Re: which dentry a page belongs to

[Jamie Lokier <jamie@shareable.org>]

Shaya Potter wrote:
> > That means you can get no vmas in i_mmap{,shared} when doing
> > perfectly normal writable shared mappings.
> 
> just realized an insanely ugly solution would be for all mapped files to
> map inode -> dentry (with code that makes sure they are still valid, i.e
> if dentry gets deleted or renamed).  Since the page always has it's
> inode host, could figure it out via that.  Since relatively few mapped
> files (compared to amount of files on fs) shouldn't be a huge memory
> overhead.

It's not ugly, it's wrong.  You'd end up verson-controlling the wrong files.

It's obvious that you want:

    1. Program A opens ("/a/path1", O_RDWR);
    1. Program B opens ("/a/path2", O_RDWR);
    2. Program A calls mmap.
    3. Program B calls mmap.
    4. You call your fs version control ioctl.
    5. Program A updates data.
    6. Program A unmaps or exits, which transfers the dirty bits.
    7. Later, ->writepage is called.
    6. You store a versioned snapshot of "/a/path1".

Your "insanely ugly solution" could easily snapshot "/a/path2" instead.

-- Jamie