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

[Andrea Arcangeli <aarcange@redhat.com>]

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.