linux-kernel - Re: [v3 PATCH 1/2] mm: mempolicy: make the behavior consistent when MPOL_MF_MOVE* and MPOL_MF

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Date:   Fri, 19 Jul 2019 09:18:41 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Vlastimil Babka <vbabka@...e.cz>, mhocko@...nel.org,
        mgorman@...hsingularity.net, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [v3 PATCH 1/2] mm: mempolicy: make the behavior consistent when
 MPOL_MF_MOVE* and MPOL_MF_STRICT were specified



On 7/19/19 5:48 AM, Vlastimil Babka wrote:
> On 7/18/19 7:17 PM, Yang Shi wrote:
>> When both MPOL_MF_MOVE* and MPOL_MF_STRICT was specified, mbind() should
>> try best to migrate misplaced pages, if some of the pages could not be
>> migrated, then return -EIO.
>>
>> There are three different sub-cases:
>> 1. vma is not migratable
>> 2. vma is migratable, but there are unmovable pages
>> 3. vma is migratable, pages are movable, but migrate_pages() fails
>>
>> If #1 happens, kernel would just abort immediately, then return -EIO,
>> after the commit a7f40cfe3b7ada57af9b62fd28430eeb4a7cfcb7 ("mm:
>> mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified").
>>
>> If #3 happens, kernel would set policy and migrate pages with best-effort,
>> but won't rollback the migrated pages and reset the policy back.
>>
>> Before that commit, they behaves in the same way.  It'd better to keep
>> their behavior consistent.  But, rolling back the migrated pages and
>> resetting the policy back sounds not feasible, so just make #1 behave as
>> same as #3.
>>
>> Userspace will know that not everything was successfully migrated (via
>> -EIO), and can take whatever steps it deems necessary - attempt rollback,
>> determine which exact page(s) are violating the policy, etc.
>>
>> Make queue_pages_range() return 1 to indicate there are unmovable pages
>> or vma is not migratable.
>>
>> The #2 is not handled correctly in the current kernel, the following
>> patch will fix it.
>>
>> Cc: Vlastimil Babka <vbabka@...e.cz>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Mel Gorman <mgorman@...hsingularity.net>
>> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> Some nits below (I guess Andrew can incorporate them, no need to resend)
>
> ...
>
>> @@ -488,15 +496,15 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>>   	struct queue_pages *qp = walk->private;
>>   	unsigned long flags = qp->flags;
>>   	int ret;
>> +	bool has_unmovable = false;
>>   	pte_t *pte;
>>   	spinlock_t *ptl;
>>   
>>   	ptl = pmd_trans_huge_lock(pmd, vma);
>>   	if (ptl) {
>>   		ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
>> -		if (ret > 0)
>> -			return 0;
>> -		else if (ret < 0)
>> +		/* THP was split, fall through to pte walk */
>> +		if (ret != 2)
>>   			return ret;
> The comment should better go here after the if, as that's where fall through
> happens.
>
>>   	}
>>   
>> @@ -519,14 +527,21 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>>   		if (!queue_pages_required(page, qp))
>>   			continue;
>>   		if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
>> -			if (!vma_migratable(vma))
>> +			/* MPOL_MF_STRICT must be specified if we get here */
>> +			if (!vma_migratable(vma)) {
>> +				has_unmovable |= true;
> '|=' is weird, just use '='
>
>>   				break;
>> +			}
>>   			migrate_page_add(page, qp->pagelist, flags);
>>   		} else
>>   			break;
>>   	}
>>   	pte_unmap_unlock(pte - 1, ptl);
>>   	cond_resched();
>> +
>> +	if (has_unmovable)
>> +		return 1;
>> +
>>   	return addr != end ? -EIO : 0;
>>   }
>>   
> ...
>> @@ -1259,11 +1286,12 @@ static long do_mbind(unsigned long start, unsigned long len,
>>   				putback_movable_pages(&pagelist);
>>   		}
>>   
>> -		if (nr_failed && (flags & MPOL_MF_STRICT))
>> +		if ((ret > 0) || (nr_failed && (flags & MPOL_MF_STRICT)))
>>   			err = -EIO;
>>   	} else
>>   		putback_movable_pages(&pagelist);
>>   
>> +up_out:
>>   	up_write(&mm->mmap_sem);
>>    mpol_out:
> The new label made the wrong identation of this one stand out, so I'd just fix
> it up while here.

Thanks, will fix all of these. I will resend this patch along with patch 
2/2 which has to be resent anyway.

Yang

> Thanks!
>
>>   	mpol_put(new);
>>