Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Oleg Nesterov wrote:
> On 04/02, Konstantin Khlebnikov wrote:
>>
>> Currently kernel does not account read-only private mappings into memory commitment.
>> But these mappings can be force-COW-ed in get_user_pages().
>
> Heh. tail -n3 Documentation/vm/overcommit-accounting
> may be you should update it then.

I just wonder how fragile this accounting...

>
> Can't really comment the patch, this is not my area. Still,
>
>> +	down_write(&mm->mmap_sem);
>> +	*pvma = vma = find_vma(mm, addr);
>> +	if (vma&&  vma->vm_start<= addr) {
>> +		ret = vma->vm_end - addr;
>> +		if ((vma->vm_flags&  (VM_ACCOUNT | VM_NORESERVE | VM_SHARED |
>> +				VM_HUGETLB | VM_MAYWRITE)) == VM_MAYWRITE) {
>> +			if (!security_vm_enough_memory_mm(mm, vma_pages(vma)))
>
> Oooooh, the whole vma. Say, gdb installs the single breakpoint into
> the huge .text mapping...

We cannot split vma right there, this will be really weird. =)

>
> I am not sure, but probably you want to check at least VM_IO/PFNMAP
> as well. We do not want to charge this memory and retry with FOLL_FORCE
> before vm_ops->access(). Say, /dev/mem

No, VM_IO/PFNMAP aren't affect accounting, there is VM_NORESERVE for this.

>
> Hmm. OTOH, if I am right then mprotect_fixup() should be fixed??

mprotect_fixup() does not account area if it already accounted, so all ok.

>
>
> We drop ->mmap_sem... Say, the task does mremap() in between and
> len == 2 * PAGE_SIZE. Then, for example, copy_to_user_page() can
> write to the same page twice. Perhaps not a problem in practice,
> I dunno.

I have an old unfinished patch which implements upgrade_read() for rw-semaphore =)

 >