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Subject[PATCH 24/41] mm: conditionally write-lock VMA in free_pgtables
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Normally free_pgtables needs to lock affected VMAs except for the case
when VMAs were isolated under VMA write-lock. munmap() does just that,
isolating while holding appropriate locks and then downgrading mmap_lock
and dropping per-VMA locks before freeing page tables.
Add a parameter to free_pgtables and unmap_region for such scenario.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
mm/internal.h | 2 +-
mm/memory.c | 6 +++++-
mm/mmap.c | 18 ++++++++++++------
3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index bcf75a8b032d..5ea4ff1a70e7 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -87,7 +87,7 @@ void folio_activate(struct folio *folio);

void free_pgtables(struct mmu_gather *tlb, struct maple_tree *mt,
struct vm_area_struct *start_vma, unsigned long floor,
- unsigned long ceiling);
+ unsigned long ceiling, bool lock_vma);
void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);

struct zap_details;
diff --git a/mm/memory.c b/mm/memory.c
index 2fabf89b2be9..9ece18548db1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -348,7 +348,7 @@ void free_pgd_range(struct mmu_gather *tlb,

void free_pgtables(struct mmu_gather *tlb, struct maple_tree *mt,
struct vm_area_struct *vma, unsigned long floor,
- unsigned long ceiling)
+ unsigned long ceiling, bool lock_vma)
{
MA_STATE(mas, mt, vma->vm_end, vma->vm_end);

@@ -366,6 +366,8 @@ void free_pgtables(struct mmu_gather *tlb, struct maple_tree *mt,
* Hide vma from rmap and truncate_pagecache before freeing
* pgtables
*/
+ if (lock_vma)
+ vma_write_lock(vma);
unlink_anon_vmas(vma);
unlink_file_vma(vma);

@@ -380,6 +382,8 @@ void free_pgtables(struct mmu_gather *tlb, struct maple_tree *mt,
&& !is_vm_hugetlb_page(next)) {
vma = next;
next = mas_find(&mas, ceiling - 1);
+ if (lock_vma)
+ vma_write_lock(vma);
unlink_anon_vmas(vma);
unlink_file_vma(vma);
}
diff --git a/mm/mmap.c b/mm/mmap.c
index be289e0b693b..0d767ce043af 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -78,7 +78,7 @@ core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
struct vm_area_struct *vma, struct vm_area_struct *prev,
struct vm_area_struct *next, unsigned long start,
- unsigned long end);
+ unsigned long end, bool lock_vma);

static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
{
@@ -2202,7 +2202,7 @@ static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
struct vm_area_struct *vma, struct vm_area_struct *prev,
struct vm_area_struct *next,
- unsigned long start, unsigned long end)
+ unsigned long start, unsigned long end, bool lock_vma)
{
struct mmu_gather tlb;

@@ -2211,7 +2211,8 @@ static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
update_hiwater_rss(mm);
unmap_vmas(&tlb, mt, vma, start, end);
free_pgtables(&tlb, mt, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
- next ? next->vm_start : USER_PGTABLES_CEILING);
+ next ? next->vm_start : USER_PGTABLES_CEILING,
+ lock_vma);
tlb_finish_mmu(&tlb);
}

@@ -2468,7 +2469,11 @@ do_mas_align_munmap(struct ma_state *mas, struct vm_area_struct *vma,
mmap_write_downgrade(mm);
}

- unmap_region(mm, &mt_detach, vma, prev, next, start, end);
+ /*
+ * We can free page tables without locking the vmas because they were
+ * isolated before we downgraded mmap_lock and dropped per-vma locks.
+ */
+ unmap_region(mm, &mt_detach, vma, prev, next, start, end, !downgrade);
/* Statistics and freeing VMAs */
mas_set(&mas_detach, start);
remove_mt(mm, &mas_detach);
@@ -2785,7 +2790,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
vma->vm_file = NULL;

/* Undo any partial mapping done by a device driver. */
- unmap_region(mm, mas.tree, vma, prev, next, vma->vm_start, vma->vm_end);
+ unmap_region(mm, mas.tree, vma, prev, next, vma->vm_start, vma->vm_end,
+ true);
if (file && (vm_flags & VM_SHARED))
mapping_unmap_writable(file->f_mapping);
free_vma:
@@ -3130,7 +3136,7 @@ void exit_mmap(struct mm_struct *mm)
mmap_write_lock(mm);
mt_clear_in_rcu(&mm->mm_mt);
free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS,
- USER_PGTABLES_CEILING);
+ USER_PGTABLES_CEILING, true);
tlb_finish_mmu(&tlb);

/*
--
2.39.0
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