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Date:   Thu, 26 Nov 2020 10:18:22 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Charan Teja Kalla <charante@...eaurora.org>
Cc:     akpm@...ux-foundation.org, david@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "vinmenon@...eaurora.org" <vinmenon@...eaurora.org>
Subject: Re: [PATCH] mm: memory_hotplug: put migration failure information
 under DEBUG_VM

On Wed 25-11-20 16:18:06, Charan Teja Kalla wrote:
> 
> 
> On 11/24/2020 1:11 PM, Michal Hocko wrote:
> > On Mon 23-11-20 20:40:40, Charan Teja Kalla wrote:
> >>
> >> Thanks Michal!
> >> On 11/23/2020 7:43 PM, Michal Hocko wrote:
> >>> On Mon 23-11-20 19:33:16, Charan Teja Reddy wrote:
> >>>> When the pages are failed to get isolate or migrate, the page owner
> >>>> information along with page info is dumped. If there are continuous
> >>>> failures in migration(say page is pinned) or isolation, the log buffer
> >>>> is simply getting flooded with the page owner information. As most of
> >>>> the times page info is sufficient to know the causes for failures of
> >>>> migration or isolation, place the page owner information under DEBUG_VM.
> >>>
> >>> I do not see why this path is any different from others that call
> >>> dump_page. Page owner can add a very valuable information to debug
> >>> the underlying reasons for failures here. It is an opt-in debugging
> >>> feature which needs to be enabled explicitly. So I would argue users
> >>> are ready to accept a lot of data in the kernel log.
> >>
> >> Just thinking how frequently failures can happen in those paths. In the
> >> memory hotplug path, we can flood the page owner logs just by making one
> >> page pinned.
> > 
> > If you are operating on a movable zone then pages shouldn't be pinned
> > for unbound amount of time. Yeah there are some ways to break this
> > fundamental assumption but this is a bigger problem that needs a
> > solution.
> > 
> >> Say If it is anonymous page, the page owner information
> >> shows is something like below, which is not really telling anything
> >> other than how the pinned page is allocated.
> > 
> > Well you can tell an anonymous page from __dump_page, all right, but
> > this is not true universally.
> > 
> >> page last allocated via order 0, migratetype Movable, gfp_mask
> >> 0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
> >>   prep_new_page+0x7c/0x1a4
> >>   get_page_from_freelist+0x1ac/0x1c4
> >>   __alloc_pages_nodemask+0x12c/0x378
> >>   do_anonymous_page+0xac/0x3b4
> >>   handle_pte_fault+0x2a4/0x3bc
> >>   __handle_speculative_fault+0x208/0x3c0
> >>   do_page_fault+0x280/0x508
> >>   do_translation_fault+0x3c/0x54
> >>   do_mem_abort+0x64/0xf4
> >>   el0_da+0x1c/0x20
> >>  page last free stack trace:
> >>   free_pcp_prepare+0x320/0x454
> >>   free_unref_page_list+0x9c/0x2a4
> >>   release_pages+0x370/0x3c8
> >>   free_pages_and_swap_cache+0xdc/0x10c
> >>   tlb_flush_mmu+0x110/0x134
> >>   tlb_finish_mmu+0x48/0xc0
> >>   unmap_region+0x104/0x138
> >>   __do_munmap+0x2ec/0x3b4
> >>   __arm64_sys_munmap+0x80/0xd8
> >>
> >> I see at some places in the kernel where they put the dump_page under
> >> DEBUG_VM, but in the end I agree that it is up to the users need. Then
> >> there are some users who don't care for these page owner logs.
> > 
> > Well, as I've said page_owner requires an explicit enabling and I would
> > expect that if somebody enables this tracking then it is expected to see
> > the information when we dump a page state.
> > 
> >> And an issue on Embedded systems with these continuous logs being
> >> printed to the console is the watchdog timeouts, because console logging
> >> happens by disabling the interrupts.
> > 
> > Are you enabling page_owner on those systems unconditionally?
> > 
> 
> Yes, We do always enable the page owner on just the internal debug
> builds for memory analysis, But never on the production kernels. And on
> these builds excessive logging, at times because of a pinned page,
> causing the watchdog timeouts, is the problem.

OK, I see but I still believe that the debugging might be useful
especially when the owner is not really obvious from the page state.
I also agree that if the output is swapping the logs then the situation
is not really great either. Would something like the below work for your
situation?

MAGIC_NUMBER would need to be somehow figured but I would start with 10
or so. 

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index b44d4c7ba73b..3da5c434fb77 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1299,6 +1299,8 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 	LIST_HEAD(source);
 
 	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
+		int dumped_page = MAGIC_NUMBER;
+
 		if (!pfn_valid(pfn))
 			continue;
 		page = pfn_to_page(pfn);
@@ -1344,7 +1346,10 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 
 		} else {
 			pr_warn("failed to isolate pfn %lx\n", pfn);
-			dump_page(page, "isolation failed");
+			if (dumped_page--) {
+				dump_page(page, "isolation failed");
+				dumped_page = true;
+			}
 		}
 		put_page(page);
 	}
@@ -1372,10 +1377,14 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
 		ret = migrate_pages(&source, alloc_migration_target, NULL,
 			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
 		if (ret) {
+			int dumped_page = MAGIC_NUMBER;
+
 			list_for_each_entry(page, &source, lru) {
 				pr_warn("migrating pfn %lx failed ret:%d ",
 				       page_to_pfn(page), ret);
-				dump_page(page, "migration failure");
+				if (dumped_page--) {
+					dump_page(page, "migration failure");
+				}
 			}
 			putback_movable_pages(&source);
 		}
-- 
Michal Hocko
SUSE Labs

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