On Sat, Oct 06, 2001 at 12:08:32PM +0200, Stephan von Krawczynski wrote:
> Hello,
> 
> the time has come again for my monthly stress-testing of new kernels :-)
> I installed 2.4.11-pre4 and ran it for a day. It was really strange for me to
> see _no_ alloc failures at all, and that's why I gave it a closer look (alan
> fooled me once by deleting the printk, which made the problem "disappear" too
> :-). And here it is (from page_alloc.c):
> 
>  rebalance:
>         page = balance_classzone(classzone, gfp_mask, order, &freed);
>         if (page)
>                 return page;  
> 
>         zone = zonelist->zones;
>         if (likely(freed)) {
>                 for (;;) {
>                         zone_t *z = *(zone++);
>                         if (!z)
>                                 break;
> 
>                         if (zone_free_pages(z, order) > z->pages_min) {
>                                 page = rmqueue(z, order);
>                                 if (page)
>                                         return page;   
>                         }
>                 }
>                 goto rebalance;
>         } else {
>                 /* 
>                  * Check that no other task is been killed meanwhile,
>                  * in such a case we can succeed the allocation.
>                  */
>                 for (;;) {
>                         zone_t *z = *(zone++);
>                         if (!z)
>                                 break;
> 
>                         if (zone_free_pages(z, order) > z->pages_min) {
>                                 page = rmqueue(z, order);
>                                 if (page)
>                                         return page;
>                         }
>                 }
> 
>                 goto rebalance;
>         }
> 
>         printk(KERN_NOTICE "%s __alloc_pages: %u-order allocation failed
> (gfp=0x%x/%i) from %p\n",
>                current->comm, order, gfp_mask, !!(current->flags &
> PF_MEMALLOC), __builtin_return_address(0));
> /* my stuff :-) */
>         if (order==0)
>                 show_trace(NULL);
>         return NULL;
> }
> 
> 
> As you can see I patched some more info output on certain alloc-failures. But
> unfortunately I did not read the lines above - up to now. As you can see the
> printk cannot be reached at all, because both if-cases jump to rebalance - not
> matter how comes.
> When I wrote the first version of this posting, I wrote: this looks like a
> possible deadlock to me, because it cycles forever when no pages are found. I
> proved myself right this time, because when I tried to send the mail and
> started a CD burn at the background, the host froze. As you may remember the CD
> burns always gave me alloc-failures during startup in earlier kernel versions.
> So it is pretty obvious I reached the deadlock this time.

You may want to read the email I sent yesterday (attached).

> Another thing I would like to kindly ask: what is the difference in the two if
> branches? As I cannot read really well (already proven in another thread :-), I

Obviously no difference, but that's nothing compared to the deadlock.

> am very willing to accept any explanation on this code ;-)
> If someone else has asked the whole thing before: shoot me.
> 
> Regards,
> Stephan


Andrea