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mm: Always downgrade mmap_lock if requested 

Now that stack growth must always hold the mmap_lock for write, we can
always downgrade the mmap_lock to read and safely unmap pages from the
page table, even if we're next to a stack.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2023-06-29 20:14:14 +01:00 committed by Linus Torvalds
parent 03f889378f
commit e4bd84c069
1 changed files with 2 additions and 13 deletions
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15
mm/mmap.c
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@ -2551,19 +2551,8 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
/* Point of no return */
mm->locked_vm -= locked_vm;
mm->map_count -= count;
/*
* Do not downgrade mmap_lock if we are next to VM_GROWSDOWN or
* VM_GROWSUP VMA. Such VMAs can change their size under
* down_read(mmap_lock) and collide with the VMA we are about to unmap.
*/
if (downgrade) {
if (next && (next->vm_flags & VM_GROWSDOWN))
downgrade = false;
else if (prev && (prev->vm_flags & VM_GROWSUP))
downgrade = false;
else
mmap_write_downgrade(mm);
}
if (downgrade)
mmap_write_downgrade(mm);
/*
* We can free page tables without write-locking mmap_lock because VMAs