linux-kernel - Re: [RFC v4 PATCH 2/9] mm/hugetlb: expose hugetlb fault mutex for use by fallocate

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Date:	Wed, 17 Jun 2015 15:05:19 -0700
From:	Mike Kravetz <mike.kravetz@...cle.com>
To:	Davidlohr Bueso <dave@...olabs.net>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC v4 PATCH 2/9] mm/hugetlb: expose hugetlb fault mutex for
 use by fallocate

On 06/11/2015 03:46 PM, Davidlohr Bueso wrote:
> On Thu, 2015-06-11 at 14:01 -0700, Mike Kravetz wrote:
>>   /* Forward declaration */
>>   static int hugetlb_acct_memory(struct hstate *h, long delta);
>> @@ -3324,7 +3324,8 @@ static u32 fault_mutex_hash(struct hstate *h, struct mm_struct *mm,
>>   	unsigned long key[2];
>>   	u32 hash;
>>
>> -	if (vma->vm_flags & VM_SHARED) {
>> +	/* !vma implies this was called from hugetlbfs fallocate code */
>> +	if (!vma || vma->vm_flags & VM_SHARED) {
>
> That !vma is icky, and really no need for it: hugetlbfs_fallocate(), for
> example, already passes [pseudo]vma->vm_flags with VM_SHARED, and you
> say it yourself in the comment. Do you see any reason why we cannot just
> keep the vma->vm_flags & VM_SHARED check?
>

Ah, I did not recall all the users of this code until I went to change
it. The other user is truncate_hugapages() which will now be used for
fallocate hole punch.  Truncate like fallocate is an inode operation
and there is no specific vma.  I can create a pseudo-vma here as well
just to pass the flag.  I guess that would at least be consistent with
the other user.

-- 
Mike Kravetz
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