Re: [PATCH 1/2] drm/i915: set O_LARGEFILE in __create_shmem()

["陈涛涛 Taotao Chen" <chentaotao@didiglobal.com>]

From: Taotao Chen <chentaotao@didiglobal.com>

Hi Andi,

> Hi Taotao,
> 
>> Reported-by: kernel test robot <oliver.sang@intel.com>
>> Closes: https://lore.kernel.org/oe-lkp/202508081029.343192ec-lkp@intel.com
>
> ...
> 
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> index e3d188455f67..2b53aad915f5 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>> @@ -514,6 +514,11 @@ static int __create_shmem(struct drm_i915_private *i915,
>>  	if (IS_ERR(filp))
>>  		return PTR_ERR(filp);
>>  
>> +	/*
>> +	 * Prevent -EFBIG by allowing large writes beyond MAX_NON_LFS on shmem
>> +	 * objects by setting O_LARGEFILE.
>> +	 */
>> +	filp->f_flags |= O_LARGEFILE;
>
> I don't have anything against this, but is it really fixing the
> issue? I thought that O_LARGEFILE is ignored in 64 bit machines,
> while here the failure is happening in 64 bit machines.

As mentioned in the commit body, without O_LARGEFILE, file->f_op->write_iter
calls generic_write_check_limits(), which enforces the 2GB (MAX_NON_LFS) limit
and causes -EFBIG on large writes.

On 64-bit systems O_LARGEFILE is still set when opening files (e.g. via open()),
so we also need to set it here for shmem objects created inside the kernel.

However, on older 32-bit systems, setting O_LARGEFILE unconditionally may be risky.
Previously I did not check this, but to reduce the risk a safer approach is to wrap
it in a check, for example:

+	if (force_o_largefile())
+		filp->f_flags |= O_LARGEFILE;

>
> Besides, where do you see in the LKP logs the -EFBIG error
> message?
>

Due to the previous return order in shmem_pwrite(), this -EFBIG was being overwritten
by -EIO on short writes. This issue will be fixed in PATCH 2/2.

Taotao

> Andi
>
>>  	obj->filp = filp;
>>  	return 0;
>>  }
>> -- 
>> 2.34.1