From: zhong jiang <zhongjiang@huawei.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: vbabka@suse.cz, qiuxishi@huawei.com, akpm@linux-foundation.org,
linux-mm@kvack.org, Mike Kravetz <mike.kravetz@oracle.com>,
Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH v2] mm/hugetlb: fix race when migrate pages
Date: Thu, 21 Jul 2016 17:25:14 +0800 [thread overview]
Message-ID: <579094FA.6070602@huawei.com> (raw)
In-Reply-To: <20160720132431.GM11249@dhcp22.suse.cz>
On 2016/7/20 21:24, Michal Hocko wrote:
> [Sorry for spammin, I though my headache would force me to not stare
> into the code more but I couldn't resist ]
> On Wed 20-07-16 15:00:55, Michal Hocko wrote:
>> On Wed 20-07-16 14:45:01, Michal Hocko wrote:
>> [...]
>>> I was talking to Mel (CCed) and he has raised a good question. So if you
>>> encounter a page under migration and fail to share the pmd with it how
>>> can you have a proper content of the target page in the end?
>> Hmm, I was staring into the code some more and it seems this would be OK
>> because we should hit hugetlb_no_page with the newel instantiated pmd
>> and associate it with a page from the radix tree. So unless I am missing
>> something the corruption shouldn't be possible.
>>
>> I believe the post pmd_populate race is still there, though, so I
>> believe the approach should be rethought.
> So I think the swap entry is OK in fact. huge_pte_alloc would return a
> shared pmd which would just happen to be swap entry. hugetlb_fault would
> then recognize that by:
> /*
> * entry could be a migration/hwpoison entry at this point, so this
> * check prevents the kernel from going below assuming that we have
> * a active hugepage in pagecache. This goto expects the 2nd page fault,
> * and is_hugetlb_entry_(migration|hwpoisoned) check will properly
> * handle it.
> */
> if (!pte_present(entry))
> goto out_mutex;
>
> We would get back from the page fault and if the page was still under
> migration on the retry we would end up waiting for the migration entry
> on the next fault.
>
> ptep = huge_pte_offset(mm, address);
> if (ptep) {
> entry = huge_ptep_get(ptep);
> if (unlikely(is_hugetlb_entry_migration(entry))) {
> migration_entry_wait_huge(vma, mm, ptep);
> return 0;
>
> Or am I missing something? If not we should just reconsider the BUG_ON
> but I still have to think wether it is useful/needed at all.
you are right. I agree.
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next prev parent reply other threads:[~2016-07-21 9:36 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-19 13:45 [PATCH v2] mm/hugetlb: fix race when migrate pages zhongjiang
2016-07-20 7:38 ` Michal Hocko
2016-07-20 10:03 ` zhong jiang
2016-07-20 12:16 ` Michal Hocko
2016-07-20 12:45 ` Michal Hocko
2016-07-20 13:00 ` Michal Hocko
2016-07-20 13:24 ` Michal Hocko
2016-07-21 9:25 ` zhong jiang [this message]
-- strict thread matches above, loose matches on Subject: below --
2016-07-19 13:04 zhongjiang
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