mm/khugepaged: fix invalid page access in release_pte_pages()

release_pte_pages() converts from a pfn to a folio by using pfn_folio(). 
If the pte is not mapped, pfn_folio() will result in undefined behavior
which ends up causing a kernel panic[1].

Only call pfn_folio() once we have validated that the pte is both valid
and mapped to fix the issue.

[1] https://lore.kernel.org/linux-mm/ff300770-afe9-908d-23ed-d23e0796e899@samsung.com/

Link: https://lkml.kernel.org/r/20230213214324.34215-1-vishal.moola@gmail.comSigned-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Fixes: 9bdfeea4 ("mm/khugepaged: convert release_pte_pages() to use folios")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Debugged-by: Alexandre Ghiti <alex@ghiti.fr>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/khugepaged.c

@@ -511,11 +511,17 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
 
 	while (--_pte >= pte) {
 		pte_t pteval = *_pte;
+		unsigned long pfn;
 
-		folio = pfn_folio(pte_pfn(pteval));
-		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)) &&
-				!folio_test_large(folio))
-			release_pte_folio(folio);
+		if (pte_none(pteval))
+			continue;
+		pfn = pte_pfn(pteval);
+		if (is_zero_pfn(pfn))
+			continue;
+		folio = pfn_folio(pfn);
+		if (folio_test_large(folio))
+			continue;
+		release_pte_folio(folio);
 	}
 
 	list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {