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Subject[PATCH v2 20/24] hugetlb/userfaultfd: Handle uffd-wp special pte in hugetlb pf handler
Date
Teach the hugetlb page fault code to understand uffd-wp special pte.  For
example, when seeing such a pte we need to convert any write fault into a read
one (which is fake - we'll retry the write later if so). Meanwhile, for
handle_userfault() we'll need to make sure we must wait for the special swap
pte too just like a none pte.

Note that we also need to teach UFFDIO_COPY about this special pte across the
code path so that we can safely install a new page at this special pte as long
as we know it's a stall entry.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
fs/userfaultfd.c | 5 ++++-
mm/hugetlb.c | 26 ++++++++++++++++++++------
mm/userfaultfd.c | 5 ++++-
3 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index b34486a88b5f3..a41e0631af512 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -245,8 +245,11 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
/*
* Lockless access: we're in a wait_event so it's ok if it
* changes under us.
+ *
+ * Regarding uffd-wp special case, please refer to comments in
+ * userfaultfd_must_wait().
*/
- if (huge_pte_none(pte))
+ if (huge_pte_none(pte) || pte_swp_uffd_wp_special(pte))
ret = true;
if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
ret = true;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 071a8429ea190..d9ff7db14175d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4529,7 +4529,8 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
struct vm_area_struct *vma,
struct address_space *mapping, pgoff_t idx,
- unsigned long address, pte_t *ptep, unsigned int flags)
+ unsigned long address, pte_t *ptep,
+ pte_t old_pte, unsigned int flags)
{
struct hstate *h = hstate_vma(vma);
vm_fault_t ret = VM_FAULT_SIGBUS;
@@ -4653,7 +4654,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,

ptl = huge_pte_lock(h, mm, ptep);
ret = 0;
- if (!huge_pte_none(huge_ptep_get(ptep)))
+ if (!pte_same(huge_ptep_get(ptep), old_pte))
goto backout;

if (anon_rmap) {
@@ -4663,6 +4664,12 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
page_dup_rmap(page, true);
new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
&& (vma->vm_flags & VM_SHARED)));
+ /*
+ * If this pte was previously wr-protected, keep it wr-protected even
+ * if populated.
+ */
+ if (unlikely(pte_swp_uffd_wp_special(old_pte)))
+ new_pte = huge_pte_wrprotect(huge_pte_mkuffd_wp(new_pte));
set_huge_pte_at(mm, haddr, ptep, new_pte);

hugetlb_count_add(pages_per_huge_page(h), mm);
@@ -4778,8 +4785,13 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
mutex_lock(&hugetlb_fault_mutex_table[hash]);

entry = huge_ptep_get(ptep);
- if (huge_pte_none(entry)) {
- ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
+ /*
+ * uffd-wp-special should be handled merely the same as pte none
+ * because it's basically a none pte with a special marker
+ */
+ if (huge_pte_none(entry) || pte_swp_uffd_wp_special(entry)) {
+ ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
+ entry, flags);
goto out_mutex;
}

@@ -4913,7 +4925,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
unsigned long size;
int vm_shared = dst_vma->vm_flags & VM_SHARED;
struct hstate *h = hstate_vma(dst_vma);
- pte_t _dst_pte;
+ pte_t _dst_pte, cur_pte;
spinlock_t *ptl;
int ret;
struct page *page;
@@ -4991,8 +5003,10 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
if (idx >= size)
goto out_release_unlock;

+ cur_pte = huge_ptep_get(dst_pte);
ret = -EEXIST;
- if (!huge_pte_none(huge_ptep_get(dst_pte)))
+ /* Please refer to shmem_mfill_atomic_pte() for uffd-wp special case */
+ if (!huge_pte_none(cur_pte) && !pte_swp_uffd_wp_special(cur_pte))
goto out_release_unlock;

if (vm_shared) {
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index ceb77ea24497e..2cd6ad5c3d8f8 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -274,6 +274,8 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
}

while (src_addr < src_start + len) {
+ pte_t pteval;
+
BUG_ON(dst_addr >= dst_start + len);

/*
@@ -296,8 +298,9 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
goto out_unlock;
}

+ pteval = huge_ptep_get(dst_pte);
if (mode != MCOPY_ATOMIC_CONTINUE &&
- !huge_pte_none(huge_ptep_get(dst_pte))) {
+ !huge_pte_none(pteval) && !pte_swp_uffd_wp_special(pteval)) {
err = -EEXIST;
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
i_mmap_unlock_read(mapping);
--
2.26.2
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