The patch below does not apply to the 6.1-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y git checkout FETCH_HEAD git cherry-pick -x 49b0638502da097c15d46cd4e871dbaa022caf7c # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2023082625-monotone-traps-6498@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

49b0638502da ("mm: enable page walking API to lock vmas during the walk") 6c21e066f925 ("mm/mempolicy: Take VMA lock before replacing policy") 24139c07f413 ("mm/ksm: unmerge and clear VM_MERGEABLE when setting PR_SET_MEMORY_MERGE=0") d7597f59d1d3 ("mm: add new api to enable ksm per process") ddc65971bb67 ("prctl: add PR_GET_AUXV to copy auxv to userspace") b507808ebce2 ("mm: implement memory-deny-write-execute as a prctl") d7c0e68dab98 ("mm/ksm: convert break_ksm() to use walk_page_range_vma()") e07cda5f232f ("mm/pagewalk: add walk_page_range_vma()") 58f595c66591 ("mm/ksm: simplify break_ksm() to not rely on VM_FAULT_WRITE") c31783eeae7b ("mm/pagewalk: don't trigger test_walk() in walk_page_vma()")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 49b0638502da097c15d46cd4e871dbaa022caf7c Mon Sep 17 00:00:00 2001 From: Suren Baghdasaryan surenb@google.com Date: Fri, 4 Aug 2023 08:27:19 -0700 Subject: [PATCH] mm: enable page walking API to lock vmas during the walk

walk_page_range() and friends often operate under write-locked mmap_lock. With introduction of vma locks, the vmas have to be locked as well during such walks to prevent concurrent page faults in these areas. Add an additional member to mm_walk_ops to indicate locking requirements for the walk.

The change ensures that page walks which prevent concurrent page faults by write-locking mmap_lock, operate correctly after introduction of per-vma locks. With per-vma locks page faults can be handled under vma lock without taking mmap_lock at all, so write locking mmap_lock would not stop them. The change ensures vmas are properly locked during such walks.

A sample issue this solves is do_mbind() performing queue_pages_range() to queue pages for migration. Without this change a concurrent page can be faulted into the area and be left out of migration.

Link: https://lkml.kernel.org/r/20230804152724.3090321-2-surenb@google.com Signed-off-by: Suren Baghdasaryan surenb@google.com Suggested-by: Linus Torvalds torvalds@linuxfoundation.org Suggested-by: Jann Horn jannh@google.com Cc: David Hildenbrand david@redhat.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: Hugh Dickins hughd@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Laurent Dufour ldufour@linux.ibm.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Michal Hocko mhocko@suse.com Cc: Michel Lespinasse michel@lespinasse.org Cc: Peter Xu peterx@redhat.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org

diff --git a/arch/powerpc/mm/book3s64/subpage_prot.c b/arch/powerpc/mm/book3s64/subpage_prot.c index 0dc85556dec5..ec98e526167e 100644 --- a/arch/powerpc/mm/book3s64/subpage_prot.c +++ b/arch/powerpc/mm/book3s64/subpage_prot.c @@ -145,6 +145,7 @@ static int subpage_walk_pmd_entry(pmd_t *pmd, unsigned long addr,

static const struct mm_walk_ops subpage_walk_ops = { .pmd_entry = subpage_walk_pmd_entry, + .walk_lock = PGWALK_WRLOCK_VERIFY, };

static void subpage_mark_vma_nohuge(struct mm_struct *mm, unsigned long addr, diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c index ea3d61de065b..161d0b34c2cb 100644 --- a/arch/riscv/mm/pageattr.c +++ b/arch/riscv/mm/pageattr.c @@ -102,6 +102,7 @@ static const struct mm_walk_ops pageattr_ops = { .pmd_entry = pageattr_pmd_entry, .pte_entry = pageattr_pte_entry, .pte_hole = pageattr_pte_hole, + .walk_lock = PGWALK_RDLOCK, };

static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask, diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c index 9c8af31be970..906a7bfc2a78 100644 --- a/arch/s390/mm/gmap.c +++ b/arch/s390/mm/gmap.c @@ -2514,6 +2514,7 @@ static int thp_split_walk_pmd_entry(pmd_t *pmd, unsigned long addr,

static const struct mm_walk_ops thp_split_walk_ops = { .pmd_entry = thp_split_walk_pmd_entry, + .walk_lock = PGWALK_WRLOCK_VERIFY, };

static inline void thp_split_mm(struct mm_struct *mm) @@ -2565,6 +2566,7 @@ static int __zap_zero_pages(pmd_t *pmd, unsigned long start,

static const struct mm_walk_ops zap_zero_walk_ops = { .pmd_entry = __zap_zero_pages, + .walk_lock = PGWALK_WRLOCK, };

/* @@ -2655,6 +2657,7 @@ static const struct mm_walk_ops enable_skey_walk_ops = { .hugetlb_entry = __s390_enable_skey_hugetlb, .pte_entry = __s390_enable_skey_pte, .pmd_entry = __s390_enable_skey_pmd, + .walk_lock = PGWALK_WRLOCK, };

int s390_enable_skey(void) @@ -2692,6 +2695,7 @@ static int __s390_reset_cmma(pte_t *pte, unsigned long addr,

static const struct mm_walk_ops reset_cmma_walk_ops = { .pte_entry = __s390_reset_cmma, + .walk_lock = PGWALK_WRLOCK, };

void s390_reset_cmma(struct mm_struct *mm) @@ -2728,6 +2732,7 @@ static int s390_gather_pages(pte_t *ptep, unsigned long addr,

static const struct mm_walk_ops gather_pages_ops = { .pte_entry = s390_gather_pages, + .walk_lock = PGWALK_RDLOCK, };

/* diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index fc744964816e..fafff1bd34cd 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -757,12 +757,14 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask, static const struct mm_walk_ops smaps_walk_ops = { .pmd_entry = smaps_pte_range, .hugetlb_entry = smaps_hugetlb_range, + .walk_lock = PGWALK_RDLOCK, };

static const struct mm_walk_ops smaps_shmem_walk_ops = { .pmd_entry = smaps_pte_range, .hugetlb_entry = smaps_hugetlb_range, .pte_hole = smaps_pte_hole, + .walk_lock = PGWALK_RDLOCK, };

/* @@ -1244,6 +1246,7 @@ static int clear_refs_test_walk(unsigned long start, unsigned long end, static const struct mm_walk_ops clear_refs_walk_ops = { .pmd_entry = clear_refs_pte_range, .test_walk = clear_refs_test_walk, + .walk_lock = PGWALK_WRLOCK, };

static ssize_t clear_refs_write(struct file *file, const char __user *buf, @@ -1621,6 +1624,7 @@ static const struct mm_walk_ops pagemap_ops = { .pmd_entry = pagemap_pmd_range, .pte_hole = pagemap_pte_hole, .hugetlb_entry = pagemap_hugetlb_range, + .walk_lock = PGWALK_RDLOCK, };

/* @@ -1934,6 +1938,7 @@ static int gather_hugetlb_stats(pte_t *pte, unsigned long hmask, static const struct mm_walk_ops show_numa_ops = { .hugetlb_entry = gather_hugetlb_stats, .pmd_entry = gather_pte_stats, + .walk_lock = PGWALK_RDLOCK, };

/* diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h index 27a6df448ee5..27cd1e59ccf7 100644 --- a/include/linux/pagewalk.h +++ b/include/linux/pagewalk.h @@ -6,6 +6,16 @@

struct mm_walk;

+/* Locking requirement during a page walk. */ +enum page_walk_lock { + /* mmap_lock should be locked for read to stabilize the vma tree */ + PGWALK_RDLOCK = 0, + /* vma will be write-locked during the walk */ + PGWALK_WRLOCK = 1, + /* vma is expected to be already write-locked during the walk */ + PGWALK_WRLOCK_VERIFY = 2, +}; + /** * struct mm_walk_ops - callbacks for walk_page_range * @pgd_entry: if set, called for each non-empty PGD (top-level) entry @@ -66,6 +76,7 @@ struct mm_walk_ops { int (*pre_vma)(unsigned long start, unsigned long end, struct mm_walk *walk); void (*post_vma)(struct mm_walk *walk); + enum page_walk_lock walk_lock; };

/* diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c index 2fcc9731528a..e0e59d420fca 100644 --- a/mm/damon/vaddr.c +++ b/mm/damon/vaddr.c @@ -386,6 +386,7 @@ static int damon_mkold_hugetlb_entry(pte_t *pte, unsigned long hmask, static const struct mm_walk_ops damon_mkold_ops = { .pmd_entry = damon_mkold_pmd_entry, .hugetlb_entry = damon_mkold_hugetlb_entry, + .walk_lock = PGWALK_RDLOCK, };

static void damon_va_mkold(struct mm_struct *mm, unsigned long addr) @@ -525,6 +526,7 @@ static int damon_young_hugetlb_entry(pte_t *pte, unsigned long hmask, static const struct mm_walk_ops damon_young_ops = { .pmd_entry = damon_young_pmd_entry, .hugetlb_entry = damon_young_hugetlb_entry, + .walk_lock = PGWALK_RDLOCK, };

static bool damon_va_young(struct mm_struct *mm, unsigned long addr, diff --git a/mm/hmm.c b/mm/hmm.c index 855e25e59d8f..277ddcab4947 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -562,6 +562,7 @@ static const struct mm_walk_ops hmm_walk_ops = { .pte_hole = hmm_vma_walk_hole, .hugetlb_entry = hmm_vma_walk_hugetlb_entry, .test_walk = hmm_vma_walk_test, + .walk_lock = PGWALK_RDLOCK, };

/** diff --git a/mm/ksm.c b/mm/ksm.c index d20d7662419b..d7b5b95e936e 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -455,6 +455,12 @@ static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long nex

static const struct mm_walk_ops break_ksm_ops = { .pmd_entry = break_ksm_pmd_entry, + .walk_lock = PGWALK_RDLOCK, +}; + +static const struct mm_walk_ops break_ksm_lock_vma_ops = { + .pmd_entry = break_ksm_pmd_entry, + .walk_lock = PGWALK_WRLOCK, };

/* @@ -470,16 +476,17 @@ static const struct mm_walk_ops break_ksm_ops = { * of the process that owns 'vma'. We also do not want to enforce * protection keys here anyway. */ -static int break_ksm(struct vm_area_struct *vma, unsigned long addr) +static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool lock_vma) { vm_fault_t ret = 0; + const struct mm_walk_ops *ops = lock_vma ? + &break_ksm_lock_vma_ops : &break_ksm_ops;

do { int ksm_page;

cond_resched(); - ksm_page = walk_page_range_vma(vma, addr, addr + 1, - &break_ksm_ops, NULL); + ksm_page = walk_page_range_vma(vma, addr, addr + 1, ops, NULL); if (WARN_ON_ONCE(ksm_page < 0)) return ksm_page; if (!ksm_page) @@ -565,7 +572,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item) mmap_read_lock(mm); vma = find_mergeable_vma(mm, addr); if (vma) - break_ksm(vma, addr); + break_ksm(vma, addr, false); mmap_read_unlock(mm); }

@@ -871,7 +878,7 @@ static void remove_trailing_rmap_items(struct ksm_rmap_item **rmap_list) * in cmp_and_merge_page on one of the rmap_items we would be removing. */ static int unmerge_ksm_pages(struct vm_area_struct *vma, - unsigned long start, unsigned long end) + unsigned long start, unsigned long end, bool lock_vma) { unsigned long addr; int err = 0; @@ -882,7 +889,7 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma, if (signal_pending(current)) err = -ERESTARTSYS; else - err = break_ksm(vma, addr); + err = break_ksm(vma, addr, lock_vma); } return err; } @@ -1029,7 +1036,7 @@ static int unmerge_and_remove_all_rmap_items(void) if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma) continue; err = unmerge_ksm_pages(vma, - vma->vm_start, vma->vm_end); + vma->vm_start, vma->vm_end, false); if (err) goto error; } @@ -2530,7 +2537,7 @@ static int __ksm_del_vma(struct vm_area_struct *vma) return 0;

if (vma->anon_vma) { - err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end); + err = unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end, true); if (err) return err; } @@ -2668,7 +2675,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start, return 0; /* just ignore the advice */

if (vma->anon_vma) { - err = unmerge_ksm_pages(vma, start, end); + err = unmerge_ksm_pages(vma, start, end, true); if (err) return err; } diff --git a/mm/madvise.c b/mm/madvise.c index 886f06066622..bfe0e06427bd 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -233,6 +233,7 @@ static int swapin_walk_pmd_entry(pmd_t *pmd, unsigned long start,

static const struct mm_walk_ops swapin_walk_ops = { .pmd_entry = swapin_walk_pmd_entry, + .walk_lock = PGWALK_RDLOCK, };

static void shmem_swapin_range(struct vm_area_struct *vma, @@ -534,6 +535,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,

static const struct mm_walk_ops cold_walk_ops = { .pmd_entry = madvise_cold_or_pageout_pte_range, + .walk_lock = PGWALK_RDLOCK, };

static void madvise_cold_page_range(struct mmu_gather *tlb, @@ -757,6 +759,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,

static const struct mm_walk_ops madvise_free_walk_ops = { .pmd_entry = madvise_free_pte_range, + .walk_lock = PGWALK_RDLOCK, };

static int madvise_free_single_vma(struct vm_area_struct *vma, diff --git a/mm/memcontrol.c b/mm/memcontrol.c index e8ca4bdcb03c..315fd5f45e3c 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6024,6 +6024,7 @@ static int mem_cgroup_count_precharge_pte_range(pmd_t *pmd,

static const struct mm_walk_ops precharge_walk_ops = { .pmd_entry = mem_cgroup_count_precharge_pte_range, + .walk_lock = PGWALK_RDLOCK, };

static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm) @@ -6303,6 +6304,7 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,

static const struct mm_walk_ops charge_walk_ops = { .pmd_entry = mem_cgroup_move_charge_pte_range, + .walk_lock = PGWALK_RDLOCK, };

static void mem_cgroup_move_charge(void) diff --git a/mm/memory-failure.c b/mm/memory-failure.c index 9a285038d765..139b31fdb678 100644 --- a/mm/memory-failure.c +++ b/mm/memory-failure.c @@ -831,6 +831,7 @@ static int hwpoison_hugetlb_range(pte_t *ptep, unsigned long hmask, static const struct mm_walk_ops hwp_walk_ops = { .pmd_entry = hwpoison_pte_range, .hugetlb_entry = hwpoison_hugetlb_range, + .walk_lock = PGWALK_RDLOCK, };

/* diff --git a/mm/mempolicy.c b/mm/mempolicy.c index c53f8beeb507..ec2eaceffd74 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -718,6 +718,14 @@ static const struct mm_walk_ops queue_pages_walk_ops = { .hugetlb_entry = queue_folios_hugetlb, .pmd_entry = queue_folios_pte_range, .test_walk = queue_pages_test_walk, + .walk_lock = PGWALK_RDLOCK, +}; + +static const struct mm_walk_ops queue_pages_lock_vma_walk_ops = { + .hugetlb_entry = queue_folios_hugetlb, + .pmd_entry = queue_folios_pte_range, + .test_walk = queue_pages_test_walk, + .walk_lock = PGWALK_WRLOCK, };

/* @@ -738,7 +746,7 @@ static const struct mm_walk_ops queue_pages_walk_ops = { static int queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end, nodemask_t *nodes, unsigned long flags, - struct list_head *pagelist) + struct list_head *pagelist, bool lock_vma) { int err; struct queue_pages qp = { @@ -749,8 +757,10 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end, .end = end, .first = NULL, }; + const struct mm_walk_ops *ops = lock_vma ? + &queue_pages_lock_vma_walk_ops : &queue_pages_walk_ops;

- err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp); + err = walk_page_range(mm, start, end, ops, &qp);

if (!qp.first) /* whole range in hole */ @@ -1078,7 +1088,7 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest, vma = find_vma(mm, 0); VM_BUG_ON(!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))); queue_pages_range(mm, vma->vm_start, mm->task_size, &nmask, - flags | MPOL_MF_DISCONTIG_OK, &pagelist); + flags | MPOL_MF_DISCONTIG_OK, &pagelist, false);

if (!list_empty(&pagelist)) { err = migrate_pages(&pagelist, alloc_migration_target, NULL, @@ -1321,12 +1331,8 @@ static long do_mbind(unsigned long start, unsigned long len, * Lock the VMAs before scanning for pages to migrate, to ensure we don't * miss a concurrently inserted page. */ - vma_iter_init(&vmi, mm, start); - for_each_vma_range(vmi, vma, end) - vma_start_write(vma); - ret = queue_pages_range(mm, start, end, nmask, - flags | MPOL_MF_INVERT, &pagelist); + flags | MPOL_MF_INVERT, &pagelist, true);

if (ret < 0) { err = ret; diff --git a/mm/migrate_device.c b/mm/migrate_device.c index 8365158460ed..d5f492356e3e 100644 --- a/mm/migrate_device.c +++ b/mm/migrate_device.c @@ -279,6 +279,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp, static const struct mm_walk_ops migrate_vma_walk_ops = { .pmd_entry = migrate_vma_collect_pmd, .pte_hole = migrate_vma_collect_hole, + .walk_lock = PGWALK_RDLOCK, };

/* diff --git a/mm/mincore.c b/mm/mincore.c index b7f7a516b26c..dad3622cc963 100644 --- a/mm/mincore.c +++ b/mm/mincore.c @@ -176,6 +176,7 @@ static const struct mm_walk_ops mincore_walk_ops = { .pmd_entry = mincore_pte_range, .pte_hole = mincore_unmapped_range, .hugetlb_entry = mincore_hugetlb, + .walk_lock = PGWALK_RDLOCK, };

/* diff --git a/mm/mlock.c b/mm/mlock.c index 0a0c996c5c21..479e09d0994c 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -371,6 +371,7 @@ static void mlock_vma_pages_range(struct vm_area_struct *vma, { static const struct mm_walk_ops mlock_walk_ops = { .pmd_entry = mlock_pte_range, + .walk_lock = PGWALK_WRLOCK_VERIFY, };

/* diff --git a/mm/mprotect.c b/mm/mprotect.c index 6f658d483704..3aef1340533a 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -568,6 +568,7 @@ static const struct mm_walk_ops prot_none_walk_ops = { .pte_entry = prot_none_pte_entry, .hugetlb_entry = prot_none_hugetlb_entry, .test_walk = prot_none_test, + .walk_lock = PGWALK_WRLOCK, };

int diff --git a/mm/pagewalk.c b/mm/pagewalk.c index 2022333805d3..9b2d23fbf4d3 100644 --- a/mm/pagewalk.c +++ b/mm/pagewalk.c @@ -400,6 +400,33 @@ static int __walk_page_range(unsigned long start, unsigned long end, return err; }

+static inline void process_mm_walk_lock(struct mm_struct *mm, + enum page_walk_lock walk_lock) +{ + if (walk_lock == PGWALK_RDLOCK) + mmap_assert_locked(mm); + else + mmap_assert_write_locked(mm); +} + +static inline void process_vma_walk_lock(struct vm_area_struct *vma, + enum page_walk_lock walk_lock) +{ +#ifdef CONFIG_PER_VMA_LOCK + switch (walk_lock) { + case PGWALK_WRLOCK: + vma_start_write(vma); + break; + case PGWALK_WRLOCK_VERIFY: + vma_assert_write_locked(vma); + break; + case PGWALK_RDLOCK: + /* PGWALK_RDLOCK is handled by process_mm_walk_lock */ + break; + } +#endif +} + /** * walk_page_range - walk page table with caller specific callbacks * @mm: mm_struct representing the target process of page table walk @@ -459,7 +486,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start, if (!walk.mm) return -EINVAL;

- mmap_assert_locked(walk.mm); + process_mm_walk_lock(walk.mm, ops->walk_lock);

vma = find_vma(walk.mm, start); do { @@ -474,6 +501,7 @@ int walk_page_range(struct mm_struct *mm, unsigned long start, if (ops->pte_hole) err = ops->pte_hole(start, next, -1, &walk); } else { /* inside vma */ + process_vma_walk_lock(vma, ops->walk_lock); walk.vma = vma; next = min(end, vma->vm_end); vma = find_vma(mm, vma->vm_end); @@ -549,7 +577,8 @@ int walk_page_range_vma(struct vm_area_struct *vma, unsigned long start, if (start < vma->vm_start || end > vma->vm_end) return -EINVAL;

- mmap_assert_locked(walk.mm); + process_mm_walk_lock(walk.mm, ops->walk_lock); + process_vma_walk_lock(vma, ops->walk_lock); return __walk_page_range(start, end, &walk); }

@@ -566,7 +595,8 @@ int walk_page_vma(struct vm_area_struct *vma, const struct mm_walk_ops *ops, if (!walk.mm) return -EINVAL;

- mmap_assert_locked(walk.mm); + process_mm_walk_lock(walk.mm, ops->walk_lock); + process_vma_walk_lock(vma, ops->walk_lock); return __walk_page_range(vma->vm_start, vma->vm_end, &walk); }

diff --git a/mm/vmscan.c b/mm/vmscan.c index 1080209a568b..3555927df9b5 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -4284,6 +4284,7 @@ static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_ static const struct mm_walk_ops mm_walk_ops = { .test_walk = should_skip_vma, .p4d_entry = walk_pud_range, + .walk_lock = PGWALK_RDLOCK, };

int err;