Re: [PATCH] mm/mprotect: reduce Committed_AS if memory protection is changed to PROT_NONE

[Andrew Morton]

On Tue, 26 Apr 2022 19:27:05 +0800 liusongtang <liusongtang@xxxxxxxxxx> wrote:

> If PROT_WRITE is set, the size of vm area will be added to Committed_AS.
> However, if memory protection is changed to PROT_NONE,
> the corresponding physical memory will not be used, but Committed_AS still
> count the size of the PROT_NONE memory.
> 
> This patch reduce Committed_AS and free the corresponding memory if
> memory protection is changed to PROT_NONE.
> 
> ...
>
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -497,6 +497,12 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
>  	}
>  
>  success:
> +	if ((newflags & (VM_READ | VM_WRITE | VM_EXEC | VM_LOCKED | VM_ACCOUNT)) == VM_ACCOUNT) {
> +		zap_page_range(vma, start, end - start);
> +		newflags &= ~VM_ACCOUNT;
> +		vm_unacct_memory((end - start) >> PAGE_SHIFT);
> +	}
> +
>  	/*
>  	 * vm_flags and vm_page_prot are protected by the mmap_lock
>  	 * held in write mode.

Surprised.  If userspace does mprotect(addr, len.  PROT_NONE) then
mprotect(addr, len.  PROT_READ), what is now at *addr?  Zeroes?