Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree

From: zhong jiang <zhongjiang@huawei.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"qiuxishi@huawei.com" <qiuxishi@huawei.com>,
	"vbabka@suse.cz" <vbabka@suse.cz>,
	"mm-commits@vger.kernel.org" <mm-commits@vger.kernel.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Mel Gorman <mgorman@suse.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Date: Tue, 26 Jul 2016 22:04:16 +0800	[thread overview]
Message-ID: <57976DE0.8020809@huawei.com> (raw)
In-Reply-To: <20160726075847.GG32462@dhcp22.suse.cz>

On 2016/7/26 15:58, Michal Hocko wrote:
> On Fri 22-07-16 07:17:37, Naoya Horiguchi wrote:
> [...]
>> I think that (src_pte != dst_pte) can happen and that's ok if there's no
>> migration entry. 
> We have discussed that with Naoya off-list and couldn't find a scenario
> when parent would have !shared pmd while child would have it. The only
> plausible scenario was that parent created and poppulated mapping smaller
> than 1G and then enlarged it later on so the child would see sharedable
> pud. This doesn't seem to be possible because vma_merge would bail out
> due to VM_SPECIAL check.
  I do not understand that the process must have vm_special flags. if vm_special enable,
 the process must not be expanded.  and   what does it matter about vma_merge ??
>> But even if we have both of normal entry and migration entry
>> for one hugepage, that still looks fine to me because the running migration
>> operation fails (because there remains mapcounts on the source hugepage),
>> and all migration entries are turned back to normal entries pointing to the
>> source hugepage.
    In one case,  try_to_unmap_one is first exec and successfully,  mapcount turn into zero.
   then we get the pte lock,  if src_pte!-dst_pte, it maybe lead to the dst_pte is from migrate pte
    to normal pte, while the normal pte turn into migaret pte,, is right ?
>  
> Agreed.
>
>> Could you try to see and share what happens on your workload with
>> Michal's patch?
> Zhong Jiang did you have chance to retest with the BUG_ON changed?
>
 

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