On 27.07.23 22:30, Peter Xu wrote:
On Thu, Jul 27, 2023 at 09:17:45PM +0200, David Hildenbrand wrote:
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.
Because the old code was written like that?
And it's not 2014 anymore. Nowadays we do have the right helper in place.
[...]
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.
It doesn't really violate with the name, IMHO - protnone can be either numa
hint or PROT_NONE for real. As long as we return NULL for a FOLL_NUMA
request we're achieving the goal we want - we guarantee a NUMA balancing to
trigger with when FOLL_NUMA provided. It doesn't need to guarantee
anything else, afaiu. The final check relies in vma_is_accessible() in the
fault paths anyway. So I don't blame the old name that much.
IMHO, the name FOLL_NUMA made sense when it still was called pte_numa.
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.
As stated above, to me FOLL_NUMA is all fine and clear. If you think
having a flag back for protnone is worthwhile no matter as-is (FOLL_NUMA)
or with reverted meaning, then that sounds all fine to me. Maybe the old
name at least makes old developers know what's that.
I don't have a strong opinion on names though; mostly never had.
I'll avoid new FOLL_ flags first and post my proposal. If many people
are unhappy with that approach, we can revert the commit and call it a day.
Thanks!
--
Cheers,
David / dhildenb
