This series brings a nice refresh to the cramfs filesystem, adding the following capabilities: - Direct memory access, bypassing the block and/or MTD layers entirely. - Ability to store individual data blocks uncompressed. - Ability to locate individual data blocks anywhere in the filesystem. The end result is a very tight filesystem that can be accessed directly from ROM without any other subsystem underneath. Also this allows for user space XIP which is a very important feature for tiny embedded systems. Why cramfs? Because cramfs is very simple and small. With CONFIG_CRAMFS_BLOCK=n and CONFIG_CRAMFS_PHYSMEM=y the cramfs driver may use as little as 3704 bytes of code. That's many times smaller than squashfs. And the runtime memory usage is also much less with cramfs than squashfs. It packs very tightly already compared to romfs which has no compression support. And the cramfs format was simple to extend, allowing for both compressed and uncompressed blocks within the same file. Why not accessing ROM via MTD? The MTD layer is nice and flexible. It also represents a huge overhead considering its core with no other enabled options weights 19KB. That's many times the size of the cramfs code for something that essentially boils down to a glorified argument parser and a call to memremap() in this case. And if someone still wants to use cramfs via MTD then it is already possible with mtdblock. Why not using DAX? DAX stands for "Direct Access" and is a generic kernel layer helping with the necessary tasks involved with XIP. It is tailored for large writable filesystems and relies on the presence of an MMU. It also has the following shortcoming: "The DAX code does not work correctly on architectures which have virtually mapped caches such as ARM, MIPS and SPARC." That makes it unsuitable for a large portion of the intended targets for this series. And due to the read-only nature of cramfs, it is possible to achieve the intended result with a much simpler approach making DAX somewhat overkill in this context. The maximum size of a cramfs image can't exceed 272MB. In practice it is likely to be much much less. Given this series is concerned with small memory systems, even in the MMU case there is always plenty of vmalloc space left to map it all and even a 272MB memremap() wouldn't be a problem. If it is then maybe your system is big enough with large resources to manage already and you're pretty unlikely to be using cramfs in the first place. Of course, while this cramfs remains backward compatible with existing filesystem images, a newer mkcramfs version is necessary to take advantage of the extended data layout. I created a version of mkcramfs that detects ELF files and marks text+rodata segments for XIP and compresses the rest of those ELF files automatically. So here it is. I'm also willing to step up as cramfs maintainer given that no sign of any maintenance activities appeared for years. This series is also available based on v4.13-rc4 via git here: http://git.linaro.org/people/nicolas.pitre/linux xipcramfs Changes from v1: - Improved mmap() support by adding the ability to partially populate a mapping and lazily split the non directly mapable pages to a separate vma at fault time (thanks to Chris Brandt for testing). - Clarified the documentation some more. diffstat: Documentation/filesystems/cramfs.txt | 42 ++ MAINTAINERS | 4 +- fs/cramfs/Kconfig | 39 +- fs/cramfs/README | 31 +- fs/cramfs/inode.c | 621 +++++++++++++++++++++++++---- include/uapi/linux/cramfs_fs.h | 20 +- init/do_mounts.c | 8 + 7 files changed, 688 insertions(+), 77 deletions(-)
