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From:   David Hildenbrand <david@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        kvm@vger.kernel.org, linux-kselftest@vger.kernel.org,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        liubo <liubo254@huawei.com>, Peter Xu <peterx@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        John Hubbard <jhubbard@nvidia.com>,
        Mel Gorman <mgorman@suse.de>, Shuah Khan <shuah@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: [PATCH v3 5/7] pgtable: improve pte_protnone() comment
Date:   Thu,  3 Aug 2023 16:32:06 +0200
Message-ID: <20230803143208.383663-6-david@redhat.com>
In-Reply-To: <20230803143208.383663-1-david@redhat.com>
References: <20230803143208.383663-1-david@redhat.com>
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Especially the "For PROT_NONE VMAs, the PTEs are not marked
_PAGE_PROTNONE" part is wrong: doing an mprotect(PROT_NONE) will end up
marking all PTEs on x86_64 as _PAGE_PROTNONE, making pte_protnone()
indicate "yes".

So let's improve the comment, so it's easier to grasp which semantics
pte_protnone() actually has.

Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/pgtable.h | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 6005b5dff0c1..222a33b9600d 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1446,12 +1446,16 @@ static inline int pud_trans_unstable(pud_t *pud)
 
 #ifndef CONFIG_NUMA_BALANCING
 /*
- * Technically a PTE can be PROTNONE even when not doing NUMA balancing but
- * the only case the kernel cares is for NUMA balancing and is only ever set
- * when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked
- * _PAGE_PROTNONE so by default, implement the helper as "always no". It
- * is the responsibility of the caller to distinguish between PROT_NONE
- * protections and NUMA hinting fault protections.
+ * In an inaccessible (PROT_NONE) VMA, pte_protnone() may indicate "yes". It is
+ * perfectly valid to indicate "no" in that case, which is why our default
+ * implementation defaults to "always no".
+ *
+ * In an accessible VMA, however, pte_protnone() reliably indicates PROT_NONE
+ * page protection due to NUMA hinting. NUMA hinting faults only apply in
+ * accessible VMAs.
+ *
+ * So, to reliably identify PROT_NONE PTEs that require a NUMA hinting fault,
+ * looking at the VMA accessibility is sufficient.
  */
 static inline int pte_protnone(pte_t pte)
 {
-- 
2.41.0