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From:   Peter Xu <peterx@redhat.com>
To:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc:     peterx@redhat.com, Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
        Christoph Hellwig <hch@lst.de>
Subject: [PATCH] mm/gup: Fix follow_devmap_p[mu]d() on page==NULL handling
Date:   Thu, 23 Nov 2023 13:02:22 -0500
Message-ID: <20231123180222.1048297-1-peterx@redhat.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

This is a bug found not by any report but only by code observations.

When GUP sees a devpmd/devpud and if page==NULL is returned, it means a
fault is probably required.  Here falling through when page==NULL can cause
unexpected behavior.

Fix both cases by catching the page==NULL cases with no_page_table().

Fixes: 3565fce3a659 ("mm, x86: get_user_pages() for dax mappings")
Fixes: 080dbb618b4b ("mm/follow_page_mask: split follow_page_mask to smaller functions.")
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 231711efa390..0a5f0e91bfec 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -710,6 +710,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
 		spin_unlock(ptl);
 		if (page)
 			return page;
+		return no_page_table(vma, flags);
 	}
 	if (likely(!pmd_trans_huge(pmdval)))
 		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
@@ -758,6 +759,7 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
 		spin_unlock(ptl);
 		if (page)
 			return page;
+		return no_page_table(vma, flags);
 	}
 	if (unlikely(pud_bad(*pud)))
 		return no_page_table(vma, flags);
-- 
2.41.0