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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: + mm-hugetlb-fix-race-when-migrate-pages.patch added to -mm tree
Date: Thu, 21 Jul 2016 08:13:55 +0000
Message-ID: <20160721081355.GB25398@hori1.linux.bs1.fc.nec.co.jp>
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 <20160721074340.GA26398@dhcp22.suse.cz>
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To: Michal Hocko <mhocko@kernel.org>
Cc: "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "zhongjiang@huawei.com" <zhongjiang@huawei.com>, "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>

On Thu, Jul 21, 2016 at 09:43:40AM +0200, Michal Hocko wrote:
> We have further discussed the patch and I believe it is not correct. See =
[1].
> I am proposing the following alternative.
>
> [1] http://lkml.kernel.org/r/20160720132431.GM11249@dhcp22.suse.cz
> ---
> From b1e9b3214f1859fdf7d134cdcb56f5871933539c Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Thu, 21 Jul 2016 09:28:13 +0200
> Subject: [PATCH] mm, hugetlb: fix huge_pte_alloc BUG_ON
>
> Zhong Jiang has reported a BUG_ON from huge_pte_alloc hitting when he
> runs his database load with memory online and offline running in
> parallel. The reason is that huge_pmd_share might detect a shared pmd
> which is currently migrated and so it has migration pte which is
> !pte_huge.
>
> There doesn't seem to be any easy way to prevent from the race and in
> fact seeing the migration swap entry is not harmful. Both callers of
> huge_pte_alloc are prepared to handle them. copy_hugetlb_page_range
> will copy the swap entry and make it COW if needed. hugetlb_fault will
> back off and so the page fault is retries if the page is still under
> migration and waits for its completion in hugetlb_fault.
>
> That means that the BUG_ON is wrong and we should update it. Let's
> simply check that all present ptes are pte_huge instead.
>
> Reported-by: zhongjiang <zhongjiang@huawei.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

In the early days of hugetlb, we had an assumption that !pte_none is
equivalent to pmd_present() because there was no valid non-present entry
on huge_pte. Situation has changed by hugepage migration and/or hwpoison,
so we have to care about the separation of these two, and make sure that
pte_present is true before checking pte_huge.

So I think this change is right. Thank you Zhong, Michal.

Acked-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>=

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