Re: [PATCH] Compound page overhaul

[Andrew Morton]

David Howells <dhowells@xxxxxxxxxx> wrote:
>
> 
> > > ugh, sorry, I'd forgotten that !MMU needs to use the fields inside
> > > pages[1].
> 
> Perhaps I misunderstood what you meant here. I _assumed_ you meant that it
> used the bits of pages[1] for compound page metadata - which it now does only
> because it's now using compound pages (if only with my patch applied).

I thought that's what you meant ;)

So why did you create a "Compound page overhaul" in the first place?  Was it
not to address some insufficiency for !MMU?

> > But !MMU really wants to treat that higher-order page as an array of
> > zero-order pages, and that requires the usual usage of the fields of
> > page[1], page[2], etc.
> 
> That's not really so. For the most part, !MMU linux treats pages identically
> to MMU linux, whether those pages are big or small.
> 
> It's only for interprocess userspace access that there's an issue, and the
> issue there is, I think, that access_process_vm() wants to pin the page in
> place to stop it going away whilst it is being fiddled with.
> 
> Normally, the page is pinned in place by its refcount and/or flags. However,
> for compound pages, the refcount in question is really on the first page of
> the batch, and so refcount accesses should be directed there, and not to a
> secondary page.

The current compound page logic should handle that quite happily, no?

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