Re: [PATCH] mm, thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings

[Stefan Priebe - Profihost AG <s.priebe@profihost.ag>]

Hi,

i had multiple memory stalls this weekend again. All kvm processes where
spinning trying to get > 100% CPU and i was not able to even login to
ssh. After 5-10 minutes i was able to login.

There were about 150GB free mem on the host.

Relevant settings (no local storage involved):
        vm.dirty_background_ratio:
            3
        vm.dirty_ratio:
            10
        vm.min_free_kbytes:
            10567004

# cat /sys/kernel/mm/transparent_hugepage/defrag
always defer [defer+madvise] madvise never

# cat /sys/kernel/mm/transparent_hugepage/enabled
[always] madvise never

After that i had the following traces on the host node:
https://pastebin.com/raw/0VhyQmAv

Thanks!

Greets,
Stefan


Am 17.09.2018 um 08:11 schrieb Michal Hocko:
> [sorry I've missed your reply]
> 
> On Wed 12-09-18 18:29:25, Mel Gorman wrote:
>> On Wed, Aug 29, 2018 at 09:24:51PM +0200, Michal Hocko wrote:
> [...]
>> I recognise that this fix means that users that expect zone_reclaim_mode==1
>> type behaviour may get burned but the users that benefit from that should
>> also be users that benefit from sizing their workload to a node. They should
>> be able to replicate that with mempolicies or at least use prepation scripts
>> to clear memory on a target node (e.g. membind a memhog to the desired size,
>> exit and then start the target workload).
> 
> As I've said in other email. We probably want to add a new mempolicy
> which has zone_reclaim_mode-like semantic.
> 
> [...]
> 
>>> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
>>> index 5228c62af416..bac395f1d00a 100644
>>> --- a/include/linux/mempolicy.h
>>> +++ b/include/linux/mempolicy.h
>>> @@ -139,6 +139,8 @@ struct mempolicy *mpol_shared_policy_lookup(struct shared_policy *sp,
>>>  struct mempolicy *get_task_policy(struct task_struct *p);
>>>  struct mempolicy *__get_vma_policy(struct vm_area_struct *vma,
>>>  		unsigned long addr);
>>> +struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
>>> +						unsigned long addr);
>>>  bool vma_policy_mof(struct vm_area_struct *vma);
>>>  
>>>  extern void numa_default_policy(void);
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index c3bc7e9c9a2a..94472bf9a31b 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -629,21 +629,30 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>>>   *	    available
>>>   * never: never stall for any thp allocation
>>>   */
>>> -static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
>>> +static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma, unsigned long addr)
>>>  {
>>>  	const bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
>>> +	gfp_t this_node = 0;
>>> +	struct mempolicy *pol;
>>> +
>>> +#ifdef CONFIG_NUMA
>>> +	/* __GFP_THISNODE makes sense only if there is no explicit binding */
>>> +	pol = get_vma_policy(vma, addr);
>>> +	if (pol->mode != MPOL_BIND)
>>> +		this_node = __GFP_THISNODE;
>>> +#endif
>>>  
>>
>> Where is the mpol_cond_put? Historically it might not have mattered
>> because THP could not be used with a shared possibility but it probably
>> matters now that tmpfs can be backed by THP.
> 
> http://lkml.kernel.org/r/20180830064732.GA2656@dhcp22.suse.cz
> 
>> The comment needs more expansion as well. Arguably it only makes sense in
>> the event we are explicitly bound to one node because if we are bound to
>> two nodes without interleaving then why not fall back? The answer to that
>> is outside the scope of the patch but the comment as-is will cause head
>> scratches in a years time.
> 
> Do you have any specific wording in mind? I have a bit hard time to come
> up with something more precise and do not go into details too much.
>  
>>>  	if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags))
>>> -		return GFP_TRANSHUGE | (vma_madvised ? 0 : __GFP_NORETRY);
>>> +		return GFP_TRANSHUGE | (vma_madvised ? 0 : __GFP_NORETRY | this_node);
>>>  	if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags))
>>> -		return GFP_TRANSHUGE_LIGHT | __GFP_KSWAPD_RECLAIM;
>>> +		return GFP_TRANSHUGE_LIGHT | __GFP_KSWAPD_RECLAIM | this_node;
>>>  	if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags))
>>>  		return GFP_TRANSHUGE_LIGHT | (vma_madvised ? __GFP_DIRECT_RECLAIM :
>>> -							     __GFP_KSWAPD_RECLAIM);
>>> +							     __GFP_KSWAPD_RECLAIM | this_node);
>>>  	if (test_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags))
>>>  		return GFP_TRANSHUGE_LIGHT | (vma_madvised ? __GFP_DIRECT_RECLAIM :
>>> -							     0);
>>> -	return GFP_TRANSHUGE_LIGHT;
>>> +							     this_node);
>>> +	return GFP_TRANSHUGE_LIGHT | this_node;
>>>  }
>>>  
>>>  /* Caller must hold page table lock. */
>>> @@ -715,7 +724,7 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>>  			pte_free(vma->vm_mm, pgtable);
>>>  		return ret;
>>>  	}
>>> -	gfp = alloc_hugepage_direct_gfpmask(vma);
>>> +	gfp = alloc_hugepage_direct_gfpmask(vma, haddr);
>>>  	page = alloc_hugepage_vma(gfp, vma, haddr, HPAGE_PMD_ORDER);
>>>  	if (unlikely(!page)) {
>>>  		count_vm_event(THP_FAULT_FALLBACK);
>>> @@ -1290,7 +1299,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
>>>  alloc:
>>>  	if (transparent_hugepage_enabled(vma) &&
>>>  	    !transparent_hugepage_debug_cow()) {
>>> -		huge_gfp = alloc_hugepage_direct_gfpmask(vma);
>>> +		huge_gfp = alloc_hugepage_direct_gfpmask(vma, haddr);
>>>  		new_page = alloc_hugepage_vma(huge_gfp, vma, haddr, HPAGE_PMD_ORDER);
>>>  	} else
>>>  		new_page = NULL;
>>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>>> index da858f794eb6..75bbfc3d6233 100644
>>> --- a/mm/mempolicy.c
>>> +++ b/mm/mempolicy.c
>>> @@ -1648,7 +1648,7 @@ struct mempolicy *__get_vma_policy(struct vm_area_struct *vma,
>>>   * freeing by another task.  It is the caller's responsibility to free the
>>>   * extra reference for shared policies.
>>>   */
>>> -static struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
>>> +struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
>>>  						unsigned long addr)
>>>  {
>>>  	struct mempolicy *pol = __get_vma_policy(vma, addr);
>>> @@ -2026,32 +2026,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>>>  		goto out;
>>>  	}
>>>  
>>> -	if (unlikely(IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hugepage)) {
>>> -		int hpage_node = node;
>>> -
>>> -		/*
>>> -		 * For hugepage allocation and non-interleave policy which
>>> -		 * allows the current node (or other explicitly preferred
>>> -		 * node) we only try to allocate from the current/preferred
>>> -		 * node and don't fall back to other nodes, as the cost of
>>> -		 * remote accesses would likely offset THP benefits.
>>> -		 *
>>> -		 * If the policy is interleave, or does not allow the current
>>> -		 * node in its nodemask, we allocate the standard way.
>>> -		 */
>>> -		if (pol->mode == MPOL_PREFERRED &&
>>> -						!(pol->flags & MPOL_F_LOCAL))
>>> -			hpage_node = pol->v.preferred_node;
>>> -
>>> -		nmask = policy_nodemask(gfp, pol);
>>> -		if (!nmask || node_isset(hpage_node, *nmask)) {
>>> -			mpol_cond_put(pol);
>>> -			page = __alloc_pages_node(hpage_node,
>>> -						gfp | __GFP_THISNODE, order);
>>> -			goto out;
>>> -		}
>>> -	}
>>> -
>>
>> The hugepage flag passed into this function is now redundant and that
>> means that callers of alloc_hugepage_vma need to move back to using
>> alloc_pages_vma() directly and remove the API entirely. This block of
>> code is about both GFP flag settings and node selection but at a glance I
>> cannot see the point of it because it's very similar to the base page code.
>> The whole point may be to get around the warning in policy_node and that
>> could just as easily be side-stepped in alloc_hugepage_direct_gfpmask
>> as you do already in this patch. There should be no reason why THP has a
>> different policy than a base page within a single VMA.
> 
> OK, I can follow up with a cleanup patch once we settle down with this
> approach to fix the issue.
> 
> Thanks!
>