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Subject[RFC 1/2] introduce f_op->{pre,post}_mmap()
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The locking order between mm->mmap_sem and inode->i_mutex (or other FS
internal lock) has a problem. The right order is i_mutex first and then
mmap_sem. But sometimes this is hard for FS which has complicated
->mmap() since it prohibits acquire i_mutex (or other FS internal lock)
otherwise it will case an AB-BA deadlock problem.
In order to allow FS to implemente complicated ->mmpa(), introduce
f_op->{pre,post}_mmap(). ->pre_mmap() is called just before acquiring
mmap_sem for ->mmap(), and ->post_mmap() is called just after releasing
mmap_sem.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
---
Documentation/filesystems/Locking | 8 ++++++++
Documentation/filesystems/vfs.txt | 7 +++++++
include/linux/fs.h | 2 ++
include/linux/mm.h | 4 ++++
mm/mmap.c | 27 ++++++++++++++++++++++++---
5 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 57d827d..1815e20 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -408,7 +408,9 @@ prototypes:
unsigned int (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
+ void (*post_mmap) (struct file *, unsigned long, unsigned long);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
@@ -466,6 +468,12 @@ components. And there are other reasons why the current interface is a mess...
->read on directories probably must go away - we should just enforce -EISDIR
in sys_read() and friends.

+->mmap has mm->mmap_sem for write. If your FS needs i_mutex for mmap(2),
+ then never acquire it in ->mmap. Instead acquire it in ->pre_mmap(),
+ and release it in ->post_mmap() since they don't have mm->mmap_sem.
+ When ->pre_mmap() returns other than zero, both of ->mmap() and
+ ->post_mmap() will not be called.
+
--------------------------- dquot_operations -------------------------------
prototypes:
int (*write_dquot) (struct dquot *);
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 88b9f55..e2a579e 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -751,7 +751,9 @@ struct file_operations {
unsigned int (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
+ void (*post_mmap) (struct file *, unsigned long, unsigned long);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
@@ -794,8 +796,13 @@ otherwise noted.
compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
are used on 64 bit kernels.

+ pre_mmap: called by the mmap(2) system call
+
mmap: called by the mmap(2) system call

+ post_mmap: called by the mmap(2) system call
+ For pre_mmap, mmap, post_mmap, read Locking.txt too.
+
open: called by the VFS when an inode should be opened. When the VFS
opens a file, it creates a new "struct file". It then calls the
open method for the newly allocated file structure. You might
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b5b9792..bce9d44 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1554,7 +1554,9 @@ struct file_operations {
unsigned int (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
+ void (*post_mmap) (struct file *, unsigned long, unsigned long);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *, fl_owner_t id);
int (*release) (struct inode *, struct file *);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9670f71..e22230c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1428,6 +1428,10 @@ extern unsigned long mmap_region(struct file *file, unsigned long addr,
unsigned long len, unsigned long flags,
vm_flags_t vm_flags, unsigned long pgoff);

+extern int pre_mmap(struct file *file, unsigned long prot, unsigned long flag);
+extern void post_mmap(struct file *file, unsigned long prot,
+ unsigned long flag);
+
static inline unsigned long do_mmap(struct file *file, unsigned long addr,
unsigned long len, unsigned long prot,
unsigned long flag, unsigned long offset)
diff --git a/mm/mmap.c b/mm/mmap.c
index 0290c8e..0dd2acb 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1089,6 +1089,25 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
}
EXPORT_SYMBOL(do_mmap_pgoff);

+int pre_mmap(struct file *file, unsigned long prot, unsigned long flag)
+{
+ int err;
+
+ err = 0;
+ if (file && file->f_op && file->f_op->pre_mmap)
+ err = file->f_op->pre_mmap(file, prot, flag);
+ if (!err)
+ down_write(&current->mm->mmap_sem);
+ return err;
+}
+
+void post_mmap(struct file *file, unsigned long prot, unsigned long flag)
+{
+ up_write(&current->mm->mmap_sem);
+ if (file && file->f_op && file->f_op->post_mmap)
+ file->f_op->post_mmap(file, prot, flag);
+}
+
SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
unsigned long, prot, unsigned long, flags,
unsigned long, fd, unsigned long, pgoff)
@@ -1120,9 +1139,11 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,

flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);

- down_write(&current->mm->mmap_sem);
- retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
- up_write(&current->mm->mmap_sem);
+ retval = pre_mmap(file, prot, flags);
+ if (!retval) {
+ retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
+ post_mmap(file, prot, flags);
+ }

if (file)
fput(file);
--
1.7.2.5


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