Re: [PATCH] vfs: Export fallocate facility to kernel modules

[Tyler Hicks <tyhicks@canonical.com>]

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On 2011-11-05 08:30:02, Tyler Hicks wrote:
> On 2011-10-31 12:14:05, Thieu Le wrote:
> > Export fallocate facility so layered file systems such as ecryptfs can
> > take advantage of this feature.  One example of the need for fallocate
> > is by ecryptfs.  ecryptfs has recently switched to a writeback cache
> > model so its dirty pages are not written to the lower file system
> > immediately.  In order to ensure that space is available when the page
> > is later written, ecryptfs can efficiently preallocate that space using
> > vfs_fallocate().
> 
> Hi Thieu - Thanks for the patch. Something like this is definitely
> needed for eCryptfs.
> 
> > 
> > vfs_fallocate() does not perform all of the checkings of do_fallocate()
> > because those checks are done higher in the stack.
> 
> After taking a closer look at the checks in do_fallocate(), I don't know
> that this statement is correct. The sanity checks around offset, len,
> and mode aren't guaranteed to be done higher up in the stack. The
> existing VFS helper functions also seem to do these types of checks. I
> think the rest of the checks are redundant.

I obviously goofed up here. Thieu states where offset and len are
checked in his following sentence. Not sure how I missed that. My
apologies.

However, I do still think that we need the fallocate mode checks in
vfs_fallocate().

Tyler

> > Checking of offset and
> > len are done by rw_verify_area() and generic_write_checks().  These
> > functions are called by VFS before ecryptfs invokes vfs_fallocate() in its
> > ecryptfs_write_end() function.  And since ecryptfs directly opens the lower
> > file, it ensures that the file is opened with the proper flags and mode.
> > 
> > Signed-off-by: Thieu Le <thieule@chromium.org>
> > ---
> >  fs/open.c          |   14 ++++++++++----
> >  include/linux/fs.h |    2 ++
> >  2 files changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/open.c b/fs/open.c
> > index f711921..8a4ba20 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -214,6 +214,15 @@ SYSCALL_ALIAS(sys_ftruncate64, SyS_ftruncate64);
> >  #endif /* BITS_PER_LONG == 32 */
> >  
> >  
> > +int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> > +{
> > +	if (!file->f_op->fallocate)
> > +		return -EOPNOTSUPP;
> > +
> > +	return file->f_op->fallocate(file, mode, offset, len);
> > +}
> > +EXPORT_SYMBOL(vfs_fallocate);
> > +
> >  int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >  {
> >  	struct inode *inode = file->f_path.dentry->d_inode;
> > @@ -263,10 +272,7 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >  	if (((offset + len) > inode->i_sb->s_maxbytes) || ((offset + len) < 0))
> >  		return -EFBIG;
> >  
> > -	if (!file->f_op->fallocate)
> > -		return -EOPNOTSUPP;
> > -
> > -	return file->f_op->fallocate(file, mode, offset, len);
> > +	return vfs_fallocate(file, mode, offset, len);
> >  }
> >  
> >  SYSCALL_DEFINE(fallocate)(int fd, int mode, loff_t offset, loff_t len)
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 277f497..c0e1225 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1504,6 +1504,8 @@ extern int vfs_link(struct dentry *, struct inode *, struct dentry *);
> >  extern int vfs_rmdir(struct inode *, struct dentry *);
> >  extern int vfs_unlink(struct inode *, struct dentry *);
> >  extern int vfs_rename(struct inode *, struct dentry *, struct inode *, struct dentry *);
> > +extern int vfs_fallocate(struct file *file, int mode, loff_t offset,
> > +			 loff_t len);
> >  
> >  /*
> >   * VFS dentry helper functions.
> > -- 
> > 1.7.3.1
> > 
> > --
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