Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race

From: Mike Kravetz <mike.kravetz@oracle.com>
To: "Longpeng (Mike)" <longpeng2@huawei.com>,
	Matthew Wilcox <willy@infradead.org>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, arei.gonglei@huawei.com,
	weidong.huang@huawei.com, weifuqiang@huawei.com,
	kvm@vger.kernel.org
Subject: Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race
Date: Tue, 18 Feb 2020 19:49:01 -0800	[thread overview]
Message-ID: <8292299c-4c5a-a8cb-22e2-d5c9051f122a@oracle.com> (raw)
In-Reply-To: <593d82a3-1d1e-d8f2-6b90-137f10441522@huawei.com>

On 2/18/20 6:09 PM, Longpeng (Mike) wrote:
> 在 2020/2/19 4:52, Matthew Wilcox 写道:
>> On Tue, Feb 18, 2020 at 08:10:25PM +0800, Longpeng(Mike) wrote:
>>>  {
>>> -	pgd_t *pgd;
>>> -	p4d_t *p4d;
>>> -	pud_t *pud;
>>> -	pmd_t *pmd;
>>> +	pgd_t *pgdp;
>>> +	p4d_t *p4dp;
>>> +	pud_t *pudp, pud;
>>> +	pmd_t *pmdp, pmd;
>>
>> Renaming the variables as part of a fix is a really bad idea.  It obscures
>> the actual fix and makes everybody's life harder.  Plus, it's not even
>> renaming to follow the normal convention -- there are only two places
>> (migrate.c and gup.c) which follow this pattern in mm/ while there are
>> 33 that do not.
>>
> Good suggestion, I've never noticed this, thanks.
> By the way, could you give an example if we use this way to fix the bug?

Matthew and others may have better suggestions for naming.  However, I would
keep the existing names and add:

pud_t pud_entry;
pmd_t pmd_entry;

Then the *_entry variables are the target of the READ_ONCE()

pud_entry = READ_ONCE(*pud);
if (sz != PUD_SIZE && pud_none(pud_entry))
...
...
pmd_entry = READ_ONCE(*pmd);
if (sz != PMD_SIZE && pmd_none(pmd_entry))
...
...

BTW, thank you for finding this issue!
-- 
Mike Kravetz