erofs: add multiple device support

In order to support multi-layer container images, add multiple
device feature to EROFS. Two ways are available to use for now:

 - Devices can be mapped into 32-bit global block address space;
 - Device ID can be specified with the chunk indexes format.

Note that it assumes no extent would cross device boundary and mkfs
should take care of it seriously.

In the future, a dedicated device manager could be introduced then
thus extra devices can be automatically scanned by UUID as well.

Link: https://lore.kernel.org/r/20211014081010.43485-1-hsiangkao@linux.alibaba.com
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

1 files changed, 15 insertions, 9 deletions

diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
index 14b747026742..addfe608d08e 100644
--- a/fs/erofs/Kconfig
+++ b/fs/erofs/Kconfig
@@ -6,16 +6,22 @@ config EROFS_FS
 	select FS_IOMAP
 	select LIBCRC32C
 	help
-	  EROFS (Enhanced Read-Only File System) is a lightweight
-	  read-only file system with modern designs (eg. page-sized
-	  blocks, inline xattrs/data, etc.) for scenarios which need
-	  high-performance read-only requirements, e.g. Android OS
-	  for mobile phones and LIVECDs.
+	  EROFS (Enhanced Read-Only File System) is a lightweight read-only
+	  file system with modern designs (e.g. no buffer heads, inline
+	  xattrs/data, chunk-based deduplication, multiple devices, etc.) for
+	  scenarios which need high-performance read-only solutions, e.g.
+	  smartphones with Android OS, LiveCDs and high-density hosts with
+	  numerous containers;
 
-	  It also provides fixed-sized output compression support,
-	  which improves storage density, keeps relatively higher
-	  compression ratios, which is more useful to achieve high
-	  performance for embedded devices with limited memory.
+	  It also provides fixed-sized output compression support in order to
+	  improve storage density as well as keep relatively higher compression
+	  ratios and implements in-place decompression to reuse the file page
+	  for compressed data temporarily with proper strategies, which is
+	  quite useful to ensure guaranteed end-to-end runtime decompression
+	  performance under extremely memory pressure without extra cost.
+
+	  See the documentation at <file:Documentation/filesystems/erofs.rst>
+	  for more details.
 
 	  If unsure, say N.