linux-kernel - Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages

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Message-ID: <df05e2e0-be29-1651-40e7-82dc686919c2@oracle.com>
Date:   Thu, 23 Aug 2018 10:56:30 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared
 pages

On 08/23/2018 10:01 AM, Mike Kravetz wrote:
> On 08/23/2018 05:48 AM, Michal Hocko wrote:
>> On Tue 21-08-18 18:10:42, Mike Kravetz wrote:
>> [...]
>>
>> OK, after burning myself when trying to be clever here it seems like
>> your proposed solution is indeed simpler.
>>
>>> +bool huge_pmd_sharing_possible(struct vm_area_struct *vma,
>>> +				unsigned long *start, unsigned long *end)
>>> +{
>>> +	unsigned long check_addr = *start;
>>> +	bool ret = false;
>>> +
>>> +	if (!(vma->vm_flags & VM_MAYSHARE))
>>> +		return ret;
>>> +
>>> +	for (check_addr = *start; check_addr < *end; check_addr += PUD_SIZE) {
>>> +		unsigned long a_start = check_addr & PUD_MASK;
>>> +		unsigned long a_end = a_start + PUD_SIZE;
>>
>> I guess this should be rather in HPAGE_SIZE * PTRS_PER_PTE units as
>> huge_pmd_unshare does.
> 
> Sure, I can do that.

On second thought, this is more similar to vma_shareable() which uses
PUD_MASK and PUD_SIZE.  In fact Kirill asked me to put in a common helper
for this and vma_shareable.  So, I would prefer to leave it as PUD* unless
you feel strongly.

IMO, it would make more sense to change this in huge_pmd_unshare as PMD
sharing is pretty explicitly tied to PUD_SIZE.  But, that is a future cleanup.

-- 
Mike Kravetz