linux-kernel - Re: [shmem] 7a7256d5f5: WARNING:possible_recursive_locking

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Date:   Fri, 21 Oct 2022 17:02:57 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Ira Weiny <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        kernel test robot <yujie.liu@...el.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        lkp@...ts.01.org, lkp@...el.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Peter Xu <peterx@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [shmem] 7a7256d5f5: WARNING:possible_recursive_locking_detected



On 10/21/22 17:00, Ira Weiny wrote:

> 
> I was about to get this patch ready to send when I found this:
> 
> commit b6ebaedb4cb1a18220ae626c3a9e184ee39dd248
> Author: Andrea Arcangeli <aarcange@...hat.com>
> Date:   Fri Sep 4 15:47:08 2015 -0700
> 
>     userfaultfd: avoid mmap_sem read recursion in mcopy_atomic
> 
>     If the rwsem starves writers it wasn't strictly a bug but lockdep
>     doesn't like it and this avoids depending on lowlevel implementation  
>     details of the lock.
>     
>     [akpm@...ux-foundation.org: delete weird BUILD_BUG_ON()]
>     Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
>     Acked-by: Pavel Emelyanov <xemul@...allels.com>
> ...
> 
> So I wonder if the true fix is something to lockdep?
> 
> Regardless I'll send the below patch because it will restore things to a
> working order.


with ; added to each + line.

> But I'm CC'ing Andrea for comments.
> 
> Ira
> 
>>
>> Ira
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 8280a5cb48df..6c8e99bf5983 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -2424,9 +2424,11 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
>>
>>                 if (!zeropage) {        /* COPY */
>>                         page_kaddr = kmap_local_folio(folio, 0);
>> +                       pagefault_disable()
>>                         ret = copy_from_user(page_kaddr,
>>                                              (const void __user *)src_addr,
>>                                              PAGE_SIZE);
>> +                       pagefault_enable()
>>                         kunmap_local(page_kaddr);
>>
>>                         /* fallback to copy_from_user outside mmap_lock */
>>
>>

-- 
~Randy