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SubjectRe: ext2 FS corruption with 2.5.59.
Oleg Drokin <green@namesys.com> wrote:
>
> Looking through 2.5.54, I found that changeset named "[PATCH] quota locking update" forwarded by you
> is the cause for the problem.

OK, it's writepage-versus-writepage and/or writepage-versus-truncate.
There's no locking between the updaters and i_blocks goes negative.
This is good ;) We have done well.

I went for the new frlocks which I dragged into -mm to fix up the xtime_lock
problems. It's not 100% appropriate, but it'll do just fine.

Stephen, we can use that frlock in the inode to protect access to the 64-bit
i_size on 32-bit platforms as well. However in the case of i_size, the
updaters do not need to take the frlock's spinlock, because writers are
serialised inode->i_sem.

So could you please take a look at adding new writer-side primitives to
frlocks to cover this requirement? See i_size_read/i_size_write from the -aa
kernel:

+static inline void i_size_write(struct inode * inode, loff_t i_size)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+#ifdef __ARCH_HAS_GET_SET_64BIT
+ set_64bit((unsigned long long *) &inode->i_size, (unsigned long long) i_size);
+#else
+ inode->i_size_version2++;
+ wmb();
+ inode->i_size = i_size;
+ wmb();
+ inode->i_size_version1++;
+ wmb(); /* make it visible ASAP */
+#endif
+#elif BITS_PER_LONG==64 || !defined(CONFIG_SMP)
+ inode->i_size = i_size;
+#endif
+}

Something like this might suffice:

diff -puN include/linux/frlock.h~frlock_write_begin include/linux/frlock.h
--- 25/include/linux/frlock.h~frlock_write_begin 2003-01-25 18:59:27.000000000 -0800
+++ 25-akpm/include/linux/frlock.h 2003-01-25 19:01:27.000000000 -0800
@@ -80,6 +80,22 @@ static inline unsigned fr_read_end(frloc
return rw->pre_sequence;
}

+static inline unsigned fr_write_begin(frlock_t *rw)
+{
+ unsigned ret = rw->pre_sequence++;
+ wmb();
+ return ret;
+}
+
+static inline unsigned fr_write_end(frlock_t *rw)
+{
+ unsgned ret;
+ wmb();
+ ret = ++(rw->post_sequence);
+ wmb();
+ return ret;
+}
+
/*
* Possible sw/hw IRQ protected versions of the interfaces.
*/
Thanks.




Since Jan removed the lock_kernel()s in inode_add_bytes() and
inode_sub_bytes(), these functions have been racy.

One problematic workload has been discovered in which concurrent writepage
and truncate on SMP quickly causes i_blocks to go negative. writepage() does
not take i_sem, and it seems that for ext2, there are no other locks in
force when inode_add_bytes() is called.

Putting the BKL back in there is not acceptable. To fix this race I have
used the new frlocks.

This is actually fairly pointless, because these fields are write-mostly and
read-rarely. But we need a spinlock anyway because of the concurrent
modifiers problem.

And we need this frlock in the inode to solve the problem of the nonatomicity
of i_size. (But some aditional frlock work is needed first: in the case of
i_size we do not need to take the spinlock because of i_sem protection).

The splitting of the used disk space into i_blocks and i_bytes is silly - we
should nuke all that and just have a bare loff_t i_usedbytes. Later.


inode.c | 1 +
stat.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
linux/fs.h | 52 +++++++++++++++++-----------------------------------
ksyms.c | 4 ++++
4 files changed, 68 insertions(+), 35 deletions(-)
diff -puN include/linux/fs.h~inode-accounting-race-fix include/linux/fs.h
--- linux-hype/include/linux/fs.h~inode-accounting-race-fix 2003-01-25 18:36:25.000000000 -0800
+++ linux-hype-akpm/include/linux/fs.h 2003-01-25 18:36:25.000000000 -0800
@@ -18,6 +18,7 @@
#include <linux/stat.h>
#include <linux/cache.h>
#include <linux/radix-tree.h>
+#include <linux/frlock.h>
#include <asm/atomic.h>

struct iovec;
@@ -371,9 +372,19 @@ struct inode {
struct timespec i_ctime;
unsigned int i_blkbits;
unsigned long i_blksize;
- unsigned long i_blocks;
unsigned long i_version;
+
+ /*
+ * i_bytes and i_blocks represent the amount of allocated disk space:
+ * (i_blocks * 512 + i_bytes) (FIXME: just use an loff_t here). They
+ * are protected by i_frlock.
+ */
+ unsigned long i_blocks;
unsigned short i_bytes;
+
+ /* Protects i_blocks, i_bytes and, later, i_size */
+ frlock_t i_frlock;
+
struct semaphore i_sem;
struct inode_operations *i_op;
struct file_operations *i_fop; /* former ->i_op->default_file_ops */
@@ -400,7 +411,7 @@ struct inode {
void *i_security;
__u32 i_generation;
union {
- void *generic_ip;
+ void *generic_ip;
} u;
};

@@ -412,39 +423,6 @@ struct fown_struct {
void *security;
};

-static inline void inode_add_bytes(struct inode *inode, loff_t bytes)
-{
- inode->i_blocks += bytes >> 9;
- bytes &= 511;
- inode->i_bytes += bytes;
- if (inode->i_bytes >= 512) {
- inode->i_blocks++;
- inode->i_bytes -= 512;
- }
-}
-
-static inline void inode_sub_bytes(struct inode *inode, loff_t bytes)
-{
- inode->i_blocks -= bytes >> 9;
- bytes &= 511;
- if (inode->i_bytes < bytes) {
- inode->i_blocks--;
- inode->i_bytes += 512;
- }
- inode->i_bytes -= bytes;
-}
-
-static inline loff_t inode_get_bytes(struct inode *inode)
-{
- return (((loff_t)inode->i_blocks) << 9) + inode->i_bytes;
-}
-
-static inline void inode_set_bytes(struct inode *inode, loff_t bytes)
-{
- inode->i_blocks = bytes >> 9;
- inode->i_bytes = bytes & 511;
-}
-
/*
* Track a single file's readahead state
*/
@@ -1277,6 +1255,10 @@ extern int page_symlink(struct inode *in
extern struct inode_operations page_symlink_inode_operations;
extern void generic_fillattr(struct inode *, struct kstat *);
extern int vfs_getattr(struct vfsmount *, struct dentry *, struct kstat *);
+void inode_add_bytes(struct inode *inode, loff_t bytes);
+void inode_sub_bytes(struct inode *inode, loff_t bytes);
+loff_t inode_get_bytes(struct inode *inode);
+void inode_set_bytes(struct inode *inode, loff_t bytes);

extern int vfs_readdir(struct file *, filldir_t, void *);

diff -puN fs/stat.c~inode-accounting-race-fix fs/stat.c
--- linux-hype/fs/stat.c~inode-accounting-race-fix 2003-01-25 18:36:25.000000000 -0800
+++ linux-hype-akpm/fs/stat.c 2003-01-25 18:36:25.000000000 -0800
@@ -316,3 +316,49 @@ asmlinkage long sys_fstat64(unsigned lon
}

#endif /* LFS-64 */
+
+void inode_add_bytes(struct inode *inode, loff_t bytes)
+{
+ fr_write_lock(&inode->i_frlock);
+ inode->i_blocks += bytes >> 9;
+ bytes &= 511;
+ inode->i_bytes += bytes;
+ if (inode->i_bytes >= 512) {
+ inode->i_blocks++;
+ inode->i_bytes -= 512;
+ }
+ fr_write_unlock(&inode->i_frlock);
+}
+
+void inode_sub_bytes(struct inode *inode, loff_t bytes)
+{
+ fr_write_lock(&inode->i_frlock);
+ inode->i_blocks -= bytes >> 9;
+ bytes &= 511;
+ if (inode->i_bytes < bytes) {
+ inode->i_blocks--;
+ inode->i_bytes += 512;
+ }
+ inode->i_bytes -= bytes;
+ fr_write_unlock(&inode->i_frlock);
+}
+
+loff_t inode_get_bytes(struct inode *inode)
+{
+ unsigned seq;
+ loff_t ret;
+
+ do {
+ seq = fr_read_begin(&inode->i_frlock);
+ ret = (((loff_t)inode->i_blocks) << 9) + inode->i_bytes;
+ } while (seq != fr_read_end(&inode->i_frlock));
+ return ret;
+}
+
+void inode_set_bytes(struct inode *inode, loff_t bytes)
+{
+ fr_write_lock(&inode->i_frlock);
+ inode->i_blocks = bytes >> 9;
+ inode->i_bytes = bytes & 511;
+ fr_write_unlock(&inode->i_frlock);
+}
diff -puN kernel/ksyms.c~inode-accounting-race-fix kernel/ksyms.c
--- linux-hype/kernel/ksyms.c~inode-accounting-race-fix 2003-01-25 18:36:25.000000000 -0800
+++ linux-hype-akpm/kernel/ksyms.c 2003-01-25 18:36:25.000000000 -0800
@@ -272,6 +272,10 @@ EXPORT_SYMBOL(vfs_fstat);
EXPORT_SYMBOL(vfs_stat);
EXPORT_SYMBOL(vfs_lstat);
EXPORT_SYMBOL(vfs_getattr);
+EXPORT_SYMBOL(inode_add_bytes);
+EXPORT_SYMBOL(inode_sub_bytes);
+EXPORT_SYMBOL(inode_get_bytes);
+EXPORT_SYMBOL(inode_set_bytes);
EXPORT_SYMBOL(lock_rename);
EXPORT_SYMBOL(unlock_rename);
EXPORT_SYMBOL(generic_read_dir);
diff -puN fs/inode.c~inode-accounting-race-fix fs/inode.c
--- linux-hype/fs/inode.c~inode-accounting-race-fix 2003-01-25 18:36:25.000000000 -0800
+++ linux-hype-akpm/fs/inode.c 2003-01-25 18:36:25.000000000 -0800
@@ -176,6 +176,7 @@ void inode_init_once(struct inode *inode
spin_lock_init(&inode->i_data.private_lock);
INIT_LIST_HEAD(&inode->i_data.i_mmap);
INIT_LIST_HEAD(&inode->i_data.i_mmap_shared);
+ frlock_init(&inode->i_frlock);
}

static void init_once(void * foo, kmem_cache_t * cachep, unsigned long flags)
_

-
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