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

[liusongtang <liusongtang@huawei.com>]

On 2022/4/27 4:34, Andrew Morton wrote:

> On Tue, 26 Apr 2022 19:27:05 +0800 liusongtang <liusongtang@huawei.com> 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?
> .

1. In the case mentioned above, I think data in *addr is invalid after
mprotect(addr, len.  PROT_NONE), so clear it will not cause a problem.
2. Another idea is we can check if this vm area is populated before reduce Committed_AS.