consistent_free()

[Paul Mackerras]

David Gibson writes:

> Well, I had a look at the ARM version and now I'm a bit confused.  As
> far as I can tell it does two things differently from us:
> 	1) After making the allocation rounded up to the appropriate
> order it frees any leftover pages.  That certainly seems worthwhile.
> 	2) It sets PageReserved on each struct page that it
> allocates/remaps.
>
> (2) seems very strange.  It doesn't seem in keeping with the meaning
> of PageReserved (well my best guess at the meaning from only slightly

PageReserved means that the VM system shouldn't try to manage this
page.  In particular, it shouldn't try to do the page aging/move to
swap cache/swap out stuff that it will do on normal pages.

> illuminating comments in page.h). Apparently it's so that
> remap_page_range() works - as indeed it wouldn't without this because
> of a test in remap_pte_range() which that calls.  But that test looks
> to be precisely inverted from what it should be.  Which makes me

No, it's right.  remap_page_range() is only meant to be used for pages
that are either memory-mapped I/O (the !VALID_PAGE(page) test) or for
pages that are "special" (the PageReserved(page) test).  You aren't
supposed to use remap_page_range on normal RAM that the VM is
managing.

> wonder how the hell anything works now, since remap_page_range() is
> apparently called from several places.

It's only called from drivers (such as /dev/mem) or from arch-specific
code AFAICS.

Paul.

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