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Message-Id: <20210115170907.24498-27-peterx@redhat.com>
Date: Fri, 15 Jan 2021 12:09:03 -0500
From: Peter Xu <peterx@...hat.com>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Mike Kravetz <mike.kravetz@...cle.com>, peterx@...hat.com,
Jerome Glisse <jglisse@...hat.com>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Hugh Dickins <hughd@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Matthew Wilcox <willy@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH RFC 26/30] hugetlb/userfaultfd: Handle uffd-wp special pte in hugetlb pf handler
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@...hat.com>
---
fs/userfaultfd.c | 5 ++++-
mm/hugetlb.c | 35 ++++++++++++++++++++++++++++-------
mm/userfaultfd.c | 3 ++-
3 files changed, 34 insertions(+), 9 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 3190dff39d6c..3264ec46242b 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -248,8 +248,11 @@ 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 a971513cdff6..9b9f71ec30e1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4253,7 +4253,8 @@ int huge_add_to_page_cache(struct page *page, struct address_space *mapping,
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;
@@ -4297,6 +4298,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
.vma = vma,
.address = haddr,
.flags = flags,
+ .orig_pte = old_pte,
/*
* Hard to debug if it ends up being
* used by a callee that assumes
@@ -4394,7 +4396,7 @@ 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) {
@@ -4404,6 +4406,11 @@ 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 (unlikely(flags & FAULT_FLAG_UFFD_WP)) {
+ WARN_ON_ONCE(flags & FAULT_FLAG_WRITE);
+ /* We should have the write bit cleared already, but be safe */
+ 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);
@@ -4485,9 +4492,16 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
if (unlikely(is_hugetlb_entry_migration(entry))) {
migration_entry_wait_huge(vma, mm, ptep);
return 0;
- } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
+ } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry))) {
return VM_FAULT_HWPOISON_LARGE |
VM_FAULT_SET_HINDEX(hstate_index(h));
+ } else if (unlikely(is_swap_special_pte(entry))) {
+ /* Must be a uffd-wp special swap pte */
+ WARN_ON_ONCE(!pte_swp_uffd_wp_special(entry));
+ flags |= FAULT_FLAG_UFFD_WP;
+ /* Emulate a read fault */
+ flags &= ~FAULT_FLAG_WRITE;
+ }
}
/*
@@ -4519,8 +4533,13 @@ 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);
+ /*
+ * FAULT_FLAG_UFFD_WP 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;
}
@@ -4651,7 +4670,7 @@ 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;
@@ -4725,8 +4744,10 @@ 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 3d49b888e3e8..1dff5b9a2c26 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -300,7 +300,8 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
err = -EEXIST;
dst_pteval = huge_ptep_get(dst_pte);
- if (!huge_pte_none(dst_pteval)) {
+ if (!huge_pte_none(dst_pteval) &&
+ !pte_swp_uffd_wp_special(dst_pteval)) {
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
i_mmap_unlock_read(mapping);
goto out_unlock;
--
2.26.2
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