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Subject: Re: [PATCH v3 2/2] mm: hugetlb: soft-offline:
 dissolve_free_huge_page() return zero on !PageHuge
To:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        xishi.qiuxishi@alibaba-inc.com,
        "Chen, Jerry T" <jerry.t.chen@intel.com>,
        "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>, linux-kernel@vger.kernel.org,
        Anshuman Khandual <anshuman.khandual@arm.com>
References: <1560761476-4651-1-git-send-email-n-horiguchi@ah.jp.nec.com>
 <1560761476-4651-3-git-send-email-n-horiguchi@ah.jp.nec.com>
From:   Mike Kravetz <mike.kravetz@oracle.com>
Message-ID: <53844c4d-966e-48d3-f174-b0d3598c180c@oracle.com>
Date:   Tue, 18 Jun 2019 10:55:27 -0700
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On 6/17/19 1:51 AM, Naoya Horiguchi wrote:
> madvise(MADV_SOFT_OFFLINE) often returns -EBUSY when calling soft offline
> for hugepages with overcommitting enabled. That was caused by the suboptimal
> code in current soft-offline code. See the following part:
> 
>     ret = migrate_pages(&pagelist, new_page, NULL, MPOL_MF_MOVE_ALL,
>                             MIGRATE_SYNC, MR_MEMORY_FAILURE);
>     if (ret) {
>             ...
>     } else {
>             /*
>              * We set PG_hwpoison only when the migration source hugepage
>              * was successfully dissolved, because otherwise hwpoisoned
>              * hugepage remains on free hugepage list, then userspace will
>              * find it as SIGBUS by allocation failure. That's not expected
>              * in soft-offlining.
>              */
>             ret = dissolve_free_huge_page(page);
>             if (!ret) {
>                     if (set_hwpoison_free_buddy_page(page))
>                             num_poisoned_pages_inc();
>             }
>     }
>     return ret;
> 
> Here dissolve_free_huge_page() returns -EBUSY if the migration source page
> was freed into buddy in migrate_pages(), but even in that case we actually
> has a chance that set_hwpoison_free_buddy_page() succeeds. So that means
> current code gives up offlining too early now.
> 
> dissolve_free_huge_page() checks that a given hugepage is suitable for
> dissolving, where we should return success for !PageHuge() case because
> the given hugepage is considered as already dissolved.
> 
> This change also affects other callers of dissolve_free_huge_page(),
> which are cleaned up together.
> 
> Reported-by: Chen, Jerry T <jerry.t.chen@intel.com>
> Tested-by: Chen, Jerry T <jerry.t.chen@intel.com>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Thanks,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz