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.
Thanks,
-Steve
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