hello, Michal
thanks for your kind reply!
On 2015年10月14日 15:41, Michal Hocko wrote:
> On Wed 14-10-15 13:58:05, Pan Xinhui wrote:
>> Hi, all
>> I am working on some debug features' development.
>> I use kmalloc in some places of *scheduler*.
>
> This sounds inherently dangerous.
>
that's how we found weird bugs. :)
>> And the gfp_flag is GFP_ATOMIC, code looks like
>> p = kmalloc(sizeof(*p), GFP_ATOMIC);
>>
>> however I notice GFP_ATOMIC is still not enough. because when system
>> is at low memory state, slub might try to wakeup kswapd. then some
>> weird issues hit.
>
> gfp flags have been reworked in the current mmotm tree so you want
> __GFP_ATOMIC here. This will be non sleeping allocation which won't even
> wake up kswapd. I guess you do not want/need to touch memory reserves
> for something like a debugging feature (so you do not have to abuse
> __GFP_HIGH)
>
In my debug patches, I need __GFP_HIGH.
There is one special test case, in boot stage, we will reserve a big range of
memory, to try to detect if all drivers can handle low-memory in correct ways.
So maybe I need my own kmalloc succeed.
what I need to do now is just clear __GFP_KSWAPD_RECLAIM in kmalloc.
> [...]
>
>> After some simple check, I change my codes. this time code looks like:
>> p = kmalloc(sizeof(*p), GFP_ATOMIC | __GFP_NO_KSWAPD);
>> I think this flag will forbid slub to call any scheduler codes. But issue
>> still hit. :(
>>
>> my test result shows that __GFP_NO_KSWAPD is cleared when slub pass gfp_flag
>> to page allocator!!!
>>
>> at last I found it is clear by codes below.
>> 1441 static struct page *new_slab(struct kmem_cache *s, gfp_t flags, int
>> node)
>> 1442 {
>> 1443 if (unlikely(flags & GFP_SLAB_BUG_MASK)) {
>> 1444 pr_emerg("gfp: %u\n", flags & GFP_SLAB_BUG_MASK);
>> 1445 BUG();
>> 1446 }
>> 1447
>> 1448 return allocate_slab(s,
>> 1449 flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK),
>> node);//all other flags will be cleared. my god!!!
>> 1450 }
>>
>> I think GFP_RECLAIM_MASK should include as many available flags as possible.
>> :)
>
> Not really. It should only contain those which are really reclaim
> related. The fact that SLUB drops other flags is an internal detail
> of the allocator. If the resulting memory doesn't match the original
> requirements (e.g. zone placing etc...) then it is certainly a bug
> but not a bug in GFP_RECLAIM_MASK.
>
oh, yes. thanks for explanation.
> Anyway you are right that GFP_RECLAIM_MASK should contain
> __GFP_NO_KSWAPD resp. its new representation which is the case in the
> current mmotm tree as pointed out in previous response.
>
I am glad to see someone has fix it, too :)
thanks
xinhui
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