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SubjectPossible bug in __alloc_pages() ?

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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