diff options
Diffstat (limited to 'fs/cramfs/README')
| -rw-r--r-- | fs/cramfs/README | 26 |
1 files changed, 13 insertions, 13 deletions
diff --git a/fs/cramfs/README b/fs/cramfs/README index 445d1c2d7646..9d4e7ea311f4 100644 --- a/fs/cramfs/README +++ b/fs/cramfs/README @@ -86,26 +86,26 @@ Block Size (Block size in cramfs refers to the size of input data that is compressed at a time. It's intended to be somewhere around -PAGE_CACHE_SIZE for cramfs_readpage's convenience.) +PAGE_SIZE for cramfs_readpage's convenience.) The superblock ought to indicate the block size that the fs was written for, since comments in <linux/pagemap.h> indicate that -PAGE_CACHE_SIZE may grow in future (if I interpret the comment +PAGE_SIZE may grow in future (if I interpret the comment correctly). -Currently, mkcramfs #define's PAGE_CACHE_SIZE as 4096 and uses that -for blksize, whereas Linux-2.3.39 uses its PAGE_CACHE_SIZE, which in +Currently, mkcramfs #define's PAGE_SIZE as 4096 and uses that +for blksize, whereas Linux-2.3.39 uses its PAGE_SIZE, which in turn is defined as PAGE_SIZE (which can be as large as 32KB on arm). This discrepancy is a bug, though it's not clear which should be changed. -One option is to change mkcramfs to take its PAGE_CACHE_SIZE from +One option is to change mkcramfs to take its PAGE_SIZE from <asm/page.h>. Personally I don't like this option, but it does require the least amount of change: just change `#define -PAGE_CACHE_SIZE (4096)' to `#include <asm/page.h>'. The disadvantage +PAGE_SIZE (4096)' to `#include <asm/page.h>'. The disadvantage is that the generated cramfs cannot always be shared between different kernels, not even necessarily kernels of the same architecture if -PAGE_CACHE_SIZE is subject to change between kernel versions +PAGE_SIZE is subject to change between kernel versions (currently possible with arm and ia64). The remaining options try to make cramfs more sharable. @@ -126,22 +126,22 @@ size. The options are: 1. Always 4096 bytes. 2. Writer chooses blocksize; kernel adapts but rejects blocksize > - PAGE_CACHE_SIZE. + PAGE_SIZE. 3. Writer chooses blocksize; kernel adapts even to blocksize > - PAGE_CACHE_SIZE. + PAGE_SIZE. It's easy enough to change the kernel to use a smaller value than -PAGE_CACHE_SIZE: just make cramfs_readpage read multiple blocks. +PAGE_SIZE: just make cramfs_readpage read multiple blocks. -The cost of option 1 is that kernels with a larger PAGE_CACHE_SIZE +The cost of option 1 is that kernels with a larger PAGE_SIZE value don't get as good compression as they can. The cost of option 2 relative to option 1 is that the code uses variables instead of #define'd constants. The gain is that people -with kernels having larger PAGE_CACHE_SIZE can make use of that if +with kernels having larger PAGE_SIZE can make use of that if they don't mind their cramfs being inaccessible to kernels with -smaller PAGE_CACHE_SIZE values. +smaller PAGE_SIZE values. Option 3 is easy to implement if we don't mind being CPU-inefficient: e.g. get readpage to decompress to a buffer of size MAX_BLKSIZE (which |