Re: [PATCH] mm/slub: Reduce memory consumption in extreme scenarios

[chenjun (AM)]

在 2023/3/20 17:12, Mike Rapoport 写道:
> On Mon, Mar 20, 2023 at 09:05:57AM +0100, Vlastimil Babka wrote:
>> On 3/19/23 08:22, chenjun (AM) wrote:
>>> 在 2023/3/17 20:06, Vlastimil Babka 写道:
>>>> On 3/17/23 12:32, chenjun (AM) wrote:
>>>>> 在 2023/3/14 22:41, Vlastimil Babka 写道:
>>>>>>>     	pc.flags = gfpflags;
>>>>>>> +
>>>>>>> +	/*
>>>>>>> +	 * when (node != NUMA_NO_NODE) && (gfpflags & __GFP_THISNODE)
>>>>>>> +	 * 1) try to get a partial slab from target node with __GFP_THISNODE.
>>>>>>> +	 * 2) if 1) failed, try to allocate a new slab from target node with
>>>>>>> +	 *    __GFP_THISNODE.
>>>>>>> +	 * 3) if 2) failed, retry 1) and 2) without __GFP_THISNODE constraint.
>>>>>>> +	 */
>>>>>>> +	if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode)
>>>>>>> +			pc.flags |= __GFP_THISNODE;
>>>>>>
>>>>>> Hmm I'm thinking we should also perhaps remove direct reclaim possibilities
>>>>>> from the attempt 2). In your qemu test it should make no difference, as it
>>>>>> fills everything with kernel memory that is not reclaimable. But in practice
>>>>>> the target node might be filled with user memory, and I think it's better to
>>>>>> quickly allocate on a different node than spend time in direct reclaim. So
>>>>>> the following should work I think?
>>>>>>
>>>>>> pc.flags = GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
>>>>>>
>>>>>
>>>>> Hmm, Should it be that:
>>>>>
>>>>> pc.flags |= GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
>>>>
>>>> No, we need to ignore the other reclaim-related flags that the caller
>>>> passed, or it wouldn't work as intended.
>>>> The danger is that we ignore some flag that would be necessary to pass, but
>>>> I don't think there's any?
>>>>
>>>>
>>>
>>> If we ignore __GFP_ZERO passed by kzalloc， kzalloc will not work.
>>> Could we just unmask __GFP_RECLAIMABLE | __GFP_RECLAIM?
>>>
>>> pc.flags &= ~(__GFP_RECLAIMABLE | __GFP_RECLAIM)
>>> pc.flags |= __GFP_THISNODE
>>
>> __GFP_RECLAIMABLE would be wrong, but also ignored as new_slab() does:
>> 	flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK)
>>
>> which would filter out __GFP_ZERO as well. That's not a problem as kzalloc()
>> will zero out the individual allocated objects, so it doesn't matter if we
>> don't zero out the whole slab page.
>>
>> But I wonder, if we're not past due time for a helper e.g.
>> gfp_opportunistic(flags) that would turn any allocation flags to a
>> GFP_NOWAIT while keeping the rest of relevant flags intact, and thus there
>> would be one canonical way to do it - I'm sure there's a number of places
>> with their own variants now?
>> With such helper we'd just add __GFP_THISNODE to the result here as that's
>> specific to this particular opportunistic allocation.
> 
> I like the idea, but maybe gfp_no_reclaim() would be clearer?
> 

#define gfp_no_reclaim(gfpflag) (gfpflag & ~__GFP_DIRECT_RECLAIM)

And here,

pc.flags = gfp_no_reclaim(gfpflags) | __GFP_THISNODE.

Do I get it right？