From: Marco Stornelli <marco.storne...@gmail.com> Documentation for PRAMFS.
Signed-off-by: Marco Stornelli <marco.storne...@gmail.com> --- diff -Nurp linux-2.6.36-orig/Documentation/filesystems/pramfs.txt linux-2.6.36/Documentation/filesystems/pramfs.txt --- linux-2.6.36-orig/Documentation/filesystems/pramfs.txt 1970-01-01 01:00:00.000000000 +0100 +++ linux-2.6.36/Documentation/filesystems/pramfs.txt 2010-10-23 09:04:32.000000000 +0200 @@ -0,0 +1,296 @@ + +PRAMFS Overview +=============== + +Many embedded systems have a block of non-volatile RAM separate from +normal system memory, i.e. of which the kernel maintains no memory page +descriptors. For such systems it would be beneficial to mount a +fast read/write filesystem over this "I/O memory", for storing frequently +accessed data that must survive system reboots and power cycles. An +example usage might be system logs under /var/log, or a user address +book in a cell phone or PDA. + +Linux traditionally had no support for a persistent, non-volatile RAM-based +filesystem, persistent meaning the filesystem survives a system reboot +or power cycle intact. The RAM-based filesystems such as tmpfs and ramfs +have no actual backing store but exist entirely in the page and buffer +caches, hence the filesystem disappears after a system reboot or +power cycle. + +A relatively straightforward solution is to write a simple block driver +for the non-volatile RAM, and mount over it any disk-based filesystem such +as ext2, ext3, ext4, etc. + +But the disk-based fs over non-volatile RAM block driver approach has +some drawbacks: + +1. Complexity of disk-based fs: disk-based filesystems such as ext2/ext3/ext4 + were designed for optimum performance on spinning disk media, so they + implement features such as block groups, which attempts to group inode data + into a contiguous set of data blocks to minimize disk seeking when accessing + files. For RAM there is no such concern; a file's data blocks can be + scattered throughout the media with no access speed penalty at all. So block + groups in a filesystem mounted over RAM just adds unnecessary + complexity. A better approach is to use a filesystem specifically + tailored to RAM media which does away with these disk-based features. + This increases the efficient use of space on the media, i.e. more + space is dedicated to actual file data storage and less to meta-data + needed to maintain that file data. + +2. Different problems between disks and RAM: Because PRAMFS attempts to avoid + filesystem corruption caused by kernel bugs, dirty pages in the page cache + are not allowed to be written back to the backing-store RAM. This way, an + errant write into the page cache will not get written back to the filesystem. + However, if the backing-store RAM is comparable in access speed to system + memory, the penalty of not using caching is minimal. With this consideration + it's better to move file data directly between the user buffers and the backing + store RAM, i.e. use direct I/O. This prevents the unnecessary populating of + the page cache with dirty pages. However direct I/O has to be enabled at + every file open. To enable direct I/O at all times for all regular files + requires either that applications be modified to include the O_DIRECT flag on + all file opens, or that the filesystem used performs direct I/O by default. + +The Persistent/Protected RAM Special Filesystem (PRAMFS) is a read/write +filesystem that has been designed to address these issues. PRAMFS is targeted +to fast I/O memory, and if the memory is non-volatile, the filesystem will be +persistent. + +In PRAMFS, direct I/O is enabled across all files in the filesystem, in other +words the O_DIRECT flag is forced on every open of a PRAMFS file. Also, file +I/O in the PRAMFS is always synchronous. There is no need to block the current +process while the transfer to/from the PRAMFS is in progress, since one of +the requirements of the PRAMFS is that the filesystem exists in fast RAM. So +file I/O in PRAMFS is always direct, synchronous, and never blocks. + +The data organization in PRAMFS can be thought of as an extremely simplified +version of ext2, such that the ratio of data to meta-data is very high. + +PRAMFS supports the execute-in-place. With XIP, instead of keeping data in the +page cache, the need to have a page cache copy is eliminated completely. +Read&write type operations are performed directly from/to the memory. For file +mappings, the RAM itself is mapped directly into userspace. XIP, in addition, +speed up the applications start-up time because it removes the needs of any +copies. + +PRAMFS is write protected. The page table entries that map the backing-store +RAM are normally marked read-only. Write operations into the filesystem +temporarily mark the affected pages as writeable, the write operation is +carried out with locks held, and then the page table entries is +marked read-only again. +This feature provides protection against filesystem corruption caused by errant +writes into the RAM due to kernel bugs for instance. In case there are systems +where the write protection is not possible (for instance the RAM cannot be +mapped with page tables), this feature can be disabled via the +CONFIG_PRAMFS_WRITE_PROTECT config option. + +PRAMFS supports extended attributes, ACLs and security labels. + +In summary, PRAMFS is a light-weight, space-efficient special filesystem that +is ideal for systems with a block of fast non-volatile RAM that need to access +data on it using a standard filesytem interface. + +Supported mount options +======================= + +The PRAMFS currently requires one mount option, and there are several +optional mount options: + +physaddr= Required. It tells PRAMFS the physical address of the + start of the RAM that makes up the filesystem. The + physical address must be located on a page boundary. + +init= Optional. It is used to initialize the memory to an + empty filesystem. Any data in an existing filesystem + will be lost if this option is given. The parameter to + "init=" is the RAM in kilo/mega/giga bytes. + +bs= Optional. It is used to specify a block size. It is + ignored if the "init=" option is not specified, since + otherwise the block size is read from the PRAMFS + super-block. The default blocksize is 2048 bytes, + and the allowed block sizes are 512, 1024, 2048, and + 4096. + +bpi= Optional. It is used to specify the bytes per inode + ratio, i.e. for every N bytes in the filesystem, an + inode will be created. This behaves the same as the "-i" + option to mke2fs. It is ignored if the "init=" option is + not specified. + +N= Optional. It is used to specify the number of inodes to + allocate in the inode table. If the option is not + specified, the bytes-per-inode ratio is used to + calculate the number of inodes. If neither the "N=" or + "bpi=" options are specified, the default behavior is to + reserve 5% of the total space in the filesystem for the + inode table. This option behaves the same as the "-N" + option to mke2fs. It is ignored if the "init=" option is + not specified. + +Examples: + +mount -t pramfs -o physaddr=0x20000000,init=1M,bs=1k none /mnt/pram + +This example locates the filesystem at physical address 0x20000000, and +also requests an empty filesystem be initialized, of total size of one +megabyte and blocksize of one kilobyte. The mount point is /mnt/pram. + +mount -t pramfs -o physaddr=0x20000000 none /mnt/pram + +This example locates the filesystem at physical address 0x20000000 as in +the first example, but uses the intact filesystem that already exists. + +Current Limitations +=================== + +- The RAM used for PRAMFS must be directly addressable. + +- PRAMFS does not support hard links. + +- PRAMFS supports only private memory mappings. This allows most + executables to run, but programs that attempt shared memory + mappings, such as X apps that use X shared memory, will fail. + +- PRAMFS does not support quota settings. + +Further Documentation +===================== + +If you are interested in the internal design of PRAMFS, there is +documentation available at the Sourceforge PRAMFS home page at +http://pramfs.sourceforge.net/. + +Please send bug reports/comments/feedback to the pramfs development +list at sourceforge: pramfs-de...@lists.sourceforge.net. + + +ChangeLog +========= + +1.2.1 + - kernel 2.6.36 + - replaced d_add with d_splice_alias in pram_lookup to support NFS + - reworked pram_get_parent + - added disable/enable irq in the pram_{memlock|memunlock}_range + in order to avoid "open windows" with writeable memory + - when xattr support was enabled, the semaphore xattr_sem wasn't + initialized + - implemented new truncate convention + - removed pram_truncate_page + - fixed a possible oops in pram_put_super, a super block field was + used after iounmap + - fixed file holes management + - super block s_size field now is 64bit + +1.2.0 + - kernel 2.6.35 + - added support for setattr callback, now truncate command works + - added support for extended attributes, ACLs, security labels + - fixed scalability problem with truncate mutex + - fixed bitmap endianess and problems with 64bit archs (bug 3013785) + - replaced functions pram_fill_new_inode and pram_fill_inode with + pram_read_inode and pram_iget + - added support for export operations, now it's possible to use + NFS to export pram mount points + - added generation field in the inode struct for file versioning + (for NFS) + - fixed the settings of fields s_maxbytes and s_time_gran in the super + block struct + - removed any reference to quota operations, they are not supported + +1.1.6 + - kernel 2.6.33 + - added preempt_enable/disable in pram_{memlock|memunlock}_range in + order to avoid race conditions + - replaced pram_unlock_* and pram_lock_* with pram_memlock_* and + pram_memunlock_* + +1.1.5 + - kernel 2.6.31 + - reworked pram_writeable in order to use the funcions + write_on_kernel_pte_range and write_off_kernel_pte_range + - added writeable_kernel_pte_range in order to compile on ARM arch + - added truncate_inode_pages() in pram_delete_inode() + - fixed an endianess conversion error in super.c, df command didn't work + - fixed an error in pram_find_data_block(), some code was relative to + 32-bit design yet + - fixed an error in pram_free_block(), endianess conversion not needed + - fixed lock usage, possible deadlock recognized by lockdep + +1.1.4 + - fs layout now endianess indipendent + - debug functions replaced with pr_* + - changed CONFIG_PRAMFS_NOWP to CONFIG_PRAMFS_WRITE_PROTECT and + revert the meaning + - removed the rootfs mounting + - redundant superblock + - crc performed with crc16 + - moved out pte manipulations + - used only flush_kernel_tlb_range instead of non standard + flush_kernel_tlb_page + - replaced BKL with a mutex + +1.1.3: + - kernel 2.6.30 + - replaced DQUOT macros with vfs_dq_* functions in inode.c + +1.1.2: + - kernel 2.6.29 + - replaced current->fsuid and current->fsgid in inode.c with + current_fsuid() and current_fsgid() macro + - replaced pram_fsync with simple_file_sync (deleted fsync.c) + - removed ioctl.c + - fix the minus inside pram_sync_super and pram_sync_inode + inline functions + - now flags must be explicit in pram_lock/unlock_inode, + pram_lock/unlock_super and in pram_lock/unlock_block + - fix compiler warning about unused flags variable + - now multiple instances of pramfs are allowed + - added management of mode, uid and gid mount options + - renamed find_pramfs_super() in pram_get_super() and changed its + behaviour to remove the dependecy from a kernel symbol not exported + - added superblock operation pram_show_options + - added pram_parse_options function + - added checks for some mount parameters to avoid kernel crashes + - now a physical address of zero is allowed + - added the use of request_mem_region_exclusive + - now phys_addr in the struct pram_sb_info is phys_addr_t type + +1.1.1: + - kernel 2.6.28.1 + - now the code is compliant to kernel coding style + - replaced typedef pram_off_t with the standard off_t + +1.1.0 + - kernel 2.6.27 + - added xip feature. + +1.0.4: + - kernel 2.6.10 + - include <asm/tlbflush.h> in wprotect.c for + flush_tlb_kernel_range(). + - fixed a bug in pram_mknod(). The pramfs inode needs + updating after calling init_special_inode() to update + the rdev. +1.0.3: + - kernel 2.6.9. + - __ioremap() definition not consistent across archs, use + ioremap() instead. + - flush_tlb_kernel_page() is only available on some archs. + - fixed bug in pram_fill_super(): root inode pointer needs + to be recalculated after remapping whole fs. +1.0.2: + - kernel 2.6.4. + - use pram_truncate() in pram_delete_inode(). + - dangling pram_lock_inode() removed in pram_truncate_blocks(). + - edits to this README + +1.0.1: + - port to kernel 2.6.3. + - implement direct_IO() method instead of custom file read/write + methods. + - do away with __ioremap_readonly() requirement. + - implement inode truncate() method. + +1.0.0: + - Started ChangeLog (kernel 2.4.22). diff -Nurp linux-2.6.36-orig/Documentation/filesystems/xip.txt linux-2.6.36/Documentation/filesystems/xip.txt --- linux-2.6.36-orig/Documentation/filesystems/xip.txt 2010-09-13 01:07:37.000000000 +0200 +++ linux-2.6.36/Documentation/filesystems/xip.txt 2010-09-14 18:49:52.000000000 +0200 @@ -49,6 +49,8 @@ This address space operation is mutually do page cache read/write operations. The following filesystems support it as of today: - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt +- pramfs: persistent and protected RAM filesystem, see + Documentation/filesystems/pramfs.txt A set of file operations that do utilize get_xip_page can be found in mm/filemap_xip.c . The following file operation implementations are provided: -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
