Re: [PATCH] mm/page_alloc: Skip non present sections on zone initialization

David Hildenbrand
2020-01-10T14:35:01Z

On 10.01.20 14:45, Kirill A. Shutemov wrote:
> On Fri, Jan 10, 2020 at 02:15:26PM +0100, David Hildenbrand wrote:
>> On 08.01.20 15:40, Michal Hocko wrote:
>>> On Mon 30-12-19 12:38:28, Kirill A. Shutemov wrote:
>>>> memmap_init_zone() can be called on the ranges with holes during the
>>>> boot. It will skip any non-valid PFNs one-by-one. It works fine as long
>>>> as holes are not too big.
>>>>
>>>> But huge holes in the memory map causes a problem. It takes over 20
>>>> seconds to walk 32TiB hole. x86-64 with 5-level paging allows for much
>>>> larger holes in the memory map which would practically hang the system.
>>>>
>>>> Deferred struct page init doesn't help here. It only works on the
>>>> present ranges.
>>>>
>>>> Skipping non-present sections would fix the issue.
>>>
>>> Makes sense to me.
>>>
>>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>>>
>>> That pfn inc back and forth is quite ugly TBH but whatever.
>>
>> Indeed, can we please rewrite the loop to fix that?
>
> Any suggestions?
>
> I don't see an obvious way to not break readablity in another place.
>

I'd probably do it like this (applied some other tweaks, untested)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cb766aac6772..a96b1ad1d74b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5859,6 +5859,22 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
return false;
}

+static inline __meminit unsigned long next_present_pfn(unsigned long pfn)
+{
+#ifdef CONFIG_SPARSEMEM
+ unsigned long section_nr = pfn_to_section_nr(pfn + 1);
+
+ /*
+ * Note: We don't check the subsection bitmap, so this can produce
+ * false positives when only subsections are present/valid. The
+ * caller should recheck if the returned pfn is valid.
+ */
+ if (!present_section_nr(section_nr))
+ return section_nr_to_pfn(next_present_section_nr(section_nr));
+#endif
+ return pfn++;
+}
+
/*
* Initially all pages are reserved - free ones are freed
* up by memblock_free_all() once the early boot process is
@@ -5892,18 +5908,22 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
}
#endif

- for (pfn = start_pfn; pfn < end_pfn; pfn++) {
+ pfn = start_pfn;
+ while (pfn < end_pfn) {
/*
* There can be holes in boot-time mem_map[]s handed to this
* function. They do not exist on hotplugged memory.
*/
if (context == MEMMAP_EARLY) {
- if (!early_pfn_valid(pfn))
+ if (!early_pfn_valid(pfn)) {
+ pfn = next_present_pfn(pfn, end_pfn);
continue;
- if (!early_pfn_in_nid(pfn, nid))
- continue;
- if (overlap_memmap_init(zone, &pfn))
+ }
+ if (!early_pfn_in_nid(pfn, nid) ||
+ overlap_memmap_init(zone, &pfn)) {
+ pfn++;
continue;
+ }
if (defer_init(nid, pfn, end_pfn))
break;
}
@@ -5929,6 +5949,7 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
set_pageblock_migratetype(page, MIGRATE_MOVABLE);
cond_resched();
}
+ pfn++;
}

I played with using a "pfn = next_init_pfn()" in the for loop instead, moving all
the checks in there, but didn't turn out too well.

--
Thanks,

David / dhildenb