mm/uffd: detect pgtable allocation failures

Before this patch, when there's any pgtable allocation issues happened
during change_protection(), the error will be ignored from the syscall. 
For shmem, there will be an error dumped into the host dmesg.  Two issues
with that:

  (1) Doing a trace dump when allocation fails is not anything close to
      grace.

  (2) The user should be notified with any kind of such error, so the user
      can trap it and decide what to do next, either by retrying, or stop
      the process properly, or anything else.

For userfault users, this will change the API of UFFDIO_WRITEPROTECT when
pgtable allocation failure happened.  It should not normally break anyone,
though.  If it breaks, then in good ways.

One man-page update will be on the way to introduce the new -ENOMEM for
UFFDIO_WRITEPROTECT.  Not marking stable so we keep the old behavior on
the 5.19-till-now kernels.

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20230104225207.1066932-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: James Houghton <jthoughton@google.com>
Acked-by: James Houghton <jthoughton@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 42 insertions, 21 deletions

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 0af22ab59ea8..92fc6f3fa512 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -330,28 +330,34 @@ uffd_wp_protect_file(struct vm_area_struct *vma, unsigned long cp_flags)
 /*
  * If wr-protecting the range for file-backed, populate pgtable for the case
  * when pgtable is empty but page cache exists.  When {pte|pmd|...}_alloc()
- * failed it means no memory, we don't have a better option but stop.
+ * failed we treat it the same way as pgtable allocation failures during
+ * page faults by kicking OOM and returning error.
  */
 #define  change_pmd_prepare(vma, pmd, cp_flags)				\
-	do {								\
+	({								\
+		long err = 0;						\
 		if (unlikely(uffd_wp_protect_file(vma, cp_flags))) {	\
-			if (WARN_ON_ONCE(pte_alloc(vma->vm_mm, pmd)))	\
-				break;					\
+			if (pte_alloc(vma->vm_mm, pmd))			\
+				err = -ENOMEM;				\
 		}							\
-	} while (0)
+		err;							\
+	})
+
 /*
  * This is the general pud/p4d/pgd version of change_pmd_prepare(). We need to
  * have separate change_pmd_prepare() because pte_alloc() returns 0 on success,
  * while {pmd|pud|p4d}_alloc() returns the valid pointer on success.
  */
 #define  change_prepare(vma, high, low, addr, cp_flags)			\
-	do {								\
+	  ({								\
+		long err = 0;						\
 		if (unlikely(uffd_wp_protect_file(vma, cp_flags))) {	\
 			low##_t *p = low##_alloc(vma->vm_mm, high, addr); \
-			if (WARN_ON_ONCE(p == NULL))			\
-				break;					\
+			if (p == NULL)					\
+				err = -ENOMEM;				\
 		}							\
-	} while (0)
+		err;							\
+	})
 
 static inline long change_pmd_range(struct mmu_gather *tlb,
 		struct vm_area_struct *vma, pud_t *pud, unsigned long addr,
@@ -367,11 +373,15 @@ static inline long change_pmd_range(struct mmu_gather *tlb,
 
 	pmd = pmd_offset(pud, addr);
 	do {
-		long this_pages;
+		long ret;
 
 		next = pmd_addr_end(addr, end);
 
-		change_pmd_prepare(vma, pmd, cp_flags);
+		ret = change_pmd_prepare(vma, pmd, cp_flags);
+		if (ret) {
+			pages = ret;
+			break;
+		}
 		/*
 		 * Automatic NUMA balancing walks the tables with mmap_lock
 		 * held for read. It's possible a parallel update to occur
@@ -401,7 +411,11 @@ static inline long change_pmd_range(struct mmu_gather *tlb,
 				 * cleared; make sure pmd populated if
 				 * necessary, then fall-through to pte level.
 				 */
-				change_pmd_prepare(vma, pmd, cp_flags);
+				ret = change_pmd_prepare(vma, pmd, cp_flags);
+				if (ret) {
+					pages = ret;
+					break;
+				}
 			} else {
 				/*
 				 * change_huge_pmd() does not defer TLB flushes,
@@ -422,9 +436,8 @@ static inline long change_pmd_range(struct mmu_gather *tlb,
 			}
 			/* fall through, the trans huge pmd just split */
 		}
-		this_pages = change_pte_range(tlb, vma, pmd, addr, next,
-					      newprot, cp_flags);
-		pages += this_pages;
+		pages += change_pte_range(tlb, vma, pmd, addr, next,
+					  newprot, cp_flags);
 next:
 		cond_resched();
 	} while (pmd++, addr = next, addr != end);
@@ -443,12 +456,14 @@ static inline long change_pud_range(struct mmu_gather *tlb,
 {
 	pud_t *pud;
 	unsigned long next;
-	long pages = 0;
+	long pages = 0, ret;
 
 	pud = pud_offset(p4d, addr);
 	do {
 		next = pud_addr_end(addr, end);
-		change_prepare(vma, pud, pmd, addr, cp_flags);
+		ret = change_prepare(vma, pud, pmd, addr, cp_flags);
+		if (ret)
+			return ret;
 		if (pud_none_or_clear_bad(pud))
 			continue;
 		pages += change_pmd_range(tlb, vma, pud, addr, next, newprot,
@@ -464,12 +479,14 @@ static inline long change_p4d_range(struct mmu_gather *tlb,
 {
 	p4d_t *p4d;
 	unsigned long next;
-	long pages = 0;
+	long pages = 0, ret;
 
 	p4d = p4d_offset(pgd, addr);
 	do {
 		next = p4d_addr_end(addr, end);
-		change_prepare(vma, p4d, pud, addr, cp_flags);
+		ret = change_prepare(vma, p4d, pud, addr, cp_flags);
+		if (ret)
+			return ret;
 		if (p4d_none_or_clear_bad(p4d))
 			continue;
 		pages += change_pud_range(tlb, vma, p4d, addr, next, newprot,
@@ -486,14 +503,18 @@ static long change_protection_range(struct mmu_gather *tlb,
 	struct mm_struct *mm = vma->vm_mm;
 	pgd_t *pgd;
 	unsigned long next;
-	long pages = 0;
+	long pages = 0, ret;
 
 	BUG_ON(addr >= end);
 	pgd = pgd_offset(mm, addr);
 	tlb_start_vma(tlb, vma);
 	do {
 		next = pgd_addr_end(addr, end);
-		change_prepare(vma, pgd, p4d, addr, cp_flags);
+		ret = change_prepare(vma, pgd, p4d, addr, cp_flags);
+		if (ret) {
+			pages = ret;
+			break;
+		}
 		if (pgd_none_or_clear_bad(pgd))
 			continue;
 		pages += change_p4d_range(tlb, vma, pgd, addr, next, newprot,