linux-kernel - Re: [PATCH] mm: clear __GFP_FS when PF_MEMALLOC

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Date:	Fri, 05 Sep 2014 13:13:59 +0800
From:	Junxiao Bi <junxiao.bi@...cle.com>
To:	Dave Chinner <david@...morbit.com>
CC:	akpm@...ux-foundation.org, xuejiufei@...wei.com,
	ming.lei@...onical.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mm: clear __GFP_FS when PF_MEMALLOC_NOIO is set

On 09/05/2014 10:32 AM, Junxiao Bi wrote:
> On 09/04/2014 05:23 PM, Dave Chinner wrote:
>> On Wed, Sep 03, 2014 at 01:54:54PM +0800, Junxiao Bi wrote:
>>> commit 21caf2fc1931 ("mm: teach mm by current context info to not do I/O during memory allocation")
>>> introduces PF_MEMALLOC_NOIO flag to avoid doing I/O inside memory allocation, __GFP_IO is cleared
>>> when this flag is set, but __GFP_FS implies __GFP_IO, it should also be cleared. Or it may still
>>> run into I/O, like in superblock shrinker.
>>>
>>> Signed-off-by: Junxiao Bi <junxiao.bi@...cle.com>
>>> Cc: joyce.xue <xuejiufei@...wei.com>
>>> Cc: Ming Lei <ming.lei@...onical.com>
>>> ---
>>>  include/linux/sched.h |    6 ++++--
>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 5c2c885..2fb2c47 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -1936,11 +1936,13 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
>>>  #define tsk_used_math(p) ((p)->flags & PF_USED_MATH)
>>>  #define used_math() tsk_used_math(current)
>>>  
>>> -/* __GFP_IO isn't allowed if PF_MEMALLOC_NOIO is set in current->flags */
>>> +/* __GFP_IO isn't allowed if PF_MEMALLOC_NOIO is set in current->flags
>>> + * __GFP_FS is also cleared as it implies __GFP_IO.
>>> + */
>>>  static inline gfp_t memalloc_noio_flags(gfp_t flags)
>>>  {
>>>  	if (unlikely(current->flags & PF_MEMALLOC_NOIO))
>>> -		flags &= ~__GFP_IO;
>>> +		flags &= ~(__GFP_IO | __GFP_FS);
>>>  	return flags;
>>>  }
>>
>> You also need to mask all the shrink_control->gfp_mask
>> initialisations in mm/vmscan.c. The current code only masks the page
>> reclaim gfp_mask, not those that are passed to the shrinkers.
> Yes, there are some shrink_control->gfp_mask not masked in vmscan.c in
> the following functions. Beside this, all seemed be masked from direct
> reclaim path by memalloc_noio_flags().
> 
> -reclaim_clean_pages_from_list()
> used by alloc_contig_range(), this function is invoked in hugetlb and
> cma, for hugetlb, it should be safe as only userspace use it. I am not
> sure about the cma.
> David & Andrew, may you share your idea about whether cma is affected?
> 
Look at CMA, it's used for device which doesn't support scatter/gather
dma and mainly used for embedded device like camera, this should not be
the case of the block device. So i think this gfp_mask doesn't need be
masked.

Thanks,
Junxiao.
> -mem_cgroup_shrink_node_zone()
> -try_to_free_mem_cgroup_pages()
> These two are used by mem cgroup, as no kernel thread can be assigned
> into such cgroup, so i think, no need mask.
> 
> -balance_pgdat()
> used by kswapd, no need mask.
> 
> -shrink_all_memory()
> used by hibernate, should be safe with GFP_FS/IO.
> 
> Thanks,
> Junxiao.
>>
>> Cheers,
>>
>> Dave.
>>
> 
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