Re: oom-killer 2.6.8.1

[Hugh Dickins]

On Wed, 18 Aug 2004, Jakob Oestergaard wrote:
> 
> Looking thru the swapfile.c and oom killer code, one thing that is
> making me scratch my head:
> 
> nr_swap_pages is a *signed* integer.  This does not make sense. There
> are even tests in swapfile.c that explicitly test "nr_swap_pages <= 0"
> instead of simply "!nr_swap_pages" - this does not make sense at all
> either - or does it?
> 
> Stephen is that your code?

I'm not Stephen, and it wasn't originally my code, but I do remember
tidying this up to stop /proc/meminfo showing negative SwapFree
(see nr_to_be_unused).

nr_swap_pages _may_ legitimately be negative, during sys_swapoff:
that does "nr_swap_pages -= p->pages", which is liable to send it
negative, before going on to "try_to_unuse", which slowly increments
nr_swap_pages back up to its final value (0 if no other swap areas),
page by page via delete_from_swap_cache's swap_free.

Surprising, I agree, but it allows swap_free to increment
nr_swap_pages without any special casing for swapoff.

Hugh

> See, if nr_swap_pages can validly be negative and some meaning is
> attached to that (some meaning other than "we're out of swap"), the
> oom_killer surely misses that, as it tests "nr_swap_pages > 0".
> 
> I don't think that nr_swap_pages can be negative (unless one adds a
> *lot* of swap in which case this will unintentionally happen all by
> itself),  but I felt I should chirp in with this comment in case
> someone's looking at it anyway  :)

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