* Possible bug in __alloc_pages() ?
@ 2002-09-17 21:48 Juan M. de la Torre
2002-09-17 22:02 ` Martin J. Bligh
0 siblings, 1 reply; 2+ messages in thread
From: Juan M. de la Torre @ 2002-09-17 21:48 UTC (permalink / raw)
To: linux-kernel
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
^ permalink raw reply [flat|nested] 2+ messages in thread* Re: Possible bug in __alloc_pages() ?
2002-09-17 21:48 Possible bug in __alloc_pages() ? Juan M. de la Torre
@ 2002-09-17 22:02 ` Martin J. Bligh
0 siblings, 0 replies; 2+ messages in thread
From: Martin J. Bligh @ 2002-09-17 22:02 UTC (permalink / raw)
To: Juan M. de la Torre, linux-kernel
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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>
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