Re: [PATCH v10 06/14] btrfs: optionally extend i_size in cow_file_range_inline()

On Fri, Aug 20, 2021 at 05:13:34PM +0800, Qu Wenruo wrote:

On 2021/8/20 下午4:51, Nikolay Borisov wrote:

quoted

On 18.08.21 г. 0:06, Omar Sandoval wrote:

quoted

From: Omar Sandoval <redacted>

Currently, an inline extent is always created after i_size is extended
from btrfs_dirty_pages(). However, for encoded writes, we only want to
update i_size after we successfully created the inline extent.

To me, the idea of write first then update isize is just going to cause
tons of inline extent related prblems.

The current example is falloc, which only update the isize after the
falloc finishes.

This behavior has already bothered me quite a lot, as it can easily
create mixed inline and regular extents.

Do you have an example of how this would happen? I have the inode and
extent bits locked during an encoded write, and I see that fallocate
does the same.

Can't we remember the old isize (with proper locking), enlarge isize
(with holes filled), do the write.

If something wrong happened, we truncate the isize back to its old isize.

quoted

quoted

Add an
update_i_size parameter to cow_file_range_inline() and
insert_inline_extent() and pass in the size of the extent rather than
determining it from i_size. Since the start parameter is always passed
as 0, get rid of it and simplify the logic in these two functions. While
we're here, let's document the requirements for creating an inline
extent.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Omar Sandoval <redacted>
---
  fs/btrfs/inode.c | 100 +++++++++++++++++++++++------------------------
  1 file changed, 48 insertions(+), 52 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 708d8ab098bc..0b5ff14aa7fd 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c

@@ -236,9 +236,10 @@ static int btrfs_init_inode_security(struct btrfs_trans_handle *trans,
  static int insert_inline_extent(struct btrfs_trans_handle *trans,
  				struct btrfs_path *path, bool extent_inserted,
  				struct btrfs_root *root, struct inode *inode,
-				u64 start, size_t size, size_t compressed_size,
+				size_t size, size_t compressed_size,
  				int compress_type,
-				struct page **compressed_pages)
+				struct page **compressed_pages,
+				bool update_i_size)
  {
  	struct extent_buffer *leaf;
  	struct page *page = NULL;

@@ -247,7 +248,7 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
  	struct btrfs_file_extent_item *ei;
  	int ret;
  	size_t cur_size = size;
-	unsigned long offset;
+	u64 i_size;

  	ASSERT((compressed_size > 0 && compressed_pages) ||
  	       (compressed_size == 0 && !compressed_pages));

@@ -260,7 +261,7 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
  		size_t datasize;

  		key.objectid = btrfs_ino(BTRFS_I(inode));
-		key.offset = start;
+		key.offset = 0;
  		key.type = BTRFS_EXTENT_DATA_KEY;

  		datasize = btrfs_file_extent_calc_inline_size(cur_size);

@@ -297,12 +298,10 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
  		btrfs_set_file_extent_compression(leaf, ei,
  						  compress_type);
  	} else {
-		page = find_get_page(inode->i_mapping,
-				     start >> PAGE_SHIFT);
+		page = find_get_page(inode->i_mapping, 0);
  		btrfs_set_file_extent_compression(leaf, ei, 0);
  		kaddr = kmap_atomic(page);
-		offset = offset_in_page(start);
-		write_extent_buffer(leaf, kaddr + offset, ptr, size);
+		write_extent_buffer(leaf, kaddr, ptr, size);
  		kunmap_atomic(kaddr);
  		put_page(page);
  	}

@@ -313,8 +312,8 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
  	 * We align size to sectorsize for inline extents just for simplicity
  	 * sake.
  	 */
-	size = ALIGN(size, root->fs_info->sectorsize);
-	ret = btrfs_inode_set_file_extent_range(BTRFS_I(inode), start, size);
+	ret = btrfs_inode_set_file_extent_range(BTRFS_I(inode), 0,
+					ALIGN(size, root->fs_info->sectorsize));
  	if (ret)
  		goto fail;

@@ -327,7 +326,13 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
  	 * before we unlock the pages.  Otherwise we
  	 * could end up racing with unlink.
  	 */
-	BTRFS_I(inode)->disk_i_size = inode->i_size;
+	i_size = i_size_read(inode);
+	if (update_i_size && size > i_size) {
+		i_size_write(inode, size);
+		i_size = size;
+	}
+	BTRFS_I(inode)->disk_i_size = i_size;
+
  fail:
  	return ret;
  }

@@ -338,35 +343,31 @@ static int insert_inline_extent(struct btrfs_trans_handle *trans,
   * does the checks required to make sure the data is small enough
   * to fit as an inline extent.
   */
-static noinline int cow_file_range_inline(struct btrfs_inode *inode, u64 start,
-					  u64 end, size_t compressed_size,
+static noinline int cow_file_range_inline(struct btrfs_inode *inode, u64 size,
+					  size_t compressed_size,
  					  int compress_type,
-					  struct page **compressed_pages)
+					  struct page **compressed_pages,
+					  bool update_i_size)
  {
  	struct btrfs_drop_extents_args drop_args = { 0 };
  	struct btrfs_root *root = inode->root;
  	struct btrfs_fs_info *fs_info = root->fs_info;
  	struct btrfs_trans_handle *trans;
-	u64 isize = i_size_read(&inode->vfs_inode);
-	u64 actual_end = min(end + 1, isize);
-	u64 inline_len = actual_end - start;
-	u64 aligned_end = ALIGN(end, fs_info->sectorsize);
-	u64 data_len = inline_len;
+	u64 data_len = compressed_size ? compressed_size : size;
  	int ret;
  	struct btrfs_path *path;

-	if (compressed_size)
-		data_len = compressed_size;
-
-	if (start > 0 ||
-	    actual_end > fs_info->sectorsize ||
+	/*
+	 * We can create an inline extent if it ends at or beyond the current
+	 * i_size, is no larger than a sector (decompressed), and the (possibly
+	 * compressed) data fits in a leaf and the configured maximum inline
+	 * size.
+	 */

Urgh, just some days ago Qu was talking about how awkward it is to have
mixed extents in a file. And now, AFAIU, you are making them more likely
since now they can be created not just at the beginning of the file but
also after i_size write. While this won't be a problem in and of itself
it goes just the opposite way of us trying to shrink the possible cases
when we can have mixed extents.

Tree-checker should reject such inline extent at non-zero offset.

This change does not allow creating inline extents at a non-zero offset.

quoted

Qu what is your take on that?

My question is, why encoded write needs to bother the inline extents at all?

My intuition of such encoded write is, it should not create inline
extents at all.

Or is there any special use-case involved for encoded write?

We create compressed inline extents with normal writes. We should be
able to send and receive them without converting them into regular
extents.