Re: [PATCH 1/3] Avoid unmapping THP when it is failed to be split.

From: Andrea Arcangeli
Date: Tue Jan 25 2011 - 17:02:33 EST

On Tue, Jan 25, 2011 at 02:45:27PM +0900, Jin Dongming wrote:
> If the THP is failed to be split,
> 1. The processes using the poisoned page could not be collected.
> (Because page_mapped_in_vma() in collect_procs_anon() always
> returns NULL.)

page_mapped_in_vma is only called after split_huge_page without
requiring this change.

> 2. The poisoned page could not be unmapped.
> (If CONFIG_DEBUG_VM is "y", VM_BUG_ON(PageTransHuge(page))
> in try_to_unmap() will be called, and system panic will be
> caused.)

This is more reasonable argument for the change because at times
collect_procs may fail kmalloc or for other reasons kill may be set to
1 without split_huge_page having run.

> 3. The processes using the poisoned page could not be killed, too.
> (Because no process is collected in 1.)

I don't see the point of the change for 1.

> So if splitting THP is failed, it is better to stop unmapping
> rather than causing panic. System might servive if the page is freed
> later.

Splitting THP can't fail unless the page is freed under
split_huge_page (like if the process quits while you're calling
split_huge_page and in turn the anon_vma was already unusable).

Patch looks good but just for point 2.
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