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Fri, 31 Oct 2025 20:10:30 -0400 (EDT) Date: Fri, 31 Oct 2025 17:10:27 -0700 From: Boris Burkov To: Mark Harmstone Cc: linux-btrfs@vger.kernel.org Subject: Re: [PATCH v4 11/16] btrfs: move existing remaps before relocating block group Message-ID: References: <20251024181227.32228-1-mark@harmstone.com> <20251024181227.32228-12-mark@harmstone.com> Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251024181227.32228-12-mark@harmstone.com> On Fri, Oct 24, 2025 at 07:12:12PM +0100, Mark Harmstone wrote: > If when relocating a block group we find that `remap_bytes` > 0 in its > block group item, that means that it has been the destination block > group for another that has been remapped. > > We need to seach the remap tree for any remap backrefs within this > range, and move the data to a third block group. This is because > otherwise btrfs_translate_remap() could end up following an unbounded > chain of remaps, which would only get worse over time. > > We only relocate one block group at a time, so `remap_bytes` will only > ever go down while we are doing this. Once we're finished we set the > REMAPPED flag on the block group, which will permanently prevent any > other data from being moved to within it. > > Signed-off-by: Mark Harmstone > --- > fs/btrfs/extent-tree.c | 6 +- > fs/btrfs/relocation.c | 481 +++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 485 insertions(+), 2 deletions(-) > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > index 1c14e0c82c03..10dc6f8d2f71 100644 > --- a/fs/btrfs/extent-tree.c > +++ b/fs/btrfs/extent-tree.c > @@ -4545,7 +4545,8 @@ static noinline int find_free_extent(struct btrfs_root *root, > block_group->cached != BTRFS_CACHE_NO) { > down_read(&space_info->groups_sem); > if (list_empty(&block_group->list) || > - block_group->ro) { > + block_group->ro || > + block_group->flags & BTRFS_BLOCK_GROUP_REMAPPED) { > /* > * someone is removing this block group, > * we can't jump into the have_block_group > @@ -4579,7 +4580,8 @@ static noinline int find_free_extent(struct btrfs_root *root, > > ffe_ctl->hinted = false; > /* If the block group is read-only, we can skip it entirely. */ > - if (unlikely(block_group->ro)) { > + if (unlikely(block_group->ro) || > + block_group->flags & BTRFS_BLOCK_GROUP_REMAPPED) { > if (ffe_ctl->for_treelog) > btrfs_clear_treelog_bg(block_group); > if (ffe_ctl->for_data_reloc) > diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c > index cd53509c2fda..d31817379078 100644 > --- a/fs/btrfs/relocation.c > +++ b/fs/btrfs/relocation.c > @@ -3987,6 +3987,481 @@ static void adjust_block_group_remap_bytes(struct btrfs_trans_handle *trans, > btrfs_inc_delayed_refs_rsv_bg_updates(fs_info); > } > > +struct reloc_io_private { > + struct completion done; > + refcount_t pending_refs; > + blk_status_t status; > +}; > + > +static void reloc_endio(struct btrfs_bio *bbio) > +{ > + struct reloc_io_private *priv = bbio->private; > + > + if (bbio->bio.bi_status) > + WRITE_ONCE(priv->status, bbio->bio.bi_status); > + > + if (refcount_dec_and_test(&priv->pending_refs)) > + complete(&priv->done); > + > + bio_put(&bbio->bio); > +} > + > +static int copy_remapped_data_io(struct btrfs_fs_info *fs_info, > + struct reloc_io_private *priv, > + struct page **pages, u64 addr, u64 length, > + bool do_write) > +{ > + struct btrfs_bio *bbio; > + unsigned long i = 0; > + blk_opf_t op = do_write ? REQ_OP_WRITE : REQ_OP_READ; > + > + init_completion(&priv->done); > + refcount_set(&priv->pending_refs, 1); > + priv->status = 0; > + > + bbio = btrfs_bio_alloc(BIO_MAX_VECS, op, fs_info, reloc_endio, > + priv); > + bbio->bio.bi_iter.bi_sector = addr >> SECTOR_SHIFT; > + > + do { > + size_t bytes = min_t(u64, length, PAGE_SIZE); > + > + if (bio_add_page(&bbio->bio, pages[i], bytes, 0) < bytes) { > + refcount_inc(&priv->pending_refs); > + btrfs_submit_bbio(bbio, 0); > + > + bbio = btrfs_bio_alloc(BIO_MAX_VECS, op, fs_info, > + reloc_endio, priv); > + bbio->bio.bi_iter.bi_sector = addr >> SECTOR_SHIFT; > + continue; > + } > + > + i++; > + addr += bytes; > + length -= bytes; > + } while (length); > + > + refcount_inc(&priv->pending_refs); > + btrfs_submit_bbio(bbio, 0); > + > + if (!refcount_dec_and_test(&priv->pending_refs)) > + wait_for_completion_io(&priv->done); > + > + return blk_status_to_errno(READ_ONCE(priv->status)); > +} > + > +static int copy_remapped_data(struct btrfs_fs_info *fs_info, u64 old_addr, > + u64 new_addr, u64 length) > +{ > + int ret; > + struct page **pages; > + unsigned int nr_pages; > + struct reloc_io_private priv; > + > + nr_pages = DIV_ROUND_UP(length, PAGE_SIZE); > + pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS); > + if (!pages) > + return -ENOMEM; > + ret = btrfs_alloc_page_array(nr_pages, pages, 0); > + if (ret) { > + ret = -ENOMEM; > + goto end; > + } > + > + ret = copy_remapped_data_io(fs_info, &priv, pages, old_addr, length, > + false); > + if (ret) > + goto end; > + > + ret = copy_remapped_data_io(fs_info, &priv, pages, new_addr, length, > + true); > + > +end: > + for (unsigned int i = 0; i < nr_pages; i++) { > + if (pages[i]) > + __free_page(pages[i]); > + } > + kfree(pages); > + > + return ret; > +} > + > +static int do_copy(struct btrfs_fs_info *fs_info, u64 old_addr, u64 new_addr, > + u64 length) > +{ > + int ret; > + > + /* Copy 1MB at a time, to avoid using too much memory. */ > + > + do { > + u64 to_copy = min_t(u64, length, SZ_1M); > + > + ret = copy_remapped_data(fs_info, old_addr, new_addr, > + to_copy); > + if (ret) > + return ret; > + > + if (to_copy == length) > + break; > + > + old_addr += to_copy; > + new_addr += to_copy; > + length -= to_copy; > + } while (true); > + > + return 0; > +} > + > +static int add_remap_item(struct btrfs_trans_handle *trans, > + struct btrfs_path *path, u64 new_addr, u64 length, > + u64 old_addr) > +{ > + struct btrfs_fs_info *fs_info = trans->fs_info; > + struct btrfs_remap remap; > + struct btrfs_key key; > + struct extent_buffer *leaf; > + int ret; > + > + key.objectid = old_addr; > + key.type = BTRFS_REMAP_KEY; > + key.offset = length; > + > + ret = btrfs_insert_empty_item(trans, fs_info->remap_root, path, > + &key, sizeof(struct btrfs_remap)); > + if (ret) > + return ret; > + > + leaf = path->nodes[0]; > + > + btrfs_set_stack_remap_address(&remap, new_addr); > + > + write_extent_buffer(leaf, &remap, > + btrfs_item_ptr_offset(leaf, path->slots[0]), > + sizeof(struct btrfs_remap)); > + > + btrfs_release_path(path); > + > + return 0; > +} > + > +static int add_remap_backref_item(struct btrfs_trans_handle *trans, > + struct btrfs_path *path, u64 new_addr, > + u64 length, u64 old_addr) > +{ > + struct btrfs_fs_info *fs_info = trans->fs_info; > + struct btrfs_remap remap; > + struct btrfs_key key; > + struct extent_buffer *leaf; > + int ret; > + > + key.objectid = new_addr; > + key.type = BTRFS_REMAP_BACKREF_KEY; > + key.offset = length; > + > + ret = btrfs_insert_empty_item(trans, fs_info->remap_root, > + path, &key, sizeof(struct btrfs_remap)); > + if (ret) > + return ret; > + > + leaf = path->nodes[0]; > + > + btrfs_set_stack_remap_address(&remap, old_addr); > + > + write_extent_buffer(leaf, &remap, > + btrfs_item_ptr_offset(leaf, path->slots[0]), > + sizeof(struct btrfs_remap)); > + > + btrfs_release_path(path); > + > + return 0; > +} > + > +static int move_existing_remap(struct btrfs_fs_info *fs_info, > + struct btrfs_path *path, > + struct btrfs_block_group *bg, u64 new_addr, > + u64 length, u64 old_addr) > +{ > + struct btrfs_trans_handle *trans; > + struct extent_buffer *leaf; > + struct btrfs_remap *remap_ptr, remap; > + struct btrfs_key key, ins; > + u64 dest_addr, dest_length, min_size; > + struct btrfs_block_group *dest_bg; > + int ret; > + bool is_data = bg->flags & BTRFS_BLOCK_GROUP_DATA; > + struct btrfs_space_info *sinfo = bg->space_info; > + bool mutex_taken = false, bg_needs_free_space; > + > + spin_lock(&sinfo->lock); > + btrfs_space_info_update_bytes_may_use(sinfo, length); > + spin_unlock(&sinfo->lock); > + > + if (is_data) > + min_size = fs_info->sectorsize; > + else > + min_size = fs_info->nodesize; > + > + ret = btrfs_reserve_extent(fs_info->fs_root, length, length, min_size, > + 0, 0, &ins, is_data, false); > + if (ret) { > + spin_lock(&sinfo->lock); > + btrfs_space_info_update_bytes_may_use(sinfo, -length); > + spin_unlock(&sinfo->lock); > + return ret; > + } > + > + dest_addr = ins.objectid; > + dest_length = ins.offset; > + > + if (!is_data && !IS_ALIGNED(dest_length, fs_info->nodesize)) { > + u64 new_length = ALIGN_DOWN(dest_length, fs_info->nodesize); > + > + btrfs_free_reserved_extent(fs_info, dest_addr + new_length, > + dest_length - new_length, 0); > + > + dest_length = new_length; > + } > + > + trans = btrfs_join_transaction(fs_info->remap_root); > + if (IS_ERR(trans)) { > + ret = PTR_ERR(trans); > + trans = NULL; > + goto end; > + } > + > + mutex_lock(&fs_info->remap_mutex); I answered my own locking question. I think this mutex protects us from the remap disappearing out from under us. I'll take another look at the deletion side of it to make sure it still makes sense if the remapping here wins the race and moves it before deletion. :) > + mutex_taken = true; > + > + /* Find old remap entry. */ > + > + key.objectid = old_addr; > + key.type = BTRFS_REMAP_KEY; > + key.offset = length; > + > + ret = btrfs_search_slot(trans, fs_info->remap_root, &key, > + path, 0, 1); > + if (ret == 1) { > + /* > + * Not a problem if the remap entry wasn't found: that means > + * that another transaction has deallocated the data. > + * move_existing_remaps() loops until the BG contains no > + * remaps, so we can just return 0 in this case. > + */ > + btrfs_release_path(path); > + ret = 0; > + goto end; > + } else if (ret) { > + goto end; > + } > + > + ret = do_copy(fs_info, new_addr, dest_addr, dest_length); > + if (ret) > + goto end; > + > + /* Change data of old remap entry. */ > + > + leaf = path->nodes[0]; > + > + remap_ptr = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_remap); > + btrfs_set_remap_address(leaf, remap_ptr, dest_addr); > + > + btrfs_mark_buffer_dirty(trans, leaf); > + > + if (dest_length != length) { > + key.offset = dest_length; > + btrfs_set_item_key_safe(trans, path, &key); > + } > + > + btrfs_release_path(path); > + > + if (dest_length != length) { > + /* Add remap item for remainder. */ > + > + ret = add_remap_item(trans, path, new_addr + dest_length, > + length - dest_length, > + old_addr + dest_length); > + if (ret) > + goto end; > + } > + > + /* Change or remove old backref. */ > + > + key.objectid = new_addr; > + key.type = BTRFS_REMAP_BACKREF_KEY; > + key.offset = length; > + > + ret = btrfs_search_slot(trans, fs_info->remap_root, &key, > + path, -1, 1); > + if (ret) { > + if (ret == 1) { > + btrfs_release_path(path); > + ret = -ENOENT; > + } > + goto end; > + } > + > + leaf = path->nodes[0]; > + > + if (dest_length == length) { > + ret = btrfs_del_item(trans, fs_info->remap_root, path); > + if (ret) { > + btrfs_release_path(path); > + goto end; > + } > + } else { > + key.objectid += dest_length; > + key.offset -= dest_length; > + btrfs_set_item_key_safe(trans, path, &key); > + > + btrfs_set_stack_remap_address(&remap, old_addr + dest_length); > + > + write_extent_buffer(leaf, &remap, > + btrfs_item_ptr_offset(leaf, path->slots[0]), > + sizeof(struct btrfs_remap)); > + } > + > + btrfs_release_path(path); > + > + /* Add new backref. */ > + > + ret = add_remap_backref_item(trans, path, dest_addr, dest_length, > + old_addr); > + if (ret) > + goto end; > + > + adjust_block_group_remap_bytes(trans, bg, -dest_length); > + > + ret = btrfs_add_to_free_space_tree(trans, new_addr, dest_length); > + if (ret) > + goto end; > + > + dest_bg = btrfs_lookup_block_group(fs_info, dest_addr); > + > + adjust_block_group_remap_bytes(trans, dest_bg, dest_length); > + > + mutex_lock(&dest_bg->free_space_lock); > + bg_needs_free_space = test_bit(BLOCK_GROUP_FLAG_NEEDS_FREE_SPACE, > + &dest_bg->runtime_flags); > + mutex_unlock(&dest_bg->free_space_lock); > + btrfs_put_block_group(dest_bg); > + > + if (bg_needs_free_space) { > + ret = btrfs_add_block_group_free_space(trans, dest_bg); > + if (ret) > + goto end; > + } > + > + ret = btrfs_remove_from_free_space_tree(trans, dest_addr, dest_length); > + if (ret) { > + btrfs_remove_from_free_space_tree(trans, new_addr, > + dest_length); > + goto end; > + } > + > + ret = 0; > + > +end: > + if (mutex_taken) > + mutex_unlock(&fs_info->remap_mutex); > + > + btrfs_dec_block_group_reservations(fs_info, dest_addr); > + > + if (ret) { > + btrfs_free_reserved_extent(fs_info, dest_addr, dest_length, 0); > + > + if (trans) { > + btrfs_abort_transaction(trans, ret); > + btrfs_end_transaction(trans); > + } > + } else { > + dest_bg = btrfs_lookup_block_group(fs_info, dest_addr); > + btrfs_free_reserved_bytes(dest_bg, dest_length, 0); > + btrfs_put_block_group(dest_bg); > + > + ret = btrfs_commit_transaction(trans); > + } > + > + return ret; > +} > + > +static int move_existing_remaps(struct btrfs_fs_info *fs_info, > + struct btrfs_block_group *bg, > + struct btrfs_path *path) > +{ > + int ret; > + struct btrfs_key key; > + struct extent_buffer *leaf; > + struct btrfs_remap *remap; > + u64 old_addr; > + > + /* Look for backrefs in remap tree. */ > + > + while (bg->remap_bytes > 0) { > + key.objectid = bg->start; > + key.type = BTRFS_REMAP_BACKREF_KEY; > + key.offset = 0; > + > + ret = btrfs_search_slot(NULL, fs_info->remap_root, &key, path, > + 0, 0); > + if (ret < 0) > + return ret; > + > + leaf = path->nodes[0]; > + > + if (path->slots[0] >= btrfs_header_nritems(leaf)) { > + ret = btrfs_next_leaf(fs_info->remap_root, path); > + if (ret < 0) { > + btrfs_release_path(path); > + return ret; > + } > + > + if (ret) { > + btrfs_release_path(path); > + break; > + } > + > + leaf = path->nodes[0]; > + } > + > + btrfs_item_key_to_cpu(leaf, &key, path->slots[0]); > + > + if (key.type != BTRFS_REMAP_BACKREF_KEY) { > + path->slots[0]++; > + > + if (path->slots[0] >= btrfs_header_nritems(leaf)) { > + ret = btrfs_next_leaf(fs_info->remap_root, path); > + if (ret < 0) { > + btrfs_release_path(path); > + return ret; > + } > + > + if (ret) { > + btrfs_release_path(path); > + break; > + } > + > + leaf = path->nodes[0]; > + } > + } > + > + remap = btrfs_item_ptr(leaf, path->slots[0], > + struct btrfs_remap); > + > + old_addr = btrfs_remap_address(leaf, remap); > + > + btrfs_release_path(path); > + > + ret = move_existing_remap(fs_info, path, bg, key.objectid, > + key.offset, old_addr); > + if (ret) > + return ret; > + } > + > + BUG_ON(bg->remap_bytes > 0); > + > + return 0; > +} > + > static int create_remap_tree_entries(struct btrfs_trans_handle *trans, > struct btrfs_path *path, > struct btrfs_block_group *bg) > @@ -4564,6 +5039,12 @@ int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start, > WARN_ON(ret && ret != -EAGAIN); > > if (*using_remap_tree) { > + if (bg->remap_bytes != 0) { > + ret = move_existing_remaps(fs_info, bg, path); > + if (ret) > + goto out; > + } > + > ret = start_block_group_remapping(fs_info, path, bg); > goto out; > } > -- > 2.49.1 >