Re: [mm-unstable PATCH v4 1/9] mm/hugetlb: check gigantic_page_runtime_supported() in return_unused_surplus_pages()

[HORIGUCHI NAOYA(堀口 直也)]

On Tue, Jul 05, 2022 at 10:16:39AM +0800, Miaohe Lin wrote:
> On 2022/7/4 9:33, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > 
> > I found a weird state of 1GB hugepage pool, caused by the following
> > procedure:
> > 
> >   - run a process reserving all free 1GB hugepages,
> >   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
> >     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
> >   - kill the reserving process.
> > 
> > , then all the hugepages are free *and* surplus at the same time.
> > 
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
> >   0
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
> >   3
> > 
> > This state is resolved by reserving and allocating the pages then
> > freeing them again, so this seems not to result in serious problem.
> > But it's a little surprising (shrinking pool suddenly fails).
> > 
> > This behavior is caused by hstate_is_gigantic() check in
> > return_unused_surplus_pages(). This was introduced so long ago in 2008
> > by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> > at that time the gigantic pages were not supposed to be allocated/freed
> > at run-time.  Now kernel can support runtime allocation/free, so let's
> > check gigantic_page_runtime_supported() together.
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> 
> This patch looks good to me with a few question below.

Thank you for reviewing.

> 
> > ---
> > v2 -> v3:
> > - Fixed typo in patch description,
> > - add !gigantic_page_runtime_supported() check instead of removing
> >   hstate_is_gigantic() check (suggested by Miaohe and Muchun)
> > - add a few more !gigantic_page_runtime_supported() check in
> >   set_max_huge_pages() (by Mike).
> > ---
> >  mm/hugetlb.c | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 2a554f006255..bdc4499f324b 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2432,8 +2432,7 @@ static void return_unused_surplus_pages(struct hstate *h,
> >  	/* Uncommit the reservation */
> >  	h->resv_huge_pages -= unused_resv_pages;
> >  
> > -	/* Cannot return gigantic pages currently */
> > -	if (hstate_is_gigantic(h))
> > +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> >  		goto out;
> >  
> >  	/*
> > @@ -3315,7 +3314,8 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
> >  	 * the user tries to allocate gigantic pages but let the user free the
> >  	 * boottime allocated gigantic pages.
> >  	 */
> > -	if (hstate_is_gigantic(h) && !IS_ENABLED(CONFIG_CONTIG_ALLOC)) {
> > +	if (hstate_is_gigantic(h) && (!IS_ENABLED(CONFIG_CONTIG_ALLOC) ||
> > +				      !gigantic_page_runtime_supported())) {
> >  		if (count > persistent_huge_pages(h)) {
> >  			spin_unlock_irq(&hugetlb_lock);
> >  			mutex_unlock(&h->resize_lock);
> > @@ -3363,6 +3363,19 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
> >  			goto out;
> >  	}
> >  
> > +	/*
> > +	 * We can not decrease gigantic pool size if runtime modification
> > +	 * is not supported.
> > +	 */
> > +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported()) {
> > +		if (count < persistent_huge_pages(h)) {
> > +			spin_unlock_irq(&hugetlb_lock);
> > +			mutex_unlock(&h->resize_lock);
> > +			NODEMASK_FREE(node_alloc_noretry);
> > +			return -EINVAL;
> > +		}
> > +	}
> 
> With above change, we're not allowed to decrease the pool size now. But it was allowed previously
> even if !gigantic_page_runtime_supported. Does this will break user?

Yes, it does. I might get the wrong idea about the definition of
gigantic_page_runtime_supported(), which shows that runtime pool *extension*
is supported or not (implying that pool shrinking is always possible).
If this is right, this new if-block is not necessary.

> 
> And it seems it's not allowed to adjust the max_huge_pages now if !gigantic_page_runtime_supported
> for gigantic huge page. Should we just return for such case as there should be nothing to do now?
> Or am I miss something?

If pool shrinking is always allowed, we need uptdate max_huge_pages so,
the above if-block should have "goto out;", but it will be removed anyway
so we don't have to care for it.

Thank you for the valuable comment.

- Naoya Horiguchi