Re: Inconsistency (bug) of vm_insert_page with high order allocations

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Sat, 30 May 2009 17:42:35 +1200
Alexey Korolev <akorolex@gmail.com> wrote:

> Kame San,
> 
> Thank you for your answers. I've decided to use split_pages function.
> >
> >  - write a patch for adding alloc_page_exact_nodemask()  // this is not difficult.
> >  - explain why you need this.
> >  - discuss.
> >
> Writing the patch is not dificult - but it will be hard to explain why
> it is necessary in kernel...
> > IMHO, considering other mmap/munmap/zap_pte, etc... page_count() and page_mapocunt()
> > should be controlled per pte. Then, you'll have to map pages one by one.
> >
> This is quite interesting. I tried to understand this code but it is
> much complicated. I clearly understand why pages have to be mapped one
> by one. By I don't understand how counters relate to this. (it is just
> a curiosity question - I won't be upset if no one answer it)
> 
The kernel/cpu cannot handle changes to multiple ptes/TLBs at once. Then,
if mapcount/count is not per pte, there will be "partially mapped/unmapped" racy
state. That's a bad and we'll need a complicated synchronization technique to
map/unmap multiple ptes/TLBs at once. It seems impossible.(and not worth to try)

Thanks
-Kame

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