Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Michal Hocko]

On Thu 01-06-17 14:00:48, Mike Rapoport wrote:
> On Wed, May 31, 2017 at 10:24:14AM +0200, Michal Hocko wrote:
> > On Wed 31-05-17 08:30:08, Vlastimil Babka wrote:
> > > On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> > > > 
> > > > I'm not sure if it should be considered a bug, the prctl is intended
> > > > to use normally by wrappers so it looks optimal as implemented this
> > > > way: affecting future vmas only, which will all be created after
> > > > execve executed by the wrapper.
> > > > 
> > > > What's the point of messing with the prctl so it mangles over the
> > > > wrapper process own vmas before exec? Messing with those vmas is pure
> > > > wasted CPUs for the wrapper use case which is what the prctl was
> > > > created for.
> > > > 
> > > > Furthermore there would be the risk a program that uses the prctl not
> > > > as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> > > > def_flags assuming the current kABI. The program could assume those
> > > > vmas that were instantiated before disabling the prctl are still with
> > > > VM_NOHUGEPAGE set (they would not after the change you propose).
> > > > 
> > > > Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> > > > on existing vmas looks more complex too and less finegrined so
> > > > probably more complex for userland to manage
> > > 
> > > I would expect the prctl wouldn't iterate all vma's, nor would it modify
> > > def_flags anymore. It would just set a flag somewhere in mm struct that
> > > would be considered in addition to the per-vma flags when deciding
> > > whether to use THP.
> > 
> > Exactly. Something like the below (not even compile tested).
>  
> I did a quick go with the patch, compiles just fine :)
> It worked for my simple examples, the THP is enabled/disabled as expected
> and the vma->vm_flags are indeed unaffected.
> 
> > > We could consider whether MADV_HUGEPAGE should be
> > > able to override the prctl or not.
> > 
> > This should be a master override to any per vma setting.
> 
> Here you've introduced a change to the current behaviour. Consider the
> following sequence:
> 
> {
> 	prctl(PR_SET_THP_DISABLE);
> 	address = mmap(...);
> 	madvise(address, len, MADV_HUGEPAGE);
> }
>
> Currently, for the vma that backs the address
> transparent_hugepage_enabled(vma) will return true, and after your patch it
> will return false.
> The new behaviour may be more correct, I just wanted to bring the change to
> attention. 

The system wide disable should override any VMA specific setting
IMHO. Why would we disable the THP for the whole process otherwise?
Anyway this needs to be discussed at linux-api mailing list. I will try
to make my change into a proper patch and post it there.
-- 
Michal Hocko
SUSE Labs