Re: [PATCH] mm/cma.c: use exact_nid true to fix possible per-numa cma leak

[Roman Gushchin <guro@fb.com>]

On Tue, Jun 30, 2020 at 07:09:31PM -0700, Andrew Morton wrote:
> On Tue, 30 Jun 2020 12:08:25 -0700 Roman Gushchin <guro@fb.com> wrote:
> 
> > On Sun, Jun 28, 2020 at 07:43:45PM +1200, Barry Song wrote:
> > > Calling cma_declare_contiguous_nid() with false exact_nid for per-numa
> > > reservation can easily cause cma leak and various confusion.
> > > For example, mm/hugetlb.c is trying to reserve per-numa cma for gigantic
> > > pages. But it can easily leak cma and make users confused when system has
> > > memoryless nodes.
> > > 
> > > In case the system has 4 numa nodes, and only numa node0 has memory.
> > > if we set hugetlb_cma=4G in bootargs, mm/hugetlb.c will get 4 cma areas
> > > for 4 different numa nodes. since exact_nid=false in current code, all
> > > 4 numa nodes will get cma successfully from node0, but hugetlb_cma[1 to 3]
> > > will never be available to hugepage will only allocate memory from
> > > hugetlb_cma[0].
> > > 
> > > In case the system has 4 numa nodes, both numa node0&2 has memory, other
> > > nodes have no memory.
> > > if we set hugetlb_cma=4G in bootargs, mm/hugetlb.c will get 4 cma areas
> > > for 4 different numa nodes. since exact_nid=false in current code, all
> > > 4 numa nodes will get cma successfully from node0 or 2, but hugetlb_cma[1]
> > > and [3] will never be available to hugepage as mm/hugetlb.c will only
> > > allocate memory from hugetlb_cma[0] and hugetlb_cma[2].
> > > This causes permanent leak of the cma areas which are supposed to be
> > > used by memoryless node.
> > > 
> > > Of cource we can workaround the issue by letting mm/hugetlb.c scan all
> > > cma areas in alloc_gigantic_page() even node_mask includes node0 only.
> > > that means when node_mask includes node0 only, we can get page from
> > > hugetlb_cma[1] to hugetlb_cma[3]. But this will cause kernel crash in
> > > free_gigantic_page() while it wants to free page by:
> > > cma_release(hugetlb_cma[page_to_nid(page)], page, 1 << order)
> > > 
> > > On the other hand, exact_nid=false won't consider numa distance, it
> > > might be not that useful to leverage cma areas on remote nodes.
> > > I feel it is much simpler to make exact_nid true to make everything
> > > clear. After that, memoryless nodes won't be able to reserve per-numa
> > > CMA from other nodes which have memory.
> > 
> > Totally agree.
> > 
> > Acked-by: Roman Gushchin <guro@fb.com>
> > 
> 
> Do we feel this merits a cc:stable?

It would be nice.

Thanks!