Re: Hard Hang with __alloc_pages: 0-order allocation failed (gfp=0x20/1) - Not out of memory

[Roger Larsson <roger.larsson@norran.net>]

Modifying the header so you find it. And write a better summary...

* When you have little memory left __alloc_pages wont allow
   allocations from interrupt context (even if PF_MEMALLOC)

	if (current->flags & PF_MEMALLOC && !in_interrupt()) {

* But in the callchain there is a loop in __kmem_cache_alloc when running SMP

> static inline void * __kmem_cache_alloc (kmem_cache_t *cachep, int flags)
> {
> 	unsigned long save_flags;
> 	void* objp;
>
> 	kmem_cache_alloc_head(cachep, flags);
> try_again:
> 	local_irq_save(save_flags);
>
> #ifdef CONFIG_SMP
> 	{
> 		cpucache_t *cc = cc_data(cachep);
>
> 		if (cc) {
> 			if (cc->avail) {
> 				STATS_INC_ALLOCHIT(cachep);
> 				objp = cc_entry(cc)[--cc->avail];
> 			} else {
> 				STATS_INC_ALLOCMISS(cachep);
> 				objp = kmem_cache_alloc_batch(cachep,cc,flags);
> 				if (!objp)
> 					goto alloc_new_slab_nolock;
>
> - - - snip - - -
> alloc_new_slab_nolock:
> #endif
> 	local_irq_restore(save_flags);
> 	if (kmem_cache_grow(cachep, flags))
> 		/* Someone may have stolen our objs.  Doesn't matter, we'll
> 		 * just come back here again.
> 		 */
> 		goto try_again;
>
> But kmem_cache_grow will return failed...
> 	-> kmem_getpages ->  _get_free_pages -> alloc_pages

But alloc_pages WILL fail every time... => infinite loop in interrupt
context... It probably should not try again in this case...

	if (!in_interrupt())
		goto try_again;

Or it should check if GFP_WAIT is set... A _busy_ wait is also a wait...

/RogerL

-- 
Roger Larsson
Skellefteå
Sweden