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

[David Hildenbrand <david@redhat.com>]

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