On Tue, 9 Jul 2024, Miaohe Lin wrote: > A kernel crash was observed when migrating hugetlb folio: > > BUG: kernel NULL pointer dereference, address: 0000000000000008 > PGD 0 P4D 0 > Oops: Oops: 0002 [#1] PREEMPT SMP NOPTI > CPU: 0 PID: 3435 Comm: bash Not tainted 6.10.0-rc6-00450-g8578ca01f21f #66 > RIP: 0010:__folio_undo_large_rmappable+0x70/0xb0 > RSP: 0018:ffffb165c98a7b38 EFLAGS: 00000097 > RAX: fffffbbc44528090 RBX: 0000000000000000 RCX: 0000000000000000 > RDX: ffffa30e000a2800 RSI: 0000000000000246 RDI: ffffa3153ffffcc0 > RBP: fffffbbc44528000 R08: 0000000000002371 R09: ffffffffbe4e5868 > R10: 0000000000000001 R11: 0000000000000001 R12: ffffa3153ffffcc0 > R13: fffffbbc44468000 R14: 0000000000000001 R15: 0000000000000001 > FS: 00007f5b3a716740(0000) GS:ffffa3151fc00000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 0000000000000008 CR3: 000000010959a000 CR4: 00000000000006f0 > Call Trace: > <TASK> > __folio_migrate_mapping+0x59e/0x950 > __migrate_folio.constprop.0+0x5f/0x120 > move_to_new_folio+0xfd/0x250 > migrate_pages+0x383/0xd70 > soft_offline_page+0x2ab/0x7f0 > soft_offline_page_store+0x52/0x90 > kernfs_fop_write_iter+0x12c/0x1d0 > vfs_write+0x380/0x540 > ksys_write+0x64/0xe0 > do_syscall_64+0xb9/0x1d0 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > RIP: 0033:0x7f5b3a514887 > RSP: 002b:00007ffe138fce68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 > RAX: ffffffffffffffda RBX: 000000000000000c RCX: 00007f5b3a514887 > RDX: 000000000000000c RSI: 0000556ab809ee10 RDI: 0000000000000001 > RBP: 0000556ab809ee10 R08: 00007f5b3a5d1460 R09: 000000007fffffff > R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000000c > R13: 00007f5b3a61b780 R14: 00007f5b3a617600 R15: 00007f5b3a616a00 > > It's because hugetlb folio is passed to __folio_undo_large_rmappable() > unexpectedly. large_rmappable flag is imperceptibly set to hugetlb folio > since commit f6a8dd98a2ce ("hugetlb: convert alloc_buddy_hugetlb_folio to > use a folio"). Then commit be9581ea8c05 ("mm: fix crashes from deferred > split racing folio migration") makes folio_migrate_mapping() call > folio_undo_large_rmappable() triggering the bug. Fix this issue by > clearing large_rmappable flag for hugetlb folios. They don't need that > flag set anyway. Gosh, thanks a lot for catching this: it had not crossed my mind that a folio which passes (folio_test_large and) folio_test_large_rmappable might not be suitable for folio_undo_large_rmappable. > > Fixes: f6a8dd98a2ce ("hugetlb: convert alloc_buddy_hugetlb_folio to use a folio") That's in 6.10-rc, isn't it? > Fixes: be9581ea8c05 ("mm: fix crashes from deferred split racing folio migration") And that's in mm-hotfixes-stable intended for 6.10 final. > Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> So if all goes to plan, this shouldn't need the Cc stable. I certainly deserve blame for not thinking of this possibility: but how was it working before my commit, when the folio_undo_large_rmappable() was being called from mem_cgroup_migrate()? I think that was just as liable to crash too. I would like to hear definitively from Matthew, whether a hugetlb page should or should not be reported as large_rmappable - is your patch here just fixing a surprise, or in danger of adding another surprise somewhere? Hugh > --- > mm/hugetlb.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 6282dd9e37e3..45fd3bc75332 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -2173,6 +2173,9 @@ static struct folio *alloc_buddy_hugetlb_folio(struct hstate *h, > nid = numa_mem_id(); > retry: > folio = __folio_alloc(gfp_mask, order, nid, nmask); > + /* Ensure hugetlb folio won't have large_rmappable flag set. */ > + if (folio) > + folio_clear_large_rmappable(folio); > > if (folio && !folio_ref_freeze(folio, 1)) { > folio_put(folio); > -- > 2.33.0