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mm/page_table_check: fix crash on ZONE_DEVICE

Not all pages may apply to pgtable check. One example is ZONE_DEVICE
pages: they map PFNs directly, and they don't allocate page_ext at all
even if there's struct page around. One may reference
devm_memremap_pages().

When both ZONE_DEVICE and page-table-check enabled, then try to map some
dax memories, one can trigger kernel bug constantly now when the kernel
was trying to inject some pfn maps on the dax device:

kernel BUG at mm/page_table_check.c:55!

While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
fault resolutions, skip all the checks if page_ext doesn't even exist in
pgtable checker, which applies to ZONE_DEVICE but maybe more.

Link: https://lkml.kernel.org/r/20240605212146.994486-1-peterx@redhat.com
Fixes: df4e817b7108 ("mm: page table check")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

Peter Xu and committed by
Andrew Morton 8bb592c2 8e5bd4ea

+10 -1
+10 -1
mm/page_table_check.c
··· 73 73 page = pfn_to_page(pfn); 74 74 page_ext = page_ext_get(page); 75 75 76 + if (!page_ext) 77 + return; 78 + 76 79 BUG_ON(PageSlab(page)); 77 80 anon = PageAnon(page); 78 81 ··· 113 110 page = pfn_to_page(pfn); 114 111 page_ext = page_ext_get(page); 115 112 113 + if (!page_ext) 114 + return; 115 + 116 116 BUG_ON(PageSlab(page)); 117 117 anon = PageAnon(page); 118 118 ··· 146 140 BUG_ON(PageSlab(page)); 147 141 148 142 page_ext = page_ext_get(page); 149 - BUG_ON(!page_ext); 143 + 144 + if (!page_ext) 145 + return; 146 + 150 147 for (i = 0; i < (1ul << order); i++) { 151 148 struct page_table_check *ptc = get_page_table_check(page_ext); 152 149