Re: Linux 2.4.17-pre3

Christoph Rohland
2001-12-06T15:20:15Z

Hi Marcelo,

On Wed, 5 Dec 2001, Marcelo Tosatti wrote:
> pre3:
>
> - Enable ppro errata workaround (Dave Jones)
> - Update tmpfs documentation (Christoph Rohland)

Unfortunately the file Documentation/filesystem/tmpfs.txt did not make
it into your patch.

Here it comes again (slightly modified)

Further on the documentation drew some attention to the completeness
of the mount option set and I added a uid and gid option. It was
easier than adding a TODO entry ;-)

And I made the error messages somewhat better understandable from a
user perspective.

Please apply
Christoph

diff -uNr 17-pre4/Documentation/filesystems/tmpfs.txt m17-pre4/Documentation/filesystems/tmpfs.txt
--- 17-pre4/Documentation/filesystems/tmpfs.txt Thu Jan 1 01:00:00 1970
+++ m17-pre4/Documentation/filesystems/tmpfs.txt Thu Dec 6 16:09:49 2001
@@ -0,0 +1,102 @@
+Tmpfs is a file system which keeps all files in virtual memory.
+
+
+Everything in tmpfs is temporary in the sense that no files will be
+created on your hard drive. If you unmount a tmpfs instance,
+everything stored therein is lost.
+
+tmpfs puts everything into the kernel internal caches and grows and
+shrinks to accommodate the files it contains and is able to swap
+unneeded pages out to swap space. It has maximum size limits which can
+be adjusted on the fly via 'mount -o remount ...'
+
+If you compare it to ramfs (which was the template to create tmpfs)
+you gain swapping and limit checking. Another similar thing is the RAM
+disk (/dev/ram*), which simulates a fixed size hard disk in physical
+RAM, where you have to create an ordinary filesystem on top. Ramdisks
+cannot swap and you do not have the possibility to resize them.
+
+Since tmpfs lives completely in the page cache and on swap, all tmpfs
+pages currently in memory will show up as cached. It will not show up
+as shared or something like that. Further on you can check the actual
+RAM+swap use of a tmpfs instance with df(1) and du(1).
+
+
+tmpfs has the following uses:
+
+1) There is always a kernel internal mount which you will not see at
+ all. This is used for shared anonymous mappings and SYSV shared
+ memory.
+
+ This mount does not depend on CONFIG_TMPFS. If CONFIG_TMPFS is not
+ set, the user visible part of tmpfs is not build. But the internal
+ mechanisms are always present.
+
+2) glibc 2.2 and above expects tmpfs to be mounted at /dev/shm for
+ POSIX shared memory (shm_open, shm_unlink). Adding the following
+ line to /etc/fstab should take care of this:
+
+ tmpfs /dev/shm tmpfs defaults 0 0
+
+ Remember to create the directory that you intend to mount tmpfs on
+ if necessary (/dev/shm is automagically created if you use devfs).
+
+ This mount is _not_ needed for SYSV shared memory. The internal
+ mount is used for that. (In the 2.3 kernel versions it was
+ necessary to mount the predecessor of tmpfs (shm fs) to use SYSV
+ shared memory)
+
+3) Some people (including me) find it very convenient to mount it
+ e.g. on /tmp and /var/tmp and have a big swap partition. But be
+ aware: loop mounts of tmpfs files do not work due to the internal
+ design. So mkinitrd shipped by most distributions will fail with a
+ tmpfs /tmp.
+
+4) And probably a lot more I do not know about :-)
+
+
+tmpfs has a couple of mount options:
+
+size: The limit of allocated bytes for this tmpfs instance. The
+ default is half of your physical RAM without swap. If you
+ oversize your tmpfs instances the machine will deadlock
+ since the OOM handler will not be able to free that memory.
+nr_blocks: The same as size, but in blocks of PAGECACHE_SIZE.
+nr_inodes: The maximum number of inodes for this instance. The default
+ is half of the number of your physical RAM pages.
+
+These parameters accept a suffix k, m or g for kilo, mega and giga and
+can be changed on remount.
+
+To specify the initial root directory you can use the following mount
+options:
+
+mode: The permissions as an octal number
+uid: The user id
+gid: The group id
+
+These options do not have any effect on remount. You can change these
+parameters with chmod(1), chown(1) and chgrp(1) on a mounted filesystem.
+
+
+So 'mount -t tmpfs -o size=10G,nr_inodes=10k,mode=700 tmpfs /mytmpfs'
+will give you tmpfs instance on /mytmpfs which can allocate 10GB
+RAM/SWAP in 10240 inodes and it is only accessible by root.
+
+
+TODOs:
+
+1) give the size option a percent semantic: If you give a mount option
+ size=50% the tmpfs instance should be able to grow to 50 percent of
+ RAM + swap. So the instance should adapt automatically if you add
+ or remove swap space.
+2) loop mounts: This is difficult since loop.c relies on the readpage
+ operation. This operation gets a page from the caller to be filled
+ with the content of the file at that position. But tmpfs always has
+ the page and thus cannot copy the content to the given page. So it
+ cannot provide this operation. The VM had to be changed seriously
+ to achieve this.
+3) Show the number of tmpfs RAM pages. (As shared?)
+
+Author:
+ Christoph Rohland <cr@sap.com>, 1.12.01
diff -uNr 17-pre4/mm/shmem.c m17-pre4/mm/shmem.c
--- 17-pre4/mm/shmem.c Mon Nov 26 16:21:58 2001
+++ m17-pre4/mm/shmem.c Thu Dec 6 16:08:26 2001
@@ -1193,7 +1193,7 @@
follow_link: shmem_follow_link,
};

-static int shmem_parse_options(char *options, int *mode, unsigned long * blocks, unsigned long *inodes)
+static int shmem_parse_options(char *options, int *mode, uid_t *uid, gid_t *gid, unsigned long * blocks, unsigned long *inodes)
{
char *this_char, *value, *rest;

@@ -1205,7 +1205,7 @@
*value++ = 0;
} else {
printk(KERN_ERR
- "shmem_parse_options: No value for option '%s'\n",
+ "tmpfs: No value for mount option '%s'\n",
this_char);
return 1;
}
@@ -1230,8 +1230,20 @@
*mode = simple_strtoul(value,&rest,8);
if (*rest)
goto bad_val;
+ } else if (!strcmp(this_char,"uid")) {
+ if (!uid)
+ continue;
+ *uid = simple_strtoul(value,&rest,0);
+ if (*rest)
+ goto bad_val;
+ } else if (!strcmp(this_char,"gid")) {
+ if (!gid)
+ continue;
+ *gid = simple_strtoul(value,&rest,0);
+ if (*rest)
+ goto bad_val;
} else {
- printk(KERN_ERR "shmem_parse_options: Bad option %s\n",
+ printk(KERN_ERR "tmpfs: Bad mount option %s\n",
this_char);
return 1;
}
@@ -1239,7 +1251,7 @@
return 0;

bad_val:
- printk(KERN_ERR "shmem_parse_options: Bad value '%s' for option '%s'\n",
+ printk(KERN_ERR "tmpfs: Bad value '%s' for mount option '%s'\n",
value, this_char);
return 1;

@@ -1251,7 +1263,7 @@
unsigned long max_blocks = sbinfo->max_blocks;
unsigned long max_inodes = sbinfo->max_inodes;

- if (shmem_parse_options (data, NULL, &max_blocks, &max_inodes))
+ if (shmem_parse_options (data, NULL, NULL, NULL, &max_blocks, &max_inodes))
return -EINVAL;
return shmem_set_size(sbinfo, max_blocks, max_inodes);
}
@@ -1268,6 +1280,8 @@
struct dentry * root;
unsigned long blocks, inodes;
int mode = S_IRWXUGO | S_ISVTX;
+ uid_t uid = current->fsuid;
+ gid_t gid = current->fsgid;
struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
struct sysinfo si;

@@ -1279,10 +1293,8 @@
blocks = inodes = si.totalram / 2;

#ifdef CONFIG_TMPFS
- if (shmem_parse_options (data, &mode, &blocks, &inodes)) {
- printk(KERN_ERR "tmpfs invalid option\n");
+ if (shmem_parse_options (data, &mode, &uid, &gid, &blocks, &inodes))
return NULL;
- }
#endif

spin_lock_init (&sbinfo->stat_lock);
@@ -1299,6 +1311,8 @@
if (!inode)
return NULL;

+ inode->i_uid = uid;
+ inode->i_gid = gid;
root = d_alloc_root(inode);
if (!root) {
iput(inode);