From: Vlastimil Babka <vbabka@suse.cz>
To: Andrea Arcangeli <aarcange@redhat.com>,
Mel Gorman <mgorman@suse.de>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Qian Cai <cai@lca.pw>
Cc: Michal Hocko <mhocko@kernel.org>,
David Hildenbrand <david@redhat.com>,
linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
Baoquan He <bhe@redhat.com>
Subject: Re: [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages
Date: Mon, 23 Nov 2020 14:01:16 +0100 [thread overview]
Message-ID: <ea911b11-945f-d2c5-5558-a3fe0bda492a@suse.cz> (raw)
In-Reply-To: <20201121194506.13464-2-aarcange@redhat.com>
On 11/21/20 8:45 PM, Andrea Arcangeli wrote:
> A corollary issue was fixed in
> e577c8b64d58fe307ea4d5149d31615df2d90861. A second issue remained in
> v5.7:
>
> https://lkml.kernel.org/r/8C537EB7-85EE-4DCF-943E-3CC0ED0DF56D@lca.pw
>
> ==
> page:ffffea0000aa0000 refcount:1 mapcount:0 mapping:000000002243743b index:0x0
> flags: 0x1fffe000001000(reserved)
> ==
>
> 73a6e474cb376921a311786652782155eac2fdf0 was applied to supposedly the
> second issue, but I still reproduced it twice with v5.9 on two
> different systems:
>
> ==
> page:0000000062b3e92f refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x39800
> flags: 0x1000(reserved)
> ==
> page:000000002a7114f8 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7a200
> flags: 0x1fff000000001000(reserved)
> ==
>
> I actually never reproduced it until v5.9, but it's still the same bug
> as it was reported first for v5.7.
>
> See the page is "reserved" in all 3 cases. In the last two crashes
> with the pfn:
>
> pfn 0x39800 -> 0x39800000 min_pfn hit non-RAM:
>
> 39639000-39814fff : Unknown E820 type
>
> pfn 0x7a200 -> 0x7a200000 min_pfn hit non-RAM:
>
> 7a17b000-7a216fff : Unknown E820 type
It would be nice to also provide a /proc/zoneinfo and how exactly the
"zone_spans_pfn" was violated. I assume we end up below zone's
start_pfn, but is it true?
> This actually seems a false positive bugcheck, the page structures are
> valid and the zones are correct, just it's non-RAM but setting
> pageblockskip should do no harm. However it's possible to solve the
> crash without lifting the bugcheck, by enforcing the invariant that
> the free_pfn cursor doesn't point to reserved pages (which would be
> otherwise implicitly achieved through the PageBuddy check, except in
> the new fast_isolate_around() path).
>
> Fixes: 5a811889de10 ("mm, compaction: use free lists to quickly locate a migration target")
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> mm/compaction.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 13cb7a961b31..d17e69549d34 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1433,7 +1433,10 @@ fast_isolate_freepages(struct compact_control *cc)
> page = pageblock_pfn_to_page(min_pfn,
> pageblock_end_pfn(min_pfn),
> cc->zone);
> - cc->free_pfn = min_pfn;
> + if (likely(!PageReserved(page)))
PageReserved check seems rather awkward solution to me. Wouldn't it be
more obvious if we made sure we don't end up below zone's start_pfn (if
my assumption is correct) in the first place?
When I check the code:
unsigned long distance;
distance = (cc->free_pfn - cc->migrate_pfn);
low_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 2));
min_pfn = pageblock_start_pfn(cc->free_pfn - (distance >> 1));
I think what can happen is that cc->free_pfn <= cc->migrate_pfn after
the very last isolate_migratepages(). Then compact_finished() detects
that in compact_zone(), but only after migrate_pages() and thus
fast_isolate_freepages() is called.
That would mean distance can be negative, or rather a large unsigned
number and low_pfn and min_pfn end up away from the zone?
Or maybe the above doesn't happen, but cc->free_pfn gets so close to
start of the zone, that the calculations above make min_pfn go below
start_pfn?
In any case I would rather make sure we stay within the expected zone
boundaries, than play tricks with PageReserved. Mel?
> + cc->free_pfn = min_pfn;
> + else
> + page = NULL;
> }
> }
> }
>
next prev parent reply other threads:[~2020-11-23 13:01 UTC|newest]
Thread overview: 61+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-23 21:25 compaction: VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn)) Qian Cai
2020-04-24 3:43 ` Baoquan He
2020-04-24 13:45 ` Qian Cai
2020-05-05 12:43 ` Baoquan He
2020-05-05 13:20 ` Qian Cai
2020-05-11 1:21 ` Baoquan He
2020-04-26 14:41 ` Mike Rapoport
2020-04-27 13:45 ` Qian Cai
2020-11-21 19:45 ` [PATCH 0/1] VM_BUG_ON_PAGE(!zone_spans_pfn) in set_pfnblock_flags_mask Andrea Arcangeli
2020-11-21 19:45 ` [PATCH 1/1] mm: compaction: avoid fast_isolate_around() to set pageblock_skip on reserved pages Andrea Arcangeli
2020-11-21 19:53 ` Andrea Arcangeli
2020-11-23 11:26 ` David Hildenbrand
2020-11-23 13:01 ` Vlastimil Babka [this message]
2020-11-24 13:32 ` Mel Gorman
2020-11-24 20:56 ` Andrea Arcangeli
2020-11-25 10:30 ` Mel Gorman
2020-11-25 17:59 ` Andrea Arcangeli
2020-11-26 10:47 ` Mel Gorman
2020-12-06 2:26 ` Andrea Arcangeli
2020-12-06 23:47 ` Mel Gorman
2020-11-25 5:34 ` Andrea Arcangeli
2020-11-25 6:45 ` David Hildenbrand
2020-11-25 8:51 ` Mike Rapoport
2020-11-25 10:39 ` Mel Gorman
2020-11-25 11:04 ` David Hildenbrand
2020-11-25 11:41 ` David Hildenbrand
2020-11-25 18:47 ` Andrea Arcangeli
2020-11-25 13:33 ` Mel Gorman
2020-11-25 13:41 ` David Hildenbrand
2020-11-25 18:28 ` Andrea Arcangeli
2020-11-25 19:27 ` David Hildenbrand
2020-11-25 20:41 ` Andrea Arcangeli
2020-11-25 21:13 ` David Hildenbrand
2020-11-25 21:04 ` Mike Rapoport
2020-11-25 21:38 ` Andrea Arcangeli
2020-11-26 9:36 ` Mike Rapoport
2020-11-26 10:05 ` David Hildenbrand
2020-11-26 17:46 ` Mike Rapoport
2020-11-29 12:32 ` Mike Rapoport
2020-12-02 0:44 ` Andrea Arcangeli
2020-12-02 17:39 ` Mike Rapoport
2020-12-03 6:23 ` Andrea Arcangeli
2020-12-03 10:51 ` Mike Rapoport
2020-12-03 17:31 ` Andrea Arcangeli
2020-12-06 8:09 ` Mike Rapoport
2020-11-26 18:15 ` Andrea Arcangeli
2020-11-26 18:29 ` Andrea Arcangeli
2020-11-26 19:44 ` Mike Rapoport
2020-11-26 20:30 ` Andrea Arcangeli
2020-11-26 21:03 ` Mike Rapoport
2020-11-26 19:21 ` Andrea Arcangeli
2020-11-25 12:08 ` Vlastimil Babka
2020-11-25 13:32 ` David Hildenbrand
2020-11-25 14:13 ` Mike Rapoport
2020-11-25 14:42 ` David Hildenbrand
2020-11-26 10:51 ` Mel Gorman
2020-11-25 19:14 ` Andrea Arcangeli
2020-11-25 19:01 ` Andrea Arcangeli
2020-11-25 19:33 ` David Hildenbrand
2020-11-26 3:40 ` Andrea Arcangeli
2020-11-26 10:43 ` Mike Rapoport
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