Re: [syzbot] WARNING in page_counter_cancel (4)

Re: [syzbot] WARNING in page_counter_cancel (4)

[Liam Howlett]

* syzbot <syzbot+28eb226ee1d37f08087a@syzkaller.appspotmail.com> [220718 03:45]:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    cb71b93c2dc3 Add linux-next specific files for 20220628
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15583826080000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=badbc1adb2d582eb
> dashboard link: https://syzkaller.appspot.com/bug?extid=28eb226ee1d37f08087a
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12222b9a080000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1560ab8c080000
> 
> The issue was bisected to:
> 
> commit 2ee236fe53a8e2ab54679c74e8a1fb77e55b29bb
> Author: Liam R. Howlett <Liam.Howlett@Oracle.com>
> Date:   Tue Jun 21 20:46:53 2022 +0000
> 
>     mm: start tracking VMAs with maple tree
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10b5b252080000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=12b5b252080000
> console output: https://syzkaller.appspot.com/x/log.txt?x=14b5b252080000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+28eb226ee1d37f08087a@syzkaller.appspotmail.com
> Fixes: 2ee236fe53a8 ("mm: start tracking VMAs with maple tree")
> 

Thank you.  Yes, it looks like failing the allocation after
vma_adjust_trans_huge() is not working out here.  What happens is a
split fails to vm_adjust, so we enter the __split_vm() error code path
which calls vm_ops->close().  The account for the hughtlb is actually
done in the close() operation.  What I can do is to avoid accounting by
setting vm_start = vm_end, and vm_pgoff = 0.  This way, the accounting
will charge 0 as apposed to removing the vma that was never added.