linux-kernel - Re: [PATCH] userfaultfd: fix a race between writeprotect and exit

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Date:   Thu, 23 Sep 2021 20:16:19 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Li Wang <liwang@...hat.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] userfaultfd: fix a race between writeprotect and
 exit_mmap()

On Wed, Sep 22, 2021 at 05:06:53PM +0800, Li Wang wrote:
> Hi,

Li,

> 
> I confirmed this patch (applied on 5.14) gets rid of the below userfaultfd
> test failure.
> 
> # ./userfaultfd anon 16 2
> nr_pages: 4096, nr_pages_per_cpu: 256
> bounces: 1, mode: rnd read, userfaults: 313 missing
> (51+34+37+26+41+28+15+20+16+12+13+7+10+2+0+1) 995 wp
> (121+79+96+53+90+104+48+61+56+82+56+41+49+26+11+22)
> bounces: 0, mode: read, userfaults: 64 missing
> (15+8+10+6+5+2+4+3+3+1+4+0+0+2+0+1) 2157 wp
> (223+274+189+141+116+132+203+153+143+126+110+114+101+66+42+24)
> testing uffd-wp with pagemap (pgsize=4096): done
> testing uffd-wp with pagemap (pgsize=2097152): done
> testing UFFDIO_ZEROPAGE: done.
> testing signal delivery: done.
> testing events (fork, remap, remove): ERROR: nr 3933 memory corruption 0 1
>  (errno=0, line=963)
> ERROR: faulting process failed (errno=0, line=1117)

Just to keep a record within this thread - my understanding is above issue is a
separate issue from what Nadav has fixed.  The other fix could be:

  https://lore.kernel.org/lkml/20210923232512.210092-1-peterx@redhat.com/

When verify with Nadav's patch, please check whether you have thp enabled
globally:

  # echo always > /sys/kernel/mm/transparent_hugepage/enabled

Thanks,

-- 
Peter Xu