ext4: introduce reserved space

Currently in ENOSPC condition when writing into unwritten space, or
punching a hole, we might need to split the extent and grow extent tree.
However since we can not allocate any new metadata blocks we'll have to
zero out unwritten part of extent or punched out part of extent, or in
the worst case return ENOSPC even though use actually does not allocate
any space.

Also in delalloc path we do reserve metadata and data blocks for the
time we're going to write out, however metadata block reservation is
very tricky especially since we expect that logical connectivity implies
physical connectivity, however that might not be the case and hence we
might end up allocating more metadata blocks than previously reserved.
So in future, metadata reservation checks should be removed since we can
not assure that we do not under reserve.

And this is where reserved space comes into the picture. When mounting
the file system we slice off a little bit of the file system space (2%
or 4096 clusters, whichever is smaller) which can be then used for the
cases mentioned above to prevent costly zeroout, or unexpected ENOSPC.

The number of reserved clusters can be set via sysfs, however it can
never be bigger than number of free clusters in the file system.

Note that this patch fixes the failure of xfstest 274 as expected.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index d91871570982..12b560435aba 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -121,6 +121,8 @@ typedef unsigned int ext4_group_t;
 #define EXT4_MB_STREAM_ALLOC		0x0800
 /* Use reserved root blocks if needed */
 #define EXT4_MB_USE_ROOT_BLOCKS		0x1000
+/* Use blocks from reserved pool */
+#define EXT4_MB_USE_RESERVED		0x2000
 
 struct ext4_allocation_request {
 	/* target inode for block we're allocating */
@@ -557,9 +559,8 @@ enum {
 #define EXT4_GET_BLOCKS_UNINIT_EXT		0x0002
 #define EXT4_GET_BLOCKS_CREATE_UNINIT_EXT	(EXT4_GET_BLOCKS_UNINIT_EXT|\
 						 EXT4_GET_BLOCKS_CREATE)
-	/* Caller is from the delayed allocation writeout path,
-	   so set the magic i_delalloc_reserve_flag after taking the
-	   inode allocation semaphore for */
+	/* Caller is from the delayed allocation writeout path
+	 * finally doing the actual allocation of delayed blocks */
 #define EXT4_GET_BLOCKS_DELALLOC_RESERVE	0x0004
 	/* caller is from the direct IO path, request to creation of an
 	unitialized extents if not allocated, split the uninitialized
@@ -571,8 +572,9 @@ enum {
 	/* Convert extent to initialized after IO complete */
 #define EXT4_GET_BLOCKS_IO_CONVERT_EXT		(EXT4_GET_BLOCKS_CONVERT|\
 					 EXT4_GET_BLOCKS_CREATE_UNINIT_EXT)
-	/* Punch out blocks of an extent */
-#define EXT4_GET_BLOCKS_PUNCH_OUT_EXT		0x0020
+	/* Eventual metadata allocation (due to growing extent tree)
+	 * should not fail, so try to use reserved blocks for that.*/
+#define EXT4_GET_BLOCKS_METADATA_NOFAIL		0x0020
 	/* Don't normalize allocation size (used for fallocate) */
 #define EXT4_GET_BLOCKS_NO_NORMALIZE		0x0040
 	/* Request will not result in inode size update (user for fallocate) */
@@ -1188,6 +1190,7 @@ struct ext4_sb_info {
 	unsigned int s_mount_flags;
 	unsigned int s_def_mount_opt;
 	ext4_fsblk_t s_sb_block;
+	atomic64_t s_resv_clusters;
 	kuid_t s_resuid;
 	kgid_t s_resgid;
 	unsigned short s_mount_state;