Re: which dentry a page belongs to

[Christoph Hellwig <hch@infradead.org>]

On Fri, Apr 23, 2004 at 12:52:54PM -0400, Shaya Potter wrote:
> > in 2.4 writepage is always the result of data dirtied by mmap.  In 2.6 it's
> > also for use for data dirtied by write.  Even in 2.4 there's no gurantee
> > the mapping that dirtied the page still exists when the page is written out
> > by the VM.
> 
> so the mapping off the page struct will be null?

No. but i_mmap and i_mmap_shared might be empty.

> so is that a yes to my understanding of "multiple dentries" i.e. a
> single inode linked into multiple places in the fs.

Yes.

> > > > It's also possible to find no dentries at all.
> > > 
> > > even if in my fs's writepage() function?
> > 
> > Yes.
> 
> so the vm_file part of vm_area_struct will be null?

You won't find a vm_area_struct at all in that case.

> > You can't do that.
> 
> Yes, I know it's evil to do and would never be accepted into the kernel
> proper, the question I'm trying to figure out if it it's possible (with
> a stackable fs) to version files on write.  

It's not evil is's fucking impossible for gods sake.

you can be in writepage with page->mapping->i_mmap{,shared} beeing empty.
No way in hell you'll ever get to a dentry.

It's not that difficult.  And if english language is so difficult to
understand ask someone to draw a diagram for you.