Re: [PATCH] ext3: Use page_mkwrite vma_operations to get mmap write notification.

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Tuesday 25 March 2008 10:01, Andrew Morton wrote:
> On Mon, 24 Mar 2008 22:34:56 +0530
>
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > We would like to get notified when we are doing a write on mmap section.
> > The changes are needed to handle ENOSPC when writing to an mmap section
> > of files with holes.
>
> umm,
>
> > diff --git a/fs/ext3/file.c b/fs/ext3/file.c
> > index acc4913..09e22e4 100644
> > --- a/fs/ext3/file.c
> > +++ b/fs/ext3/file.c
> > @@ -106,6 +106,23 @@ force_commit:
> >  	return ret;
> >  }
> >
> > +static struct vm_operations_struct ext3_file_vm_ops = {
> > +	.fault		= filemap_fault,
> > +	.page_mkwrite   = ext3_page_mkwrite,
> > +};
> > +
> > +static int ext3_file_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > +	struct address_space *mapping = file->f_mapping;
> > +
> > +	if (!mapping->a_ops->readpage)
> > +		return -ENOEXEC;
> > +	file_accessed(file);
> > +	vma->vm_ops = &ext3_file_vm_ops;
> > +	vma->vm_flags |= VM_CAN_NONLINEAR;
> > +	return 0;
> > +}
> > +
> >  const struct file_operations ext3_file_operations = {
> >  	.llseek		= generic_file_llseek,
> >  	.read		= do_sync_read,
> > @@ -116,7 +133,7 @@ const struct file_operations ext3_file_operations = {
> >  #ifdef CONFIG_COMPAT
> >  	.compat_ioctl	= ext3_compat_ioctl,
> >  #endif
> > -	.mmap		= generic_file_mmap,
> > +	.mmap		= ext3_file_mmap,
> >  	.open		= generic_file_open,
> >  	.release	= ext3_release_file,
> >  	.fsync		= ext3_sync_file,
> > diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> > index eb95670..2293506 100644
> > --- a/fs/ext3/inode.c
> > +++ b/fs/ext3/inode.c
> > @@ -3306,3 +3306,8 @@ int ext3_change_inode_journal_flag(struct inode
> > *inode, int val)
> >
> >  	return err;
> >  }
> > +
> > +int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> > +{
> > +	return block_page_mkwrite(vma, page, ext3_get_block);
> > +}
>
> This gets called within the pagefault handler.
>
> And block_page_mkwrite() does lock_page().
>
> But the pagefault handler can be called with a page already locked, from
> generic_perform_write().
>
> Nick, why are we not vulnerable to A-A or to AB-BA deadlocks here?

Pagefault handler unlocks the page before calling page_mkwrite. This is
kind of crap, because the caller invariably has to lock the page again
to check that it has not been truncated away anyway.... I voiced my
concerns about this page_mkwrite API, but no, as always, people want to
be really clever first and have understandable locking schemes second ;)

I'm hoping to one day clean this up and fold it all into ->fault()...


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