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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        Zi Yan <ziy@nvidia.com>, Michal Hocko <mhocko@kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH] mm, migrate: Check return value of try_to_unmap()
References: <20200303033645.280694-1-ying.huang@intel.com>
        <alpine.LSU.2.11.2003022150540.1344@eggly.anvils>
Date:   Tue, 03 Mar 2020 14:19:18 +0800
In-Reply-To: <alpine.LSU.2.11.2003022150540.1344@eggly.anvils> (Hugh Dickins's
        message of "Mon, 2 Mar 2020 22:11:45 -0800")
Message-ID: <87blpe55ax.fsf@yhuang-dev.intel.com>
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Hi, Hugh,

Hugh Dickins <hughd@google.com> writes:
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 3900044cfaa6..981f8374a6ef 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1116,8 +1116,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>  		/* Establish migration ptes */
>>  		VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
>>  				page);
>> -		try_to_unmap(page,
>> -			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
>> +		if (!try_to_unmap(page,
>> +			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS)) {
>> +			rc = -EIO;
>> +			goto out_unlock_both;
>
> No: even if try_to_unmap() says that it did not entirely succeed,
> it may have unmapped some ptes, inserting migration entries in their
> place. Those need to be replaced by proper ptes before the page is
> unlocked, which page_was_mapped 1 and remove_migration_ptes() do;
> but this goto skips those.

Yes.  You are right.  I misunderstand the original code.  Please ignore
this patch.

Best Regards,
Huang, Ying