1318,7 @@ static int find_extent_clone(struct send_ctx *sctx, struct clone_root *cur_clone_root; struct btrfs_key found_key; struct btrfs_path *tmp_path; + struct btrfs_extent_item *ei; int compressed; u32 i;

@@ -1357,7 +1366,6 @@ static int find_extent_clone(struct send_ctx *sctx, ret = extent_from_logical(fs_info, disk_byte, tmp_path, &found_key, &flags); up_read(&fs_info->commit_root_sem); - btrfs_release_path(tmp_path);

if (ret < 0) goto out; @@ -1366,6 +1374,21 @@ static int find_extent_clone(struct send_ctx *sctx, goto out; }

+ ei = btrfs_item_ptr(tmp_path->nodes[0], tmp_path->slots[0], + struct btrfs_extent_item); + /* + * Backreference walking (iterate_extent_inodes() below) is currently + * too expensive when an extent has a large number of references, both + * in time spent and used memory. So for now just fallback to write + * operations instead of clone operations when an extent has more than + * a certain amount of references. + */ + if (btrfs_extent_refs(tmp_path->nodes[0], ei) > SEND_MAX_EXTENT_REFS) { + ret = -ENOENT; + goto out; + } + btrfs_release_path(tmp_path); + /* * Setup the clone roots. */