mm: ptep_get() conversion

Convert all instances of direct pte_t* dereferencing to instead use
ptep_get() helper.  This means that by default, the accesses change from a
C dereference to a READ_ONCE().  This is technically the correct thing to
do since where pgtables are modified by HW (for access/dirty) they are
volatile and therefore we should always ensure READ_ONCE() semantics.

But more importantly, by always using the helper, it can be overridden by
the architecture to fully encapsulate the contents of the pte.  Arch code
is deliberately not converted, as the arch code knows best.  It is
intended that arch code (arm64) will override the default with its own
implementation that can (e.g.) hide certain bits from the core code, or
determine young/dirty status by mixing in state from another source.

Conversion was done using Coccinelle:

----

// $ make coccicheck \
//          COCCI=ptepget.cocci \
//          SPFLAGS="--include-headers" \
//          MODE=patch

virtual patch

@ depends on patch @
pte_t *v;
@@

- *v
+ ptep_get(v)

----

Then reviewed and hand-edited to avoid multiple unnecessary calls to
ptep_get(), instead opting to store the result of a single call in a
variable, where it is correct to do so.  This aims to negate any cost of
READ_ONCE() and will benefit arch-overrides that may be more complex.

Included is a fix for an issue in an earlier version of this patch that
was pointed out by kernel test robot.  The issue arose because config
MMU=n elides definition of the ptep helper functions, including
ptep_get().  HUGETLB_PAGE=n configs still define a simple
huge_ptep_clear_flush() for linking purposes, which dereferences the ptep.
So when both configs are disabled, this caused a build error because
ptep_get() is not defined.  Fix by continuing to do a direct dereference
when MMU=n.  This is safe because for this config the arch code cannot be
trying to virtualize the ptes because none of the ptep helpers are
defined.

Link: https://lkml.kernel.org/r/20230612151545.3317766-4-ryan.roberts@arm.com
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202305120142.yXsNEo6H-lkp@intel.com/
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 12 insertions, 9 deletions

diff --git a/mm/gup.c b/mm/gup.c
index 838db6c0bfc2..38986e522d34 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -477,13 +477,14 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 		pte_t *pte, unsigned int flags)
 {
 	if (flags & FOLL_TOUCH) {
-		pte_t entry = *pte;
+		pte_t orig_entry = ptep_get(pte);
+		pte_t entry = orig_entry;
 
 		if (flags & FOLL_WRITE)
 			entry = pte_mkdirty(entry);
 		entry = pte_mkyoung(entry);
 
-		if (!pte_same(*pte, entry)) {
+		if (!pte_same(orig_entry, entry)) {
 			set_pte_at(vma->vm_mm, address, pte, entry);
 			update_mmu_cache(vma, address, pte);
 		}
@@ -549,7 +550,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
 	if (!ptep)
 		return no_page_table(vma, flags);
-	pte = *ptep;
+	pte = ptep_get(ptep);
 	if (!pte_present(pte))
 		goto no_page;
 	if (pte_protnone(pte) && !gup_can_follow_protnone(flags))
@@ -821,6 +822,7 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 	pud_t *pud;
 	pmd_t *pmd;
 	pte_t *pte;
+	pte_t entry;
 	int ret = -EFAULT;
 
 	/* user gate pages are read-only */
@@ -844,16 +846,17 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 	pte = pte_offset_map(pmd, address);
 	if (!pte)
 		return -EFAULT;
-	if (pte_none(*pte))
+	entry = ptep_get(pte);
+	if (pte_none(entry))
 		goto unmap;
 	*vma = get_gate_vma(mm);
 	if (!page)
 		goto out;
-	*page = vm_normal_page(*vma, address, *pte);
+	*page = vm_normal_page(*vma, address, entry);
 	if (!*page) {
-		if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(*pte)))
+		if ((gup_flags & FOLL_DUMP) || !is_zero_pfn(pte_pfn(entry)))
 			goto unmap;
-		*page = pte_page(*pte);
+		*page = pte_page(entry);
 	}
 	ret = try_grab_page(*page, gup_flags);
 	if (unlikely(ret))
@@ -2496,7 +2499,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
 		}
 
 		if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
-		    unlikely(pte_val(pte) != pte_val(*ptep))) {
+		    unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
 			gup_put_folio(folio, 1, flags);
 			goto pte_unmap;
 		}
@@ -2693,7 +2696,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	if (!folio)
 		return 0;
 
-	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
+	if (unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
 		gup_put_folio(folio, refs, flags);
 		return 0;
 	}