Re: Possible bug in __alloc_pages() ?

Martin J. Bligh (mbligh@aracnet.com)
Tue, 17 Sep 2002 15:02:15 -0700


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" <jmtorre@gmx.net> 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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