linux-kernel - Re: [syzbot] general protection fault in PageHeadHuge

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Date:   Sat, 24 Sep 2022 12:01:16 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        syzbot <syzbot+152d76c44ba142f8992b@...kaller.appspotmail.com>,
        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
Subject: Re: [syzbot] general protection fault in PageHeadHuge

On 09/24/22 11:06, Peter Xu wrote:
> 
> Sorry I forgot to reply on this one.
> 
> I didn't try linux-next, but I can easily reproduce this with mm-unstable
> already, and I verified that Hugh's patch fixes the problem for shmem.
> 
> When I was testing I found hugetlb selftest is broken too but with some
> other errors:
> 
> $ sudo ./userfaultfd hugetlb 100 10  
> ...
> bounces: 6, mode: racing ver read, ERROR: unexpected write fault (errno=0, line=779)
> 
> The failing check was making sure all MISSING events are not triggered by
> writes, but frankly I don't really know why it's required, and that check
> existed since the 1st commit when test was introduced.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c47174fc362a089b1125174258e53ef4a69ce6b8
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/vm/userfaultfd.c?id=c47174fc362a089b1125174258e53ef4a69ce6b8#n291
> 
> And obviously some recent hugetlb-related change caused that to happen.
> 
> Dropping that check can definitely work, but I'll have a closer look soon
> too to make sure I didn't miss something.  Mike, please also let me know if
> you are aware of this problem.
> 

Peter, I am not aware of this problem.  I really should make running ALL
hugetlb tests part of my regular routine.

If you do not beat me to it, I will take a look in the next few days.
-- 
Mike Kravetz