linux-kernel - Re: [syzbot] WARNING in hugetlb

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Date:   Sat, 29 Oct 2022 17:35:13 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     syzbot <syzbot+f0b97304ef90f0d0b1dc@...kaller.appspotmail.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, llvm@...ts.linux.dev, nathan@...nel.org,
        ndesaulniers@...gle.com, songmuchun@...edance.com,
        syzkaller-bugs@...glegroups.com, trix@...hat.com,
        David Hildenbrand <david@...hat.com>
Subject: Re: [syzbot] WARNING in hugetlb_wp

On 10/28/22 22:15, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    247f34f7b803 Linux 6.1-rc2
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14a9efd2880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a66c6c673fb555e8
> dashboard link: https://syzkaller.appspot.com/bug?extid=f0b97304ef90f0d0b1dc
> 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=112217b4880000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105f4616880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/de212436b09b/disk-247f34f7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/63c4feda220f/vmlinux-247f34f7.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+f0b97304ef90f0d0b1dc@...kaller.appspotmail.com
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 3612 at mm/hugetlb.c:5313 hugetlb_wp+0x20a/0x1af0 mm/hugetlb.c:5313

This warning was added with commit 1d8d14641fd94 ("mm/hugetlb: support write-faults
in shared mappings").  It is there 'by design' as this this specific
type of write fault is not supported.

	/*
	 * hugetlb does not support FOLL_FORCE-style write faults that keep the
	 * PTE mapped R/O such as maybe_mkwrite() would do.
	 */
	if (WARN_ON_ONCE(!unshare && !(vma->vm_flags & VM_WRITE)))
		return VM_FAULT_SIGSEGV;

If there is an actual use case for this support, we can look at adding it.
-- 
Mike Kravetz