linux-kernel - [PATCH 1/2] mm: avoid spurious 'bad pmd' warning messages

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Date:   Wed, 17 May 2017 11:16:38 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Pawel Lebioda <pawel.lebioda@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Xiong Zhou <xzhou@...hat.com>, Eryu Guan <eguan@...hat.com>,
        stable@...r.kernel.org
Subject: [PATCH 1/2] mm: avoid spurious 'bad pmd' warning messages

When the pmd_devmap() checks were added by:

commit 5c7fb56e5e3f ("mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd")

to add better support for DAX huge pages, they were all added to the end of
if() statements after existing pmd_trans_huge() checks.  So, things like:

-       if (pmd_trans_huge(*pmd))
+       if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd))

When further checks were added after pmd_trans_unstable() checks by:

commit 7267ec008b5c ("mm: postpone page table allocation until we have page
to map")

they were also added at the end of the conditional:

+       if (pmd_trans_unstable(fe->pmd) || pmd_devmap(*fe->pmd))

This ordering is fine for pmd_trans_huge(), but doesn't work for
pmd_trans_unstable().  This is because DAX huge pages trip the bad_pmd()
check inside of pmd_none_or_trans_huge_or_clear_bad() (called by
pmd_trans_unstable()), which prints out a warning and returns 1.  So, we do
end up doing the right thing, but only after spamming dmesg with suspicious
looking messages:

mm/pgtable-generic.c:39: bad pmd ffff8808daa49b88(84000001006000a5)

Reorder these checks so that pmd_devmap() is checked first, avoiding the
error messages.

Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
Fixes: commit 7267ec008b5c ("mm: postpone page table allocation until we have page to map")
Cc: stable@...r.kernel.org
---
 mm/memory.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 6ff5d72..1ee269d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3061,7 +3061,7 @@ static int pte_alloc_one_map(struct vm_fault *vmf)
 	 * through an atomic read in C, which is what pmd_trans_unstable()
 	 * provides.
 	 */
-	if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
+	if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd))
 		return VM_FAULT_NOPAGE;
 
 	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
@@ -3690,7 +3690,7 @@ static int handle_pte_fault(struct vm_fault *vmf)
 		vmf->pte = NULL;
 	} else {
 		/* See comment in pte_alloc_one_map() */
-		if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
+		if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd))
 			return 0;
 		/*
 		 * A regular pmd is established and it can't morph into a huge
-- 
2.9.4