[PATCH v7 24/70] mm/mmap: Change do_brk_flags() to expand existing VMA and add do_brk_munmap()

Liam Howlett
2022-04-04T14:37:09Z

Avoid allocating a new VMA when it a vma modification can occur. When a
brk() can expand or contract a VMA, then the single store operation will
only modify one index of the maple tree instead of causing a node to
split or coalesce. This avoids unnecessary allocations/frees of maple
tree nodes and VMAs.

Move some limit & flag verifications out of the do_brk_flags() function
to use only relevant checks in the code path of bkr() and
vm_brk_flags().

Set the vma to check if it can expand in vm_brk_flags() if extra
criteria are met.

Drop userfaultfd from do_brk_flags() path and only use it in
vm_brk_flags() path since that is the only place a munmap will happen.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
mm/mmap.c | 283 +++++++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 227 insertions(+), 56 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 7b4db8decfd2..e04614d7eaba 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -189,17 +189,40 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
return next;
}

-static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
- struct list_head *uf);
+/*
+ * check_brk_limits() - Use platform specific check of range & verify mlock
+ * limits.
+ * @addr: The address to check
+ * @len: The size of increase.
+ *
+ * Return: 0 on success.
+ */
+static int check_brk_limits(unsigned long addr, unsigned long len)
+{
+ unsigned long mapped_addr;
+
+ mapped_addr = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
+ if (IS_ERR_VALUE(mapped_addr))
+ return mapped_addr;
+
+ return mlock_future_check(current->mm, current->mm->def_flags, len);
+}
+static int do_brk_munmap(struct ma_state *mas, struct vm_area_struct *vma,
+ unsigned long newbrk, unsigned long oldbrk,
+ struct list_head *uf);
+static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *brkvma,
+ unsigned long addr, unsigned long request,
+ unsigned long flags);
SYSCALL_DEFINE1(brk, unsigned long, brk)
{
unsigned long newbrk, oldbrk, origbrk;
struct mm_struct *mm = current->mm;
- struct vm_area_struct *next;
+ struct vm_area_struct *brkvma, *next = NULL;
unsigned long min_brk;
bool populate;
bool downgraded = false;
LIST_HEAD(uf);
+ MA_STATE(mas, &mm->mm_mt, 0, 0);

if (mmap_write_lock_killable(mm))
return -EINTR;
@@ -241,35 +264,52 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)

/*
* Always allow shrinking brk.
- * __do_munmap() may downgrade mmap_lock to read.
+ * do_brk_munmap() may downgrade mmap_lock to read.
*/
if (brk <= mm->brk) {
int ret;

+ /* Search one past newbrk */
+ mas_set(&mas, newbrk);
+ brkvma = mas_find(&mas, oldbrk);
+ BUG_ON(brkvma == NULL);
+ if (brkvma->vm_start >= oldbrk)
+ goto out; /* mapping intersects with an existing non-brk vma. */
/*
- * mm->brk must to be protected by write mmap_lock so update it
- * before downgrading mmap_lock. When __do_munmap() fails,
- * mm->brk will be restored from origbrk.
+ * mm->brk must be protected by write mmap_lock.
+ * do_brk_munmap() may downgrade the lock, so update it
+ * before calling do_brk_munmap().
*/
mm->brk = brk;
- ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
- if (ret < 0) {
- mm->brk = origbrk;
- goto out;
- } else if (ret == 1) {
+ mas.last = oldbrk - 1;
+ ret = do_brk_munmap(&mas, brkvma, newbrk, oldbrk, &uf);
+ if (ret == 1) {
downgraded = true;
- }
- goto success;
+ goto success;
+ } else if (!ret)
+ goto success;
+
+ mm->brk = origbrk;
+ goto out;
}

- /* Check against existing mmap mappings. */
- next = find_vma(mm, oldbrk);
+ if (check_brk_limits(oldbrk, newbrk - oldbrk))
+ goto out;
+
+ /*
+ * Only check if the next VMA is within the stack_guard_gap of the
+ * expansion area
+ */
+ mas_set(&mas, oldbrk);
+ next = mas_find(&mas, newbrk - 1 + PAGE_SIZE + stack_guard_gap);
if (next && newbrk + PAGE_SIZE > vm_start_gap(next))
goto out;

+ brkvma = mas_prev(&mas, mm->start_brk);
/* Ok, looks good - let it rip. */
- if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
+ if (do_brk_flags(&mas, brkvma, oldbrk, newbrk - oldbrk, 0) < 0)
goto out;
+
mm->brk = brk;

success:
@@ -2706,38 +2746,115 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
}

/*
- * this is really a simplified "do_mmap". it only handles
- * anonymous maps. eventually we may be able to do some
- * brk-specific accounting here.
+ * brk_munmap() - Unmap a parital vma.
+ * @mas: The maple tree state.
+ * @vma: The vma to be modified
+ * @newbrk: the start of the address to unmap
+ * @oldbrk: The end of the address to unmap
+ * @uf: The userfaultfd list_head
+ *
+ * Returns: 1 on success.
+ * unmaps a partial VMA mapping. Does not handle alignment, downgrades lock if
+ * possible.
*/
-static int do_brk_flags(unsigned long addr, unsigned long len,
- unsigned long flags, struct list_head *uf)
+static int do_brk_munmap(struct ma_state *mas, struct vm_area_struct *vma,
+ unsigned long newbrk, unsigned long oldbrk,
+ struct list_head *uf)
{
- struct mm_struct *mm = current->mm;
- struct vm_area_struct *vma, *prev;
- pgoff_t pgoff = addr >> PAGE_SHIFT;
- int error;
- unsigned long mapped_addr;
- validate_mm_mt(mm);
+ struct mm_struct *mm = vma->vm_mm;
+ struct vm_area_struct unmap;
+ unsigned long unmap_pages;
+ int ret = 1;
+
+ arch_unmap(mm, newbrk, oldbrk);
+
+ if (likely((vma->vm_end < oldbrk) ||
+ ((vma->vm_start == newbrk) && (vma->vm_end == oldbrk)))) {
+ /* remove entire mapping(s) */
+ mas_set(mas, newbrk);
+ if (vma->vm_start != newbrk)
+ mas_reset(mas); /* cause a re-walk for the first overlap. */
+ ret = __do_munmap(mm, newbrk, oldbrk - newbrk, uf, true);
+ goto munmap_full_vma;
+ }
+
+ vma_init(&unmap, mm);
+ unmap.vm_start = newbrk;
+ unmap.vm_end = oldbrk;
+ ret = userfaultfd_unmap_prep(&unmap, newbrk, oldbrk, uf);
+ if (ret)
+ return ret;
+ ret = 1;

- /* Until we need other flags, refuse anything except VM_EXEC. */
- if ((flags & (~VM_EXEC)) != 0)
- return -EINVAL;
- flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
+ /* Change the oldbrk of vma to the newbrk of the munmap area */
+ vma_adjust_trans_huge(vma, vma->vm_start, newbrk, 0);
+ if (vma->anon_vma) {
+ anon_vma_lock_write(vma->anon_vma);
+ anon_vma_interval_tree_pre_update_vma(vma);
+ }

- mapped_addr = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
- if (IS_ERR_VALUE(mapped_addr))
- return mapped_addr;
+ vma->vm_end = newbrk;
+ if (vma_mas_remove(&unmap, mas))
+ goto mas_store_fail;

- error = mlock_future_check(mm, mm->def_flags, len);
- if (error)
- return error;
+ vmacache_invalidate(vma->vm_mm);
+ if (vma->anon_vma) {
+ anon_vma_interval_tree_post_update_vma(vma);
+ anon_vma_unlock_write(vma->anon_vma);
+ }

- /* Clear old maps, set up prev and uf */
- if (munmap_vma_range(mm, addr, len, &prev, uf))
- return -ENOMEM;
+ unmap_pages = vma_pages(&unmap);
+ if (vma->vm_flags & VM_LOCKED) {
+ mm->locked_vm -= unmap_pages;
+ munlock_vma_pages_range(&unmap, newbrk, oldbrk);
+ }
+
+ mmap_write_downgrade(mm);
+ unmap_region(mm, &unmap, vma, newbrk, oldbrk);
+ /* Statistics */
+ vm_stat_account(mm, vma->vm_flags, -unmap_pages);
+ if (vma->vm_flags & VM_ACCOUNT)
+ vm_unacct_memory(unmap_pages);

- /* Check against address space limits *after* clearing old maps... */
+munmap_full_vma:
+ validate_mm_mt(mm);
+ return ret;
+
+mas_store_fail:
+ vma->vm_end = oldbrk;
+ if (vma->anon_vma) {
+ anon_vma_interval_tree_post_update_vma(vma);
+ anon_vma_unlock_write(vma->anon_vma);
+ }
+ return -ENOMEM;
+}
+
+/*
+ * do_brk_flags() - Increase the brk vma if the flags match.
+ * @mas: The maple tree state.
+ * @addr: The start address
+ * @len: The length of the increase
+ * @vma: The vma,
+ * @flags: The VMA Flags
+ *
+ * Extend the brk VMA from addr to addr + len. If the VMA is NULL or the flags
+ * do not match then create a new anonymous VMA. Eventually we may be able to
+ * do some brk-specific accounting here.
+ */
+static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *vma,
+ unsigned long addr, unsigned long len,
+ unsigned long flags)
+{
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *prev = NULL;
+ validate_mm_mt(mm);
+
+
+ /*
+ * Check against address space limits by the changed size
+ * Note: This happens *after* clearing old mappings in some code paths.
+ */
+ flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
return -ENOMEM;

@@ -2747,28 +2864,51 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
return -ENOMEM;

- /* Can we just expand an old private anonymous mapping? */
- vma = vma_merge(mm, prev, addr, addr + len, flags,
- NULL, NULL, pgoff, NULL, NULL_VM_UFFD_CTX, NULL);
- if (vma)
- goto out;
-
/*
- * create a vma struct for an anonymous mapping
+ * Expand the existing vma if possible; Note that singular lists do not
+ * occur after forking, so the expand will only happen on new VMAs.
*/
- vma = vm_area_alloc(mm);
- if (!vma) {
- vm_unacct_memory(len >> PAGE_SHIFT);
- return -ENOMEM;
+ if (vma &&
+ (!vma->anon_vma || list_is_singular(&vma->anon_vma_chain)) &&
+ ((vma->vm_flags & ~VM_SOFTDIRTY) == flags)) {
+ mas->index = vma->vm_start;
+ mas->last = addr + len - 1;
+ vma_adjust_trans_huge(vma, addr, addr + len, 0);
+ if (vma->anon_vma) {
+ anon_vma_lock_write(vma->anon_vma);
+ anon_vma_interval_tree_pre_update_vma(vma);
+ }
+ vma->vm_end = addr + len;
+ vma->vm_flags |= VM_SOFTDIRTY;
+ mas_store_gfp(mas, vma, GFP_KERNEL);
+
+ if (vma->anon_vma) {
+ anon_vma_interval_tree_post_update_vma(vma);
+ anon_vma_unlock_write(vma->anon_vma);
+ }
+ khugepaged_enter_vma_merge(vma, flags);
+ goto out;
}
+ prev = vma;
+
+ /* create a vma struct for an anonymous mapping */
+ vma = vm_area_alloc(mm);
+ if (!vma)
+ goto vma_alloc_fail;

vma_set_anonymous(vma);
vma->vm_start = addr;
vma->vm_end = addr + len;
- vma->vm_pgoff = pgoff;
+ vma->vm_pgoff = addr >> PAGE_SHIFT;
vma->vm_flags = flags;
vma->vm_page_prot = vm_get_page_prot(flags);
- vma_link(mm, vma, prev);
+ vma_mas_store(vma, mas);
+
+ if (!prev)
+ prev = mas_prev(mas, 0);
+
+ __vma_link_list(mm, vma, prev);
+ mm->map_count++;
out:
perf_event_mmap(vma);
mm->total_vm += len >> PAGE_SHIFT;
@@ -2778,15 +2918,21 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
vma->vm_flags |= VM_SOFTDIRTY;
validate_mm_mt(mm);
return 0;
+
+vma_alloc_fail:
+ vm_unacct_memory(len >> PAGE_SHIFT);
+ return -ENOMEM;
}

int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
{
struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma = NULL;
unsigned long len;
int ret;
bool populate;
LIST_HEAD(uf);
+ MA_STATE(mas, &mm->mm_mt, addr, addr);

len = PAGE_ALIGN(request);
if (len < request)
@@ -2797,13 +2943,38 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
if (mmap_write_lock_killable(mm))
return -EINTR;

- ret = do_brk_flags(addr, len, flags, &uf);
+ /* Until we need other flags, refuse anything except VM_EXEC. */
+ if ((flags & (~VM_EXEC)) != 0)
+ return -EINVAL;
+
+ ret = check_brk_limits(addr, len);
+ if (ret)
+ goto limits_failed;
+
+ if (find_vma_intersection(mm, addr, addr + len))
+ ret = do_munmap(mm, addr, len, &uf);
+
+ if (ret)
+ goto munmap_failed;
+
+ vma = mas_prev(&mas, 0);
+ if (!vma || vma->vm_end != addr || vma_policy(vma) ||
+ !can_vma_merge_after(vma, flags, NULL, NULL,
+ addr >> PAGE_SHIFT,NULL_VM_UFFD_CTX, NULL))
+ vma = NULL;
+
+ ret = do_brk_flags(&mas, vma, addr, len, flags);
populate = ((mm->def_flags & VM_LOCKED) != 0);
mmap_write_unlock(mm);
userfaultfd_unmap_complete(mm, &uf);
if (populate && !ret)
mm_populate(addr, len);
return ret;
+
+munmap_failed:
+limits_failed:
+ mmap_write_unlock(mm);
+ return ret;
}
EXPORT_SYMBOL(vm_brk_flags);

--
2.34.1