linux-kernel - Re: [PATCH 0/3] migrate_pages: fix deadlock in batched synchronous migration

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Date:   Sat, 25 Feb 2023 20:55:43 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Huang Ying <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        "Xu, Pengfei" <pengfei.xu@...el.com>,
        Christoph Hellwig <hch@....de>,
        Stefan Roesch <shr@...kernel.io>, Tejun Heo <tj@...nel.org>,
        Xin Hao <xhao@...ux.alibaba.com>, Zi Yan <ziy@...dia.com>,
        Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 0/3] migrate_pages: fix deadlock in batched synchronous
 migration

On Fri, 24 Feb 2023 22:11:42 +0800 Huang Ying <ying.huang@...el.com> wrote:

> Two deadlock bugs were reported for the migrate_pages() batching
> series.

"migrate_pages(): batch TLB flushing"

>  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.