RE: [PATCH] mm/hugetlb: avoid hardcoding while checking if cma is reserved

["Song Bao Hua (Barry Song)" <song.bao.hua@xxxxxxxxxxxxx>]

> -----Original Message-----
> From: Roman Gushchin [mailto:guro@xxxxxx]
> Sent: Tuesday, July 7, 2020 9:48 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Linuxarm <linuxarm@xxxxxxxxxx>; Mike
> Kravetz <mike.kravetz@xxxxxxxxxx>; Jonathan Cameron
> <jonathan.cameron@xxxxxxxxxx>
> Subject: Re: [PATCH] mm/hugetlb: avoid hardcoding while checking if cma is
> reserved
> 
> On Mon, Jul 06, 2020 at 08:44:05PM +1200, Barry Song wrote:
> 
> Hello, Barry!
> 
> > hugetlb_cma[0] can be NULL due to various reasons, for example, node0 has
> > no memory. Thus, NULL hugetlb_cma[0] doesn't necessarily mean cma is not
> > enabled. gigantic pages might have been reserved on other nodes.
> 
> Just curious, is it a real-life problem you've seen? If so, I wonder how
> you're using the hugetlb_cma option, and what's the outcome?

Yes. It is kind of stupid but I once got a board on which node0 has no DDR
though node1 and node3 have memory.

I actually prefer we get cma size of per node by:
cma size of one node = hugetlb_cma/ (nodes with memory)
rather than:
cma size of one node = hugetlb_cma/ (all online nodes)

but unfortunately, or the N_MEMORY infrastructures are not ready yet. I mean:

for_each_node_state(nid, N_MEMORY) {
		int res;

		size = min(per_node, hugetlb_cma_size - reserved);
		size = round_up(size, PAGE_SIZE << order);

		res = cma_declare_contiguous_nid(0, size, 0, PAGE_SIZE << order,
						 0, false, "hugetlb",
						 &hugetlb_cma[nid], nid);
		...
	}

> 
> >
> > Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages
> using cma")
> > Cc: Roman Gushchin <guro@xxxxxx>
> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Cc: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
> > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 57ece74e3aae..603aa854aa89 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2571,9 +2571,21 @@ static void __init
> hugetlb_hstate_alloc_pages(struct hstate *h)
> >
> >  	for (i = 0; i < h->max_huge_pages; ++i) {
> >  		if (hstate_is_gigantic(h)) {
> > -			if (IS_ENABLED(CONFIG_CMA) && hugetlb_cma[0]) {
> > -				pr_warn_once("HugeTLB: hugetlb_cma is enabled, skip
> boot time allocation\n");
> > -				break;
> > +			if (IS_ENABLED(CONFIG_CMA)) {
> > +				int nid;
> > +				bool cma_reserved = false;
> > +
> > +				for_each_node_state(nid, N_ONLINE) {
> > +					if (hugetlb_cma[nid]) {
> > +						pr_warn_once("HugeTLB: hugetlb_cma is
> reserved,"
> > +								"skip boot time allocation\n");
> > +						cma_reserved = true;
> > +						break;
> > +					}
> > +				}
> > +
> > +				if (cma_reserved)
> > +					break;
> 
> It's a valid problem, and I like to see it fixed. But I wonder if it would be better
> to introduce a new helper bool hugetlb_cma_enabled()? And move both
> IS_ENABLED(CONFIG_CMA)
> and hugetlb_cma[nid] checks there?

Yep. that would be more readable.

> 
> Thank you!

Thanks
Barry