Re: [PATCH v2 3/5] mm: madvise: implement lightweight guard page mechanism

[David Hildenbrand <david@redhat.com>]

On 21.10.24 22:25, Vlastimil Babka wrote:
> On 10/21/24 22:17, David Hildenbrand wrote:
>> On 21.10.24 22:11, Vlastimil Babka wrote:
>>> On 10/20/24 18:20, Lorenzo Stoakes wrote:
>>>
>>> <snip>
>>>
>>>> +static long madvise_guard_poison(struct vm_area_struct *vma,
>>>> +				 struct vm_area_struct **prev,
>>>> +				 unsigned long start, unsigned long end)
>>>> +{
>>>> +	long err;
>>>> +
>>>> +	*prev = vma;
>>>> +	if (!is_valid_guard_vma(vma, /* allow_locked = */false))
>>>> +		return -EINVAL;
>>>> +
>>>> +	/*
>>>> +	 * If we install poison markers, then the range is no longer
>>>> +	 * empty from a page table perspective and therefore it's
>>>> +	 * appropriate to have an anon_vma.
>>>> +	 *
>>>> +	 * This ensures that on fork, we copy page tables correctly.
>>>> +	 */
>>>> +	err = anon_vma_prepare(vma);
>>>> +	if (err)
>>>> +		return err;
>>>> +
>>>> +	/*
>>>> +	 * Optimistically try to install the guard poison pages first. If any
>>>> +	 * non-guard pages are encountered, give up and zap the range before
>>>> +	 * trying again.
>>>> +	 */
>>>
>>> Should the page walker become powerful enough to handle this in one go? :)
>>> But sure, if it's too big a task to teach it to zap ptes with all the tlb
>>> flushing etc (I assume it's something page walkers don't do today), it makes
>>> sense to do it this way.
>>> Or we could require userspace to zap first (MADV_DONTNEED), but that would
>>> unnecessarily mean extra syscalls for the use case of an allocator debug
>>> mode that wants to turn freed memory to guards to catch use after free.
>>> So this seems like a good compromise...
>>
>> Yes please, KIS.
> 
> You mean "require userspace to zap first (MADV_DONTNEED)" ?

Yes, I see from Lorenzo's reply that there is apparently some history to 
this (maybe it's all nicely summarized in the cover letter / this patch, 
have to dig further).

Not sure yet what the problem is, I would have thought it's all 
protected by the PTL, and concurrent faults are user space doing 
something stupid and we'd detect it.

Have to do some more reading on this.

> 
> I'd normally agree with the KIS principle, but..
> 
>> We can always implement support for that later if
> 
> it would either mean later we change behavior (installing guards on
> non-zapped PTEs would have to be an error now but maybe start working later,
> which is user observable change thus can break somebody)
> 
>> really required (leave behavior open when documenting).
> 
> and leaving it open when documenting doesn't really mean anything for the
> "we don't break userspace" promise vs what the implementation actually does.

Not quite I think. You could start return -EEXIST or -EOPNOTSUPP and 
document that this can change in the future to succeed if there is 
something. User space can sense support.

Something failing that at one point starts working is not really 
breaking user space, unless someone really *wants* to fail if there is 
already something (e.g., concurrent fault -> bail out instead of hiding it).

Of course, a more elegant solution would be GUARD_INSTALL vs. 
GUARD_FORCE_INSTALL.

.. but again, there seems to be more history to this.

-- 
Cheers,

David / dhildenb