On Mon, Oct 21, 2024 at 07:01:59PM +0200, Vlastimil Babka wrote:
> On 10/21/24 18:48, Roman Gushchin wrote:
> > Syzbot reported [1] a bad page state problem caused by a page
> > being freed using free_page() still having a mlocked flag at
> > free_pages_prepare() stage:
> >
> > BUG: Bad page state in process syz.0.15 pfn:1137bb
> > page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff8881137bb870 pfn:0x1137bb
> > flags: 0x400000000080000(mlocked|node=0|zone=1)
> > raw: 0400000000080000 0000000000000000 dead000000000122 0000000000000000
> > raw: ffff8881137bb870 0000000000000000 00000000ffffffff 0000000000000000
> > page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
> > page_owner tracks the page as allocated
> > page last allocated via order 0, migratetype Unmovable, gfp_mask
> > 0x400dc0(GFP_KERNEL_ACCOUNT|__GFP_ZERO), pid 3005, tgid
> > 3004 (syz.0.15), ts 61546 608067, free_ts 61390082085
> > set_page_owner include/linux/page_owner.h:32 [inline]
> > post_alloc_hook+0x1f3/0x230 mm/page_alloc.c:1537
> > prep_new_page mm/page_alloc.c:1545 [inline]
> > get_page_from_freelist+0x3008/0x31f0 mm/page_alloc.c:3457
> > __alloc_pages_noprof+0x292/0x7b0 mm/page_alloc.c:4733
> > alloc_pages_mpol_noprof+0x3e8/0x630 mm/mempolicy.c:2265
> > kvm_coalesced_mmio_init+0x1f/0xf0 virt/kvm/coalesced_mmio.c:99
> > kvm_create_vm virt/kvm/kvm_main.c:1235 [inline]
> > kvm_dev_ioctl_create_vm virt/kvm/kvm_main.c:5500 [inline]
> > kvm_dev_ioctl+0x13bb/0x2320 virt/kvm/kvm_main.c:5542
> > vfs_ioctl fs/ioctl.c:51 [inline]
> > __do_sys_ioctl fs/ioctl.c:907 [inline]
> > __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
> > do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> > do_syscall_64+0x69/0x110 arch/x86/entry/common.c:83
> > entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > page last free pid 951 tgid 951 stack trace:
> > reset_page_owner include/linux/page_owner.h:25 [inline]
> > free_pages_prepare mm/page_alloc.c:1108 [inline]
> > free_unref_page+0xcb1/0xf00 mm/page_alloc.c:2638
> > vfree+0x181/0x2e0 mm/vmalloc.c:3361
> > delayed_vfree_work+0x56/0x80 mm/vmalloc.c:3282
> > process_one_work kernel/workqueue.c:3229 [inline]
> > process_scheduled_works+0xa5c/0x17a0 kernel/workqueue.c:3310
> > worker_thread+0xa2b/0xf70 kernel/workqueue.c:3391
> > kthread+0x2df/0x370 kernel/kthread.c:389
> > ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
> > ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> >
> > The problem was originally introduced by
> > commit b109b87050df ("mm/munlock: replace clear_page_mlock() by final
> > clearance"): it was handling focused on handling pagecache
> > and anonymous memory and wasn't suitable for lower level
> > get_page()/free_page() API's used for example by KVM, as with
> > this reproducer.
>
> Does that mean KVM is mlocking pages that are not pagecache nor anonymous,
> thus not LRU? How and why (and since when) is that done?
KVM allows to mmap and mlock several pages allocated directly.
Please, take a look at the reproducer:
https://syzkaller.appspot.com/x/repro.c?x=1437939f980000
>
> > Fix it by moving the mlocked flag clearance down to
> > free_page_prepare().
> >
> > The bug itself if fairly old and harmless (aside from generating these
> > warnings), so the stable backport is likely not justified.
>
> But since there's a Cc: stable below, it will be backported :)
My bad, I changed my mind in the last minute and added Cc: stable but
forgot to drop this sentence.
>
> > Closes: https://syzkaller.appspot.com/x/report.txt?x=169a47d0580000
> > Fixes: b109b87050df ("mm/munlock: replace clear_page_mlock() by final clearance")
> > Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > ---
> > mm/page_alloc.c | 9 +++++++++
> > mm/swap.c | 14 --------------
> > 2 files changed, 9 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index bc55d39eb372..24200651ad92 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1044,6 +1044,7 @@ __always_inline bool free_pages_prepare(struct page *page,
> > bool skip_kasan_poison = should_skip_kasan_poison(page);
> > bool init = want_init_on_free();
> > bool compound = PageCompound(page);
> > + struct folio *folio = page_folio(page);
> >
> > VM_BUG_ON_PAGE(PageTail(page), page);
> >
> > @@ -1053,6 +1054,14 @@ __always_inline bool free_pages_prepare(struct page *page,
> > if (memcg_kmem_online() && PageMemcgKmem(page))
> > __memcg_kmem_uncharge_page(page, order);
> >
> > + if (unlikely(folio_test_mlocked(folio))) {
> > + long nr_pages = folio_nr_pages(folio);
> > +
> > + __folio_clear_mlocked(folio);
> > + zone_stat_mod_folio(folio, NR_MLOCK, -nr_pages);
> > + count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
> > + }
>
> Why drop the useful comment?
Agree. Sounds like I need to restore the comment, drop no stable backport
recommendation and send v2.
Thank you for taking a look!
