Re: [PATCH v4 4/4] hugetlb: allow to free gigantic pages regardless of the configuration

[Alexandre Ghiti]

On 03/01/2019 07:25 AM, Alex Ghiti wrote:
> On 2/28/19 5:26 PM, Mike Kravetz wrote:
> > On 2/28/19 12:23 PM, Dave Hansen wrote:
> > > On 2/28/19 11:50 AM, Mike Kravetz wrote:
> > > > On 2/28/19 11:13 AM, Dave Hansen wrote:
> > > > > > +ÂÂÂ if (hstate_is_gigantic(h) && 
> > > > > > !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
> > > > > > +ÂÂÂÂÂÂÂ spin_lock(&hugetlb_lock);
> > > > > > +ÂÂÂÂÂÂÂ if (count > persistent_huge_pages(h)) {
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ spin_unlock(&hugetlb_lock);
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ return -EINVAL;
> > > > > > +ÂÂÂÂÂÂÂ }
> > > > > > +ÂÂÂÂÂÂÂ goto decrease_pool;
> > > > > > +ÂÂÂ }
> > > > > This choice confuses me. The "Decrease the pool size" code already
> > > > > works and the code just falls through to it after skipping all the
> > > > > "Increase the pool size" code.
> > > > >
> > > > > Why did did you need to add this case so early? Why not just let it
> > > > > fall through like before?
> > > > I assume you are questioning the goto, right? You are correct in that
> > > > it is unnecessary and we could just fall through.
> > > Yeah, it just looked odd to me.
>
> (Dave I do not receive your answers, I don't know why).

I collected mistakes here: domain name expired and no mailing list added :)
Really sorry about that, I missed the whole discussion (if any).
Could someone forward it to me (if any) ? Thanks !

> I'd rather avoid useless checks when we already know they won't
> be met and I think that makes the code more understandable.
>
> But that's up to you for the next version.
>
> Thanks
> > > > However, I wonder if we might want to consider a wacky condition 
> > > > that the
> > > > above check would prevent. Consider a system/configuration with 5 
> > > > gigantic
> > > > pages allocated at boot time. Also CONFIG_CONTIG_ALLOC is not 
> > > > enabled, so
> > > > it is not possible to allocate gigantic pages after boot.
> > > >
> > > > Suppose the admin decreased the number of gigantic pages to 3. 
> > > > However, all
> > > > gigantic pages were in use. So, 2 gigantic pages are now 'surplus'.
> > > > h->nr_huge_pages == 5 and h->surplus_huge_pages == 2, so
> > > > persistent_huge_pages() == 3.
> > > >
> > > > Now suppose the admin wanted to increase the number of gigantic 
> > > > pages to 5.
> > > > The above check would prevent this. However, we do not need to really
> > > > 'allocate' two gigantic pages. We can simply convert the surplus 
> > > > pages.
> > > >
> > > > I admit this is a wacky condition. The ability to 'free' gigantic 
> > > > pages
> > > > at runtime if !CONFIG_CONTIG_ALLOC makes it possible. I don't 
> > > > necessairly
> > > > think we should consider this. hugetlbfs code just makes me think of
> > > > wacky things. :)
> > > I think you're saying that the newly-added check is overly-restrictive.
> > > Â If we "fell through" like I was suggesting we would get better 
> > > behavior.
> > At first, I did not think it overly restrictive. But, I believe we can
> > just eliminate that check for gigantic pages. If 
> > !CONFIG_CONTIG_ALLOC and
> > this is a request to allocate more gigantic pages, 
> > alloc_pool_huge_page()
> > should return NULL.
> >
> > The only potential issue I see is that in the past we have returned 
> > EINVAL
> > if !CONFIG_CONTIG_ALLOC and someone attempted to increase the pool size.
> > Now, we will not increase the pool and will not return an error. Not 
> > sure
> > if that is an acceptable change in user behavior.
>
> If I may, I think that this is the kind of info the user wants to have 
> and we should
> return an error when it is not possible to allocate runtime huge pages.
> I already noticed that if someone asks for 10 huge pages, and only 5 
> are allocated,
> no error is returned to the user and I found that surprising.
>
> > If we go down this path, then we could remove this change as well:
>
> I agree that in that path, we do not need the following change neither.
>
> > > @@ -2428,7 +2442,9 @@ static ssize_t 
> > > __nr_hugepages_store_common(bool obey_mempolicy,
> > > ÂÂÂÂÂ } else
> > > ÂÂÂÂÂÂÂÂÂ nodes_allowed = &node_states[N_MEMORY];
> > > Â -ÂÂÂ h->max_huge_pages = set_max_huge_pages(h, count, nodes_allowed);
> > > +ÂÂÂ err = set_max_huge_pages(h, count, nodes_allowed);
> > > +ÂÂÂ if (err)
> > > +ÂÂÂÂÂÂÂ goto out;
> > > Â ÂÂÂÂÂ if (nodes_allowed != &node_states[N_MEMORY])
> > > ÂÂÂÂÂÂÂÂÂ NODEMASK_FREE(nodes_allowed);
> > Do note that I beleive there is a bug the above change. The code after
> > the out label is:
> >
> > out:
> > ÂÂÂÂÂÂÂÂ NODEMASK_FREE(nodes_allowed);
> > ÂÂÂÂÂÂÂÂ return err;
> > }
> >
> > With the new goto, we need the same
> > if (nodes_allowed != &node_states[N_MEMORY]) before NODEMASK_FREE().
> >
> > Sorry, I missed this in previous versions.
>
> Oh right, I'm really sorry I missed that, thank you for noticing.