Re: [PATCH v2] mm/hugetlb: fix memory offline failure due to hwpoisoned file hugetlb

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/21/26 03:10, Jinjiang Tu wrote:
> When a file hugetlb folio triggers UCE, me_huge_page() will keep the
> hugetlb folio in pagcahe with refcount increased and PG_hwpoison set. Even
> after the hugetlb file is deleted, the hugetlb folio is still leaked.
> 
> If we want to offline the memory block that the hwpoisoned hugetlb folio
> belongs to, it fails in dissolve_free_hugetlb_folios() due to the
> hwpoisoned hugetlb folio isn't free.
> 
> I can reproduce this issue with the following steps in qemu:
>  1) echo offline >/sys/devices/system/memory/auto_online_blocks
>  2) in qemu monitor:
>        object_add memory-backend-ram,id=mem10,size=1G
>        device_add pc-dimm,id=dimm1,memdev=mem10,node=2
>  3) echo online_movable > /sys/devices/system/node/node2/memory136/state
>  4) echo 5 > /sys/devices/system/node/node2/hugepages/hugepages-2048kB/nr_hugepages
>  5) run ./hugetlb_file. This process will receive SIGBUS.
>  6) remove the hugetlbfs file.
>  7) echo offline > /sys/devices/system/node/node2/memory136/state
> 
> hugetlb_file.c:
>   fd = open("/dev/hugepages/my_hugepage_file", O_CREAT | O_RDWR, 0755);
>   fallocate(fd, 0, 0, HUGEPAGE_SIZE * 2);
>   addr = mmap(NULL, HUGEPAGE_SIZE * 2, PROT_READ | PROT_WRITE,
> 		MAP_SHARED | MAP_HUGETLB, fd, 0);
>   memset(addr, 0xaa, HUGEPAGE_SIZE * 2);
>   madvise(addr, HUGEPAGE_SIZE, MADV_HWPOISON);
> 
> To fix it, force to put ref of hwpoisoned hugetlb in memory offline, the
> hwpoisoned hugetlb will be freed and succeeds to be dissolved. We couldn't
> avoid races here, just like commit b023f46813cd ("memory-hotplug: skip
> HWPoisoned page when offlining pages"), which force to skip hwpoisoned
> page regardless of refcount.

I always considered that handling quite dubious. Just because a page has
hwpoisoned set doesn't mean that we can just offline it.

I think that mus be cleaned up at some point.

But not sure how to do this cleanly.

Why do we even care about offlining memory with hwpoisoned pages? What
is the use case for your change?

I know, it's very desirable to do it, but I much rather have it not
working then having something that is likely mostly broken and actually
might cause harm.

-- 
Cheers,

David