Re: [linux-next:master] [mm] bbcbf2a3f0: kernel_BUG_at_mm/memory.c

[David Hildenbrand]

On 21.12.23 22:58, Andrew Morton wrote:
> On Thu, 21 Dec 2023 12:32:04 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 21.12.23 12:23, Oliver Sang wrote:
> > > hi, Andrew Morton,
> > >
> > > On Wed, Dec 20, 2023 at 02:11:35PM -0800, Andrew Morton wrote:
> > > > On Tue, 19 Dec 2023 23:46:50 +0800 kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> > > >
> > > > > Hello,
> > > > >
> > > > > kernel test robot noticed "kernel_BUG_at_mm/memory.c" on:
> > > > >
> > > > > commit: bbcbf2a3f05f74f9d268eab57abbdce6a65a94ad ("mm: convert ksm_might_need_to_copy() to work on folios")
> > > >
> > > > I assume this is a bisection result, so it's quite repeatable?
> > >
> > > yes, we bisect to this commit, it's quite repeatable:
> > >
> > > ddd06bb63d9793ce bbcbf2a3f05f74f9d268eab57ab
> > > ---------------- ---------------------------
> > >          fail:runs  %reproduction    fail:runs
> > >              |             |             |
> > >              :6          100%           6:6     dmesg.Kernel_panic-not_syncing:Fatal_exception
> > >              :6          100%           6:6     dmesg.RIP:do_swap_page
> > >              :6          100%           6:6     dmesg.invalid_opcode:#[##]
> > >              :6          100%           6:6     dmesg.kernel_BUG_at_mm/memory.c
> >
> > Can you try with the snipped I sent? Please let me know if you need a
> > full patch for testing purposes.
>
> I think a full patch would be better, please.

From b82e309096abde6c0f24bba50a281e8d3855c132 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@xxxxxxxxxx>
Date: Thu, 21 Dec 2023 23:04:30 +0100
Subject: [PATCH] Fixup: mm: convert ksm_might_need_to_copy() to work on folios

We must only adjust the page if the folio changed. Otherwise, if we
had a large folio in the swapcache and the folio didn't change, we'd
suddenly change the page to-be-mapped.

Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202312192319.fa8f5709-oliver.sang@xxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 149f779910fd5..2f9668d357f5c 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3952,7 +3952,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			folio = swapcache;
 			goto out_page;
 		}
-		page = folio_page(folio, 0);
+		if (folio != swapcache)
+			page = folio_page(folio, 0);
 
 		/*
 		 * If we want to map a page that's in the swapcache writable, we
--
2.43.0


--
Cheers,

David / dhildenb