Re: which dentry a page belongs to

[Shaya Potter <spotter@cs.columbia.edu>]

On Fri, 2004-04-23 at 16:14 +0100, Jamie Lokier wrote:
> Shaya Potter wrote:
> > It would seem that since the
> > address_space object contains the vm_area_struct's of i_mmap and
> > i_mmap_shared I should then be able to get the appropriate file and
> > dentry object's through
> > 
> > page->mapping->i_mmap->vm_file->f_dentry
> > 
> > or
> > 
> > page->mapping->i_mmap_shared->vm_file->f_dentry
> > 
> > 1) Is this correct logic?  I'm assuming the only things that matters in
> > choosing which list is used if the page is map'd shared or not? is that
> > correct as well?
> 
> No, no and no.
> 
> i_mmap and i_mmap_shared are lists.  They can both be empty, or both
> non-empty.  A page can be mapped shared *and* non-shared at the same
> time.  A page might not be mapped at all.

yes, they can be empty for "generic" pages, but I'm looking at a
specific case of file system pages, so they shouldn't be empty.  i.e.
otherwise my fs's writepage() shouldn't be called, I would think.

> Also, a page is often mapped in a _subset_ of the mappings which are
> found in i_mmap and i_mmap_shared: it depends on its offset, and the
> vma offsets, and non-linear mapping offsets.

ok, this I don't understand.  need to look into this, so it's not as
simple a dereference as I thought.

> It is possible to find multiple dentries which are currently being
> used to map a page.

a single page can have multiple dentries?  but it has only one inode?
(i.e. host) So I can imagine if the single inode is linked in multiple
places (for my purposes I don't care about that directly) but can it
really have multiple inodes?

> It's also possible to find no dentries at all.

even if in my fs's writepage() function?

> Your question is extremely ill-formed.  What do you mean by "the
> dentry corresponding to a page"?  What do you want the value for?

When my writepage() is called, I want to be able to possibly do dentry
based operations (rename, d_path....) to be told what files are actually
getting written to via writepage() (as opposed to the file system's
write() functionality).

i.e. there are 2 ways for a file to be written to a file system (at
least as far as I understand, could easily be wrong) writepage() and
write().

in write() one know's the dentry (file->f_dentry)

I'm trying to figure out how I can get the same knowledge for
writepage() from the page passed to it.