Re: Possible bug in __alloc_pages() ?

[Martin J. Bligh]

This is deliberate, and is to discourage fallback. I'm not desperately fond
of the method, but I'm told it's not an accidental typo / bug.

M.

--On Tuesday, September 17, 2002 23:48:04 +0200 "Juan M. de la Torre" <[e-mail]> wrote:

> 
>  Hi, this code appears at the beggining of __page_alloc() (kernel 2.4.19):
> 
>         min = 1UL << order;
>         for (;;) {
>                 zone_t *z = *(zone++);
>                 if (!z)
>                         break;
> 
>                 min += z->pages_low;
>                 if (z->free_pages > min) {
>                         page = rmqueue(z, order);
>                         if (page)
>                                 return page;
>                 }
>         }
> 
>  AFAIK, what this code does is to try to alloc the requested pages from
> the first zone in a zone_list (passed as argument) which have enought free 
> pages.
> 
>  A zone is considered to have enought free pages if z->free_pages is greater 
> than (number_of_requested_pages + z->pages_low).
> 
>  In the loop shown, the first iteration is OK, but in the second iteration
> (which only occurs if the first zone in the zone_list hasn't enought free
> pages) the zone will only be considered to have enought free pages if
> z->free_pages is greater that (number_of_requested_pages + z->pages_low
> + PREV_ZONE->pages_low). 
> 
>  I think this is a bug, but i'm not sure (i'm not a VM hacker).
> 
>  If it is a bug, there are other two loops in the same function which
> are buggy.
> 
> 
> Possible patch:
> 
> --- linux/mm/page_alloc.c.orig  Tue Sep 17 23:45:02 2002
> +++ linux/mm/page_alloc.c       Tue Sep 17 23:46:45 2002
> @@ -330,8 +330,7 @@
>                 if (!z)
>                         break;
> 
> -               min += z->pages_low;
> -               if (z->free_pages > min) {
> +               if (z->free_pages > min + z->pages_low) {
>                         page = rmqueue(z, order);
>                         if (page)
>                                 return page;
> @@ -354,8 +353,8 @@
>                 local_min = z->pages_min;
>                 if (!(gfp_mask & __GFP_WAIT))
>                         local_min >>= 2;
> -               min += local_min;
> -               if (z->free_pages > min) {
> +
> +               if (z->free_pages > min + local_min) {
>                         page = rmqueue(z, order);
>                         if (page)
>                                 return page;
> @@ -394,8 +393,7 @@
>                 if (!z)
>                         break;
> 
> -               min += z->pages_min;
> -               if (z->free_pages > min) {
> +               if (z->free_pages > min + z->pages_min) {
>                         page = rmqueue(z, order);
>                         if (page)
>                                 return page;
> 
> Regards,
> Juanma
> 
> -- 
> /jm
> 
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