docs: filesystems: convert sysv-fs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/5b96a6[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/index.rst

···
    romfs
    squashfs
    sysfs
+   sysv-fs
    virtiofs
    vfat

Documentation/filesystems/sysv-fs.txt Documentation/filesystems/sysv-fs.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+SystemV Filesystem
+==================
+
 It implements all of
   - Xenix FS,
   - SystemV/386 FS,
   - Coherent FS.
 
 To install:
+
 * Answer the 'System V and Coherent filesystem support' question with 'y'
   when configuring the kernel.
-* To mount a disk or a partition, use
+* To mount a disk or a partition, use::
+
     mount [-r] -t sysv device mountpoint
-  The file system type names
+
+  The file system type names::
+
                -t sysv
                -t xenix
                -t coherent
+
   may be used interchangeably, but the last two will eventually disappear.
 
 Bugs in the present implementation:
+
 - Coherent FS:
+
   - The "free list interleave" n:m is currently ignored.
   - Only file systems with no filesystem name and no pack name are recognized.
-  (See Coherent "man mkfs" for a description of these features.)
+    (See Coherent "man mkfs" for a description of these features.)
+
 - SystemV Release 2 FS:
+
   The superblock is only searched in the blocks 9, 15, 18, which
   corresponds to the beginning of track 1 on floppy disks. No support
   for this FS on hard disk yet.
···
 These filesystems are rather similar. Here is a comparison with Minix FS:
 
 * Linux fdisk reports on partitions
+
   - Minix FS     0x81 Linux/Minix
   - Xenix FS     ??
   - SystemV FS   ??
   - Coherent FS  0x08 AIX bootable
 
 * Size of a block or zone (data allocation unit on disk)
+
   - Minix FS     1024
   - Xenix FS     1024 (also 512 ??)
   - SystemV FS   1024 (also 512 and 2048)
···
   all the block numbers (including the super block) are offset by one track.
 
 * Byte ordering of "short" (16 bit entities) on disk:
+
   - Minix FS     little endian  0 1
   - Xenix FS     little endian  0 1
   - SystemV FS   little endian  0 1
   - Coherent FS  little endian  0 1
+
   Of course, this affects only the file system, not the data of files on it!
 
 * Byte ordering of "long" (32 bit entities) on disk:
+
   - Minix FS     little endian  0 1 2 3
   - Xenix FS     little endian  0 1 2 3
   - SystemV FS   little endian  0 1 2 3
   - Coherent FS  PDP-11         2 3 0 1
+
   Of course, this affects only the file system, not the data of files on it!
 
 * Inode on disk: "short", 0 means non-existent, the root dir ino is:
-  - Minix FS                            1
-  - Xenix FS, SystemV FS, Coherent FS   2
+
+  =================================  ==
+  Minix FS                            1
+  Xenix FS, SystemV FS, Coherent FS   2
+  =================================  ==
 
 * Maximum number of hard links to a file:
-  - Minix FS     250
-  - Xenix FS     ??
-  - SystemV FS   ??
-  - Coherent FS  >=10000
+
+  ===========  =========
+  Minix FS     250
+  Xenix FS     ??
+  SystemV FS   ??
+  Coherent FS  >=10000
+  ===========  =========
 
 * Free inode management:
-  - Minix FS                             a bitmap
+
+  - Minix FS
+      a bitmap
   - Xenix FS, SystemV FS, Coherent FS
       There is a cache of a certain number of free inodes in the super-block.
       When it is exhausted, new free inodes are found using a linear search.
 
 * Free block management:
-  - Minix FS                             a bitmap
+
+  - Minix FS
+      a bitmap
   - Xenix FS, SystemV FS, Coherent FS
       Free blocks are organized in a "free list". Maybe a misleading term,
       since it is not true that every free block contains a pointer to
···
       0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
 
 * Super-block location:
-  - Minix FS     block 1 = bytes 1024..2047
-  - Xenix FS     block 1 = bytes 1024..2047
-  - SystemV FS   bytes 512..1023
-  - Coherent FS  block 1 = bytes 512..1023
+
+  ===========  ==========================
+  Minix FS     block 1 = bytes 1024..2047
+  Xenix FS     block 1 = bytes 1024..2047
+  SystemV FS   bytes 512..1023
+  Coherent FS  block 1 = bytes 512..1023
+  ===========  ==========================
 
 * Super-block layout:
-  - Minix FS
+
+  - Minix FS::
+
                     unsigned short s_ninodes;
                     unsigned short s_nzones;
                     unsigned short s_imap_blocks;
···
                     unsigned short s_log_zone_size;
                     unsigned long s_max_size;
                     unsigned short s_magic;
-  - Xenix FS, SystemV FS, Coherent FS
+
+  - Xenix FS, SystemV FS, Coherent FS::
+
                     unsigned short s_firstdatazone;
                     unsigned long  s_nzones;
                     unsigned short s_fzone_count;
···
                     unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
                     char           s_fname[6];
                     char           s_fpack[6];
+
     then they differ considerably:
-        Xenix FS
+
+        Xenix FS::
+
                     char           s_clean;
                     char           s_fill[371];
                     long           s_magic;
                     long           s_type;
-        SystemV FS
+
+        SystemV FS::
+
                     long           s_fill[12 or 14];
                     long           s_state;
                     long           s_magic;
                     long           s_type;
-        Coherent FS
+
+        Coherent FS::
+
                     unsigned long  s_unique;
+
     Note that Coherent FS has no magic.
 
 * Inode layout:
-  - Minix FS
+
+  - Minix FS::
+
                     unsigned short i_mode;
                     unsigned short i_uid;
                     unsigned long  i_size;
···
                     unsigned char  i_gid;
                     unsigned char  i_nlinks;
                     unsigned short i_zone[7+1+1];
-  - Xenix FS, SystemV FS, Coherent FS
+
+  - Xenix FS, SystemV FS, Coherent FS::
+
                     unsigned short i_mode;
                     unsigned short i_nlink;
                     unsigned short i_uid;
···
                     unsigned long  i_mtime;
                     unsigned long  i_ctime;
 
-* Regular file data blocks are organized as
-  - Minix FS
-               7 direct blocks
-               1 indirect block (pointers to blocks)
-               1 double-indirect block (pointer to pointers to blocks)
-  - Xenix FS, SystemV FS, Coherent FS
-              10 direct blocks
-               1 indirect block (pointers to blocks)
-               1 double-indirect block (pointer to pointers to blocks)
-               1 triple-indirect block (pointer to pointers to pointers to blocks)
 
-* Inode size, inodes per block
-  - Minix FS        32   32
-  - Xenix FS        64   16
-  - SystemV FS      64   16
-  - Coherent FS     64    8
+* Regular file data blocks are organized as
+
+  - Minix FS:
+
+             - 7 direct blocks
+	     - 1 indirect block (pointers to blocks)
+             - 1 double-indirect block (pointer to pointers to blocks)
+
+  - Xenix FS, SystemV FS, Coherent FS:
+
+             - 10 direct blocks
+             -  1 indirect block (pointers to blocks)
+             -  1 double-indirect block (pointer to pointers to blocks)
+             -  1 triple-indirect block (pointer to pointers to pointers to blocks)
+
+
+  ===========  ==========   ================
+               Inode size   inodes per block
+  ===========  ==========   ================
+  Minix FS        32        32
+  Xenix FS        64        16
+  SystemV FS      64        16
+  Coherent FS     64        8
+  ===========  ==========   ================
 
 * Directory entry on disk
-  - Minix FS
+
+  - Minix FS::
+
                     unsigned short inode;
                     char name[14/30];
-  - Xenix FS, SystemV FS, Coherent FS
+
+  - Xenix FS, SystemV FS, Coherent FS::
+
                     unsigned short inode;
                     char name[14];
 
-* Dir entry size, dir entries per block
-  - Minix FS     16/32    64/32
-  - Xenix FS     16       64
-  - SystemV FS   16       64
-  - Coherent FS  16       32
+  ===========    ==============    =====================
+                 Dir entry size    dir entries per block
+  ===========    ==============    =====================
+  Minix FS       16/32             64/32
+  Xenix FS       16                64
+  SystemV FS     16                64
+  Coherent FS    16                32
+  ===========    ==============    =====================
 
 * How to implement symbolic links such that the host fsck doesn't scream:
+
   - Minix FS     normal
   - Xenix FS     kludge: as regular files with  chmod 1000
   - SystemV FS   ??