Re: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

[Michal Hocko]

On Wed 11-10-17 13:37:50, Michael Ellerman wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > On Tue 10-10-17 23:05:08, Michael Ellerman wrote:
> >> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> >> 
> >> > From: Michal Hocko <mhocko@xxxxxxxx>
> >> >
> >> > Memory offlining can fail just too eagerly under a heavy memory pressure.
> >> >
> >> > [ 5410.336792] page:ffffea22a646bd00 count:255 mapcount:252 mapping:ffff88ff926c9f38 index:0x3
> >> > [ 5410.336809] flags: 0x9855fe40010048(uptodate|active|mappedtodisk)
> >> > [ 5410.336811] page dumped because: isolation failed
> >> > [ 5410.336813] page->mem_cgroup:ffff8801cd662000
> >> > [ 5420.655030] memory offlining [mem 0x18b580000000-0x18b5ffffffff] failed
> >> >
> >> > Isolation has failed here because the page is not on LRU. Most probably
> >> > because it was on the pcp LRU cache or it has been removed from the LRU
> >> > already but it hasn't been freed yet. In both cases the page doesn't look
> >> > non-migrable so retrying more makes sense.
> >> 
> >> This breaks offline for me.
> >> 
> >> Prior to this commit:
> >>   /sys/devices/system/memory/memory0# time echo 0 > online
> >>   -bash: echo: write error: Device or resource busy
> >>   
> >>   real	0m0.001s
> >>   user	0m0.000s
> >>   sys	0m0.001s
> >> 
> >> After:
> >>   /sys/devices/system/memory/memory0# time echo 0 > online
> >>   -bash: echo: write error: Device or resource busy
> >>   
> >>   real	2m0.009s
> >>   user	0m0.000s
> >>   sys	1m25.035s
> >> 
> >> 
> >> There's no way that block can be removed, it contains the kernel text,
> >> so it should instantly fail - which it used to.
> >
> > OK, that means that start_isolate_page_range should have failed but it
> > hasn't for some reason. I strongly suspect has_unmovable_pages is doing
> > something wrong. Is the kernel text marked somehow? E.g. PageReserved?
> 
> I'm not sure how the text is marked, will have to dig into that.
> 
> > In other words, does the diff below helps?
> 
> No that doesn't help.

This is really strange! As you write in other email the page is
reserved. That means that some of the earlier checks 
	if (zone_idx(zone) == ZONE_MOVABLE)
		return false;
	mt = get_pageblock_migratetype(page);
	if (mt == MIGRATE_MOVABLE || is_migrate_cma(mt))
		return false;
has bailed out early. I would be quite surprised if the kernel text was
sitting in the zone movable. The migrate type check is more fishy
AFAICS. I can imagine that the kernel text can share the movable or CMA
mt block. I am not really familiar with this function but it looks
suspicious. So does it help to remove this check?
--- 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3badcedf96a7..5b4d85ae445c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7355,9 +7355,6 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 	 */
 	if (zone_idx(zone) == ZONE_MOVABLE)
 		return false;
-	mt = get_pageblock_migratetype(page);
-	if (mt == MIGRATE_MOVABLE || is_migrate_cma(mt))
-		return false;
 
 	pfn = page_to_pfn(page);
 	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
@@ -7368,6 +7365,9 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
 
 		page = pfn_to_page(check);
 
+		if (PageReserved(page))
+			return true;
+
 		/*
 		 * Hugepages are not in LRU lists, but they're movable.
 		 * We need not scan over tail pages bacause we don't

-- 
Michal Hocko
SUSE Labs