[PATCH v8 07/23] mm/shmem: Persist uffd-wp bit across zapping for file-backed

Peter Xu
2022-04-05T02:41:55Z

File-backed memory is prone to being unmapped at any time. It means all
information in the pte will be dropped, including the uffd-wp flag.

To persist the uffd-wp flag, we'll use the pte markers. This patch teaches the
zap code to understand uffd-wp and know when to keep or drop the uffd-wp bit.

Add a new flag ZAP_FLAG_DROP_MARKER and set it in zap_details when we don't
want to persist such an information, for example, when destroying the whole
vma, or punching a hole in a shmem file. For the rest cases we should never
drop the uffd-wp bit, or the wr-protect information will get lost.

The new ZAP_FLAG_DROP_MARKER needs to be put into mm.h rather than memory.c
because it'll be further referenced in hugetlb files later.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
include/linux/mm.h | 10 ++++++++
include/linux/mm_inline.h | 43 ++++++++++++++++++++++++++++++++++
mm/memory.c | 49 ++++++++++++++++++++++++++++++++++++---
mm/rmap.c | 8 +++++++
4 files changed, 107 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 26428ff262fc..857bc8f7af45 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3422,4 +3422,14 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
}
#endif

+typedef unsigned int __bitwise zap_flags_t;
+
+/*
+ * Whether to drop the pte markers, for example, the uffd-wp information for
+ * file-backed memory. This should only be specified when we will completely
+ * drop the page in the mm, either by truncation or unmapping of the vma. By
+ * default, the flag is not set.
+ */
+#define ZAP_FLAG_DROP_MARKER ((__force zap_flags_t) BIT(0))
+
#endif /* _LINUX_MM_H */
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index ac32125745ab..7b25b53c474a 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -6,6 +6,8 @@
#include <linux/huge_mm.h>
#include <linux/swap.h>
#include <linux/string.h>
+#include <linux/userfaultfd_k.h>
+#include <linux/swapops.h>

/**
* folio_is_file_lru - Should the folio be on a file LRU or anon LRU?
@@ -316,5 +318,46 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
return atomic_read(&mm->tlb_flush_pending) > 1;
}

+/*
+ * If this pte is wr-protected by uffd-wp in any form, arm the special pte to
+ * replace a none pte. NOTE! This should only be called when *pte is already
+ * cleared so we will never accidentally replace something valuable. Meanwhile
+ * none pte also means we are not demoting the pte so tlb flushed is not needed.
+ * E.g., when pte cleared the caller should have taken care of the tlb flush.
+ *
+ * Must be called with pgtable lock held so that no thread will see the none
+ * pte, and if they see it, they'll fault and serialize at the pgtable lock.
+ *
+ * This function is a no-op if PTE_MARKER_UFFD_WP is not enabled.
+ */
+static inline void
+pte_install_uffd_wp_if_needed(struct vm_area_struct *vma, unsigned long addr,
+ pte_t *pte, pte_t pteval)
+{
+#ifdef CONFIG_PTE_MARKER_UFFD_WP
+ bool arm_uffd_pte = false;
+
+ /* The current status of the pte should be "cleared" before calling */
+ WARN_ON_ONCE(!pte_none(*pte));
+
+ if (vma_is_anonymous(vma) || !userfaultfd_wp(vma))
+ return;
+
+ /* A uffd-wp wr-protected normal pte */
+ if (unlikely(pte_present(pteval) && pte_uffd_wp(pteval)))
+ arm_uffd_pte = true;
+
+ /*
+ * A uffd-wp wr-protected swap pte. Note: this should even cover an
+ * existing pte marker with uffd-wp bit set.
+ */
+ if (unlikely(pte_swp_uffd_wp_any(pteval)))
+ arm_uffd_pte = true;
+
+ if (unlikely(arm_uffd_pte))
+ set_pte_at(vma->vm_mm, addr, pte,
+ make_pte_marker(PTE_MARKER_UFFD_WP));
+#endif
+}

#endif
diff --git a/mm/memory.c b/mm/memory.c
index 21abb8a30553..1144845ff734 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -74,6 +74,7 @@
#include <linux/perf_event.h>
#include <linux/ptrace.h>
#include <linux/vmalloc.h>
+#include <linux/mm_inline.h>

#include <trace/events/kmem.h>

@@ -1306,6 +1307,7 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
struct zap_details {
struct folio *single_folio; /* Locked folio to be unmapped */
bool even_cows; /* Zap COWed private pages too? */
+ zap_flags_t zap_flags; /* Extra flags for zapping */
};

/* Whether we should zap all COWed (private) pages too */
@@ -1334,6 +1336,29 @@ static inline bool should_zap_page(struct zap_details *details, struct page *pag
return !PageAnon(page);
}

+static inline bool zap_drop_file_uffd_wp(struct zap_details *details)
+{
+ if (!details)
+ return false;
+
+ return details->zap_flags & ZAP_FLAG_DROP_MARKER;
+}
+
+/*
+ * This function makes sure that we'll replace the none pte with an uffd-wp
+ * swap special pte marker when necessary. Must be with the pgtable lock held.
+ */
+static inline void
+zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
+ unsigned long addr, pte_t *pte,
+ struct zap_details *details, pte_t pteval)
+{
+ if (zap_drop_file_uffd_wp(details))
+ return;
+
+ pte_install_uffd_wp_if_needed(vma, addr, pte, pteval);
+}
+
static unsigned long zap_pte_range(struct mmu_gather *tlb,
struct vm_area_struct *vma, pmd_t *pmd,
unsigned long addr, unsigned long end,
@@ -1371,6 +1396,8 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
ptent = ptep_get_and_clear_full(mm, addr, pte,
tlb->fullmm);
tlb_remove_tlb_entry(tlb, pte, addr);
+ zap_install_uffd_wp_if_needed(vma, addr, pte, details,
+ ptent);
if (unlikely(!page))
continue;

@@ -1401,6 +1428,13 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
page = pfn_swap_entry_to_page(entry);
if (unlikely(!should_zap_page(details, page)))
continue;
+ /*
+ * Both device private/exclusive mappings should only
+ * work with anonymous page so far, so we don't need to
+ * consider uffd-wp bit when zap. For more information,
+ * see zap_install_uffd_wp_if_needed().
+ */
+ WARN_ON_ONCE(!vma_is_anonymous(vma));
rss[mm_counter(page)]--;
if (is_device_private_entry(entry))
page_remove_rmap(page, vma, false);
@@ -1417,8 +1451,10 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
if (!should_zap_page(details, page))
continue;
rss[mm_counter(page)]--;
- } else if (is_pte_marker_entry(entry)) {
- /* By default, simply drop all pte markers when zap */
+ } else if (pte_marker_entry_uffd_wp(entry)) {
+ /* Only drop the uffd-wp marker if explicitly requested */
+ if (!zap_drop_file_uffd_wp(details))
+ continue;
} else if (is_hwpoison_entry(entry)) {
if (!should_zap_cows(details))
continue;
@@ -1427,6 +1463,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
WARN_ON_ONCE(1);
}
pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+ zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
} while (pte++, addr += PAGE_SIZE, addr != end);

add_mm_rss_vec(mm, rss);
@@ -1637,12 +1674,17 @@ void unmap_vmas(struct mmu_gather *tlb,
unsigned long end_addr)
{
struct mmu_notifier_range range;
+ struct zap_details details = {
+ .zap_flags = ZAP_FLAG_DROP_MARKER,
+ /* Careful - we need to zap private pages too! */
+ .even_cows = true,
+ };

mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
start_addr, end_addr);
mmu_notifier_invalidate_range_start(&range);
for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next)
- unmap_single_vma(tlb, vma, start_addr, end_addr, NULL);
+ unmap_single_vma(tlb, vma, start_addr, end_addr, &details);
mmu_notifier_invalidate_range_end(&range);
}

@@ -3438,6 +3480,7 @@ void unmap_mapping_folio(struct folio *folio)

details.even_cows = false;
details.single_folio = folio;
+ details.zap_flags = ZAP_FLAG_DROP_MARKER;

i_mmap_lock_read(mapping);
if (unlikely(!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root)))
diff --git a/mm/rmap.c b/mm/rmap.c
index 208b2c683cec..69416072b1a6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -73,6 +73,7 @@
#include <linux/page_idle.h>
#include <linux/memremap.h>
#include <linux/userfaultfd_k.h>
+#include <linux/mm_inline.h>

#include <asm/tlbflush.h>

@@ -1538,6 +1539,13 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
pteval = ptep_clear_flush(vma, address, pvmw.pte);
}

+ /*
+ * Now the pte is cleared. If this pte was uffd-wp armed,
+ * we may want to replace a none pte with a marker pte if
+ * it's file-backed, so we don't lose the tracking info.
+ */
+ pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
+
/* Set the dirty flag on the folio now the pte is gone. */
if (pte_dirty(pteval))
folio_mark_dirty(folio);
--
2.32.0