Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

[Blaisorblade]

On Wednesday 07 March 2007 11:02, Nick Piggin wrote:
> On Wed, Mar 07, 2007 at 10:49:47AM +0100, Nick Piggin wrote:
> > On Wed, Mar 07, 2007 at 01:44:20AM -0800, Bill Irwin wrote:
> > > On Wed, Mar 07, 2007 at 10:28:21AM +0100, Nick Piggin wrote:
> > > > Depending on whether anyone wants it, and what features they want, we
> > > > could emulate the old syscall, and make a new restricted one which is
> > > > much less intrusive.
> > > > For example, if we can operate only on MAP_ANONYMOUS memory and
> > > > specify that nonlinear mappings effectively mlock the pages, then we
> > > > can get rid of all the objrmap and unmap_mapping_range handling,
> > > > forget about the writeout and msync problems...
> > >
> > > Anonymous-only would make it a doorstop for Oracle, since its entire
> > > motive for using it is to window into objects larger than user virtual
> >
> > Uh, duh yes I don't mean MAP_ANONYMOUS, I was just thinking of the shmem
> > inode that sits behind MAP_ANONYMOUS|MAP_SHARED. Of course if you don't
> > have a file descriptor to get a pgoff, then remap_file_pages is a
> > doorstop for everyone ;)
> >
> > > address spaces (this likely also applies to UML, though they should
> > > really chime in to confirm). Restrictions to tmpfs and/or ramfs would
> > > likely be liveable, though I suspect some things might want to do it to
> > > shm segments (I'll ask about that one). There's definitely no need for
> > > a persistent backing store for the object to be remapped in Oracle's
> > > case, in any event. It's largely the in-core destination and source of
> > > IO, not something saved on-disk itself.
> >
> > Yeah, tmpfs/shm segs are what I was thinking about. If UML can live with
> > that as well, then I think it might be a good option.
>
> Oh, hmm.... if you can truncate these things then you still need to
> force unmap so you still need i_mmap_nonlinear.

Well, we don't need truncate(), but MADV_REMOVE for memory hotunplug, which is 
way similar I guess.

About the restriction to tmpfs, I have just discovered 
'[PATCH] mm: tracking shared dirty pages' (commit 
d08b3851da41d0ee60851f2c75b118e1f7a5fc89), which already partially conflicts 
with remap_file_pages for file-based mmaps (and that's fully fine, for now).

Even if UML does not need it, till now if there is a VMA protection and a page 
hasn't been remapped with remap_file_pages, the VMA protection is used (just 
because it makes sense).

However, it is only used when the PTE is first created - we can never change 
protections on a VMA  - so it vma_wants_writenotify() is true (on all 
file-based and on no shmfs based mapping, right?), and we write-protect the 
VMA, it will always be write-protected.

That's no problem for UML, but for any other user (I guess I'll have to 
prevent callers from trying such stuff - I started from a pretty generic 
patch).

> But come to think of it, I still don't think nonlinear mappings are
> too bad as they are ;)

Btw, I really like removing ->populate and merging the common code together. 
filemap_populate and shmem_populate are so obnoxiously different that I 
already wanted to do that (after merging remap_file_pages() core).

Also, I'm curious. Since my patches are already changing remap_file_pages() 
code, should they be absolutely merged after yours?
-- 
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
Chiacchiera con i tuoi amici in tempo reale! 
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