Re: [PATCH 05/15] mm/hugetlb: keep surplus folios when the pool shrinks

[Muchun Song]

> On Oct 29, 2026, at 04:13, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
> Surplus folios are accounted per node. When shrinking the pool,
> set_max_huge_pages() removes free folios through
> remove_pool_hugetlb_folio() and accounts them as persistent folios.
> 
> It can select a node whose folios are all surplus and leave its surplus
> count larger than its total count. Free surplus folios can back
> reservations created by gather_surplus_pages().
> 
> For example, with nr_overcommit_hugepages = 8 on two nodes:
> 
>  (node columns show total/free/surplus)
> 
>                                      total free surp   node0   node1
>  mmap() of one page, bound to node 0     1    1    1   1 1 1   0 0 0
>  node1/nr_hugepages = 7                  8    8    1   1 1 1   7 7 0
>  nr_hugepages = 6                        7    7    1   0 0 1   7 7 0
>  munmap()                                7    7    1   0 0 1   7 7 0
>  nr_hugepages = 0                        1    1    1   0 0 1   1 1 0
> 
> After shrinking to six persistent folios, node 0 has no folios but its
> surplus count is still one. Releasing the reservation cannot clear that
> count, and shrinking the pool to zero leaves one free page on node 1.
> 
> A later write to node0/nr_hugepages underflows the unsigned per-node
> persistent count: 0 - 1, resulting in allocation of many folios.
> 
> Use persistent_huge_pages_node() to skip nodes with no persistent folios,
> as adjust_pool_surplus() does. This also excludes temporary folios.
> 
> Fixes: e4e574b767ba ("hugetlb: Try to grow hugetlb pool for MAP_SHARED mappings")
> Cc: stable@xxxxxxxxxx
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> [ david: cleanup code and description ]
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.