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@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxxxxxxxx: delete weird BUILD_BUG_ON()]
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Acked-by: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> ...
>
> 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
