Re: [PATCH] smaps: Fix the abnormal memory statistics obtained through /proc/pid/smaps

[David Hildenbrand <david@redhat.com>]

On 27.07.23 20:59, Peter Xu wrote:
> On Thu, Jul 27, 2023 at 07:27:02PM +0200, David Hildenbrand wrote:
>>>>
>>>> This was wrong from the very start. If we're not in GUP, we shouldn't call
>>>> GUP functions.
>>>
>>> My understanding is !GET && !PIN is also called gup.. otherwise we don't
>>> need GET and it can just be always implied.
>>
>> That's not the point. The point is that _arbitrary_ code shouldn't call into
>> GUP internal helper functions, where they bypass, for example, any sanity
>> checks.
> 
> What's the sanity checks that you're referring to?
> 

For example in follow_page()

if (vma_is_secretmem(vma))
	return NULL;

if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
	return NULL;


Maybe you can elaborate why you think we should *not* be using 
vm_normal_page_pmd() and instead some arbitrary GUP internal helper? I 
don't get it.

>>
>>>
>>> The other proof is try_grab_page() doesn't fail hard on !GET && !PIN.  So I
>>> don't know whether that's "wrong" to be used..
>>>
>>
>> To me, that is arbitrary code using a GUP internal helper and, therefore,
>> wrong.
>>
>>> Back to the topic: I'd say either of the patches look good to solve the
>>> problem.  If p2pdma pages are mapped as PFNMAP/MIXEDMAP (?), I guess
>>> vm_normal_page_pmd() proposed here will also work on it, so nothing I see
>>> wrong on 2nd one yet.
>>>
>>> It looks nicer indeed to not have FOLL_FORCE here, but it also makes me
>>> just wonder whether we should document NUMA behavior for FOLL_* somewhere,
>>> because we have an implication right now on !FOLL_FORCE over NUMA, which is
>>> not obvious to me..
>>
>> Yes, we probably should. For get_use_pages() and friends that behavior was
>> always like that and it makes sense: usually it represent application
>> behavior.
>>
>>>
>>> And to look more over that aspect, see follow_page(): previously we can
>>> follow a page for protnone (as it never applies FOLL_NUMA) but now it won't
>>> (it never applies FOLL_FORCE, either, so it seems "accidentally" implies
>>> FOLL_NUMA now).  Not sure whether it's intended, though..
>>
>> That was certainly an oversight, thanks for spotting that. That patch was
>> not supposed to change semantics:
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 76d222ccc3ff..ac926e19ff72 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -851,6 +851,13 @@ struct page *follow_page(struct vm_area_struct *vma,
>> unsigned long address,
>>          if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
>>                  return NULL;
>>
>> +       /*
>> +        * In contrast to get_user_pages() and friends, we don't want to
>> +        * fail if the PTE is PROT_NONE: see gup_can_follow_protnone().
>> +        */
>> +       if (!(foll_flags & FOLL_WRITE))
>> +               foll_flags |= FOLL_FORCE;
>> +
>>          page = follow_page_mask(vma, address, foll_flags, &ctx);
>>          if (ctx.pgmap)
>>                  put_dev_pagemap(ctx.pgmap);
> 
> This seems to be slightly against your other solution though for smaps,
> where we want to avoid abusing FOLL_FORCE.. isn't it..

This is GUP internal, not some arbitrary code, so to me a *completely* 
different discussion.

> 
> Why read only?  That'll always attach FOLL_FORCE to all follow page call
> sites indeed for now, but just curious - logically "I want to fetch the
> page even if protnone" is orthogonal to do with write permission here to
> me.

Historical these were not the semantics, so I won't change them.

FOLL_FORCE | FOLL_WRITE always had a special taste to it (COW ...).

> 
> I still worry about further abuse of FOLL_FORCE, I believe you also worry
> that so you proposed the other way for the smaps issue.
> 
> Do you think we can just revive FOLL_NUMA?  That'll be very clear to me
> from that aspect that we do still have valid use cases for it.

FOLL_NUMA naming was nowadays wrong to begin with (not to mention, 
confusing a we learned). There are other reasons why we have PROT_NONE 
-- mprotect(), for example.

We could have a flag that goes the other way around: 
FOLL_IGNORE_PROTNONE ... which surprisingly then ends up being exactly 
what FOLL_FORCE means without FOLL_WRITE, and what this patch does.

Does that make sense to you?


> 
> The very least is if with above we should really document FOLL_FORCE - we
> should mention NUMA effects.  But that's ... really confusing. Thinking
> about that I personally prefer a revival of FOLL_NUMA, then smaps issue all
> go away.

smaps needs to be changed in any case IMHO. And I'm absolutely not in 
favor of revicing FOLL_NUMA.

-- 
Cheers,

David / dhildenb