On Tue, 6 Nov 2012 09:31:57 +0800
Jiang Liu <jiang.liu@xxxxxxxxxx> wrote:
> Changeset 7f1290f2f2 tries to fix a issue when calculating
> zone->present_pages, but it causes a regression to 32bit systems with
> HIGHMEM. With that changeset, function reset_zone_present_pages()
> resets all zone->present_pages to zero, and fixup_zone_present_pages()
> is called to recalculate zone->present_pages when boot allocator frees
> core memory pages into buddy allocator. Because highmem pages are not
> freed by bootmem allocator, all highmem zones' present_pages becomes
> zero.
>
> Actually there's no need to recalculate present_pages for highmem zone
> because bootmem allocator never allocates pages from them. So fix the
> regression by skipping highmem in function reset_zone_present_pages()
> and fixup_zone_present_pages().
>
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6108,7 +6108,8 @@ void reset_zone_present_pages(void)
> for_each_node_state(nid, N_HIGH_MEMORY) {
> for (i = 0; i < MAX_NR_ZONES; i++) {
> z = NODE_DATA(nid)->node_zones + i;
> - z->present_pages = 0;
> + if (!is_highmem(z))
> + z->present_pages = 0;
> }
> }
> }
> @@ -6123,10 +6124,11 @@ void fixup_zone_present_pages(int nid, unsigned long start_pfn,
>
> for (i = 0; i < MAX_NR_ZONES; i++) {
> z = NODE_DATA(nid)->node_zones + i;
> + if (is_highmem(z))
> + continue;
> +
> zone_start_pfn = z->zone_start_pfn;
> zone_end_pfn = zone_start_pfn + z->spanned_pages;
> -
> - /* if the two regions intersect */
> if (!(zone_start_pfn >= end_pfn || zone_end_pfn <= start_pfn))
> z->present_pages += min(end_pfn, zone_end_pfn) -
> max(start_pfn, zone_start_pfn);
This ... isn't very nice. It is embeds within
reset_zone_present_pages() and fixup_zone_present_pages() knowledge
about their caller's state. Or, more specifically, it is emebedding
knowledge about the overall state of the system when these functions
are called.
I mean, a function called "reset_zone_present_pages" should reset
->present_pages!
The fact that fixup_zone_present_page() has multiple call sites makes
this all even more risky. And what are the interactions between this
and memory hotplug?
Can we find a cleaner fix?
Please tell us more about what's happening here. Is it the case that
reset_zone_present_pages() is being called *after* highmem has been
populated? If so, then fixup_zone_present_pages() should work
correctly for highmem? Or is it the case that highmem hasn't yet been
setup? IOW, what is the sequence of operations here?
Is the problem that we're *missing* a call to
fixup_zone_present_pages(), perhaps? If we call
fixup_zone_present_pages() after highmem has been populated,
fixup_zone_present_pages() should correctly fill in the highmem zone's
->present_pages?
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