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mm/shmem: persist uffd-wp bit across zapping for file-backed 

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.

Link: https://lkml.kernel.org/r/20220405014847.14295-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Peter Xu 2022-05-12 20:22:53 -07:00 committed by Andrew Morton
parent 9c28a205c0
commit 999dad824c
4 changed files with 107 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
include/linux/mm.h
View File

@ -3428,4 +3428,14 @@ 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 */

43
include/linux/mm_inline.h
View File

@ -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

49
mm/memory.c
View File

@ -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 @@ again:
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 @@ again:
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 @@ again:
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 @@ again:
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)))

8
mm/rmap.c
View File

@ -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>
@ -1585,6 +1586,13 @@ 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);