On Fri, Nov 09, 2012 at 07:10:04PM +0000, Sears, Steven wrote:
> I have a memory subsystem design question that I'm hoping someone can answer.
>
> I've been looking at a machine that is completely out of memory, as in
>
> v_free_count = 0,
> v_cache_count = 0,
>
> I wondered how a machine could completely run out of memory like this,
> especially after finding a lack of interrupt storms or other pathologies that
> would tend to overcommit memory. So I started investigating.
>
> Most allocators come down to vm_page_alloc(), which has this guard:
>
> if ((curproc == pageproc) && (page_req != VM_ALLOC_INTERRUPT)) {
> page_req = VM_ALLOC_SYSTEM;
> };
>
> if (cnt.v_free_count + cnt.v_cache_count > cnt.v_free_reserved ||
> (page_req == VM_ALLOC_SYSTEM &&
> cnt.v_free_count + cnt.v_cache_count > cnt.v_interrupt_free_min) ||
> (page_req == VM_ALLOC_INTERRUPT &&
> cnt.v_free_count + cnt.v_cache_count > 0)) {
>
> The key observation is if VM_ALLOC_INTERRUPT is set, it will allocate every
> last page.
>
> >From the name one might expect VM_ALLOC_INTERRUPT to be somewhat rare,
> >perhaps only used from interrupt threads. Not so, see kmem_malloc() or
> >uma_small_alloc() which both contain this mapping:
>
> if ((flags & (M_NOWAIT|M_USE_RESERVE)) == M_NOWAIT)
> pflags = VM_ALLOC_INTERRUPT | VM_ALLOC_WIRED;
> else
> pflags = VM_ALLOC_SYSTEM | VM_ALLOC_WIRED;
>
> Note that M_USE_RESERVE has been deprecated and is used in just a handful of
> places. Also note that lots of code paths come through these routines.
>
> What this means is essentially _any_ allocation using M_NOWAIT will bypass
> whatever reserves have been held back and will take every last page available.
>
> There is no documentation stating M_NOWAIT has this side effect of
> essentially being privileged, so any innocuous piece of code that can't block
> will use it. And of course M_NOWAIT is literally used all over.
>
> It looks to me like the design goal of the BSD allocators is on recovery; it
> will give all pages away knowing it can recover.
>
> Am I missing anything? I would have expected some small number of pages to be
> held in reserve just in case. And I didn't expect M_NOWAIT to be a sort of
> back door for grabbing memory.
> Your analysis is right, there is nothing to add or correct. This is the reason to strongly prefer M_WAITOK.
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