Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[David Hildenbrand]

On 06.09.22 16:30, Jason Gunthorpe wrote:
> On Tue, Sep 06, 2022 at 03:57:30PM +0200, David Hildenbrand wrote:
>
> > > READ_ONCE primarily is a marker that the data being read is unstable
> > > and that the compiler must avoid all instability when reading it. eg
> > > in this case the compiler could insanely double read the value, even
> > > though the 'if' requires only a single read. This would result in
> > > corrupt calculation.
> >
> > As we have a full memory barrier + compile barrier, the compiler might
> > indeed do double reads and all that stuff. BUT, it has to re-read after we
> > incremented the refcount, and IMHO that's the important part to detect the
> > change.
>
> Yes, it is important, but it is not the only important part.
>
> The compiler still has to exectute "if (*a != b)" *correctly*.
>
> This is what READ_ONCE is for. It doesn't set order, it doesn't
> implement a barrier, it tells the compiler that '*a' is unstable data
> and the compiler cannot make assumptions based on the idea that
> reading '*a' multiple times will always return the same value.
>
> If the compiler makes those assumptions then maybe even though 'if (*a
> != b)' is the reality, it could mis-compute '*a == b'. You enter into
> undefined behavior here.
>
> Though it is all very unlikely, the general memory model standard is
> to annotate with READ_ONCE.

The only thing I could see going wrong in the comparison once the stars 
alingn would be something like the following:

if (*a != b)

implemented as

if ((*a).lower != b.lower && (*a).higher != b.higher)


This could only go wrong if we have more than one change such that:

Original:

*a = 0x00000000ffffffffull;


First modification:
*a = 0xffffffffffffffffull;

Second modification:
*a = 0x00000000eeeeeeeeull;


If we race with both modifications, we could see that ffffffff matches, 
and could see that 00000000 matches as well.


So I agree that we should change it, but not necessarily as an urgent 
fix and not necessarily in this patch. It's best to adjust all gup_* 
functions in one patch.

... I do wonder if we want to reuse ptep_get_lockless() instead of the 
READ_ONCE(). CONFIG_GUP_GET_PTE_LOW_HIGH is confusing.

--
Thanks,

David / dhildenb