From:   "Huang, Ying" <ying.huang@intel.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Hugh Dickins <hughd@google.com>,
        "Xu, Pengfei" <pengfei.xu@intel.com>,
        Christoph Hellwig <hch@lst.de>,
        Stefan Roesch <shr@devkernel.io>, Tejun Heo <tj@kernel.org>,
        Xin Hao <xhao@linux.alibaba.com>, Zi Yan <ziy@nvidia.com>,
        Yang Shi <shy828301@gmail.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [PATCH 0/3] migrate_pages: fix deadlock in batched synchronous
 migration
References: <20230224141145.96814-1-ying.huang@intel.com>
        <20230225205543.5d56714d2145ac45da25369e@linux-foundation.org>
Date:   Mon, 27 Feb 2023 09:25:35 +0800
In-Reply-To: <20230225205543.5d56714d2145ac45da25369e@linux-foundation.org>
        (Andrew Morton's message of "Sat, 25 Feb 2023 20:55:43 -0800")
Message-ID: <874jr7dhm8.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Andrew Morton <akpm@linux-foundation.org> writes:

> On Fri, 24 Feb 2023 22:11:42 +0800 Huang Ying <ying.huang@intel.com> wrote:
>
>> Two deadlock bugs were reported for the migrate_pages() batching
>> series.
>
> "migrate_pages(): batch TLB flushing"

Yes.  Should have written as that.

>>  Thanks Hugh and Pengfei.  Analysis shows that if we have
>> locked some other folios except the one we are migrating, it's not
>> safe in general to wait synchronously, for example, to wait the
>> writeback to complete or wait to lock the buffer head.
>> 
>> So 1/3 fixes the deadlock in a simple way, where the batching support
>> for the synchronous migration is disabled.  The change is
>> straightforward and easy to be understood.  While 3/3 re-introduce the
>> batching for synchronous migration via trying to migrate
>> asynchronously in batch optimistically, then fall back to migrate
>> synchronously one by one for fail-to-migrate folios.  Test shows that
>> this can restore the TLB flushing batching performance for synchronous
>> migration effectively.
>
> If anyone backports the "migrate_pages(): batch TLB flushing" series
> into their kernels, they will want to know about such fixes.  So we can
> help them by providing suitable Link: tags.
>
> Such a Link: may also be helpful to people who are performing git
> bisection searches for some issue but who keep stumbling over the
> issues which this series addresses.
>
> Being lazy, I slapped
>
> Fixes: 6f7d760e86fa ("migrate_pages: move THP/hugetlb migration support check to simplify code")
>
> on all three, as this was the final patch in that series.  Inaccurate,
> but it means that these fixes will land in a suitable place if anyone
> needs them.

Sorry.  I should have added the "Fixes:" tag.  I will be more careful
in the future.  And, I will add proper "Link:" tag too.

Best Regards,
Huang, Ying