Re: VM_RESERVED [was Re: mapping user space buffer to kernel addressspace]

[Linus Torvalds]

On Fri, 20 Oct 2000, Andrea Arcangeli wrote:
> 
> The page count of the mapped pages should be ok, it seems those mapped pages
> have a reference count of 1 just from the vmalloc allocation and they use
> PG_reserved just to skip swap_out, but I feel safer too if the bttv maintainers
> will check it and send it to you themself after checking it's correct. (I only
> verified that it was compiling correctly)

Note that the page count should _not_ be one: the page could should be
1+nr_of_mappings.

Basically, the "nopage()" function has to do a "get_page(page)". But if
the page is marked PG_reserved, that would hide a bug in a drievr that
doesn't do that part.

Also, I suspect some drivers do the "remap_page_range() one page at a
time", and again exactly due to page count issues remap_page_range() will
refuse to touch pages that aren't marked PG_reserved. So I wholeheartedly
agree with your change, but I just suspect it will break drivers that have
depended on the fact that PG_reserved means that they can be lazy and not
bother about getting all the details right.

Not that I mind breaking drivers in general, but not right now.

                Linus

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