linux-kernel - Re: [PATCH -v5 0/9] migrate

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Message-ID: <Y/RI4s45PZ6Bv2ZR@casper.infradead.org>
Date:   Tue, 21 Feb 2023 04:30:26 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Zi Yan <ziy@...dia.com>, Yang Shi <shy828301@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Bharata B Rao <bharata@....com>,
        Alistair Popple <apopple@...dia.com>,
        Xin Hao <xhao@...ux.alibaba.com>,
        Minchan Kim <minchan@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        "Xu, Pengfei" <pengfei.xu@...el.com>,
        Christoph Hellwig <hch@....de>,
        Stefan Roesch <shr@...kernel.io>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH -v5 0/9] migrate_pages(): batch TLB flushing

On Mon, Feb 20, 2023 at 06:48:38PM -0800, Hugh Dickins wrote:
> Yes, that's a good principle, that we should avoid to lock/wait
> synchronously once we have locked one folio (hmm, above you say
> "more than one": I think we mean the same thing, we're just
> stating it differently, given how the code runs at present).

I suspect the migrate page code is disobeying the locking ordering
rules for multiple folios.  if two folios belong to the same file,
they must be locked by folio->index order, low to high.  If two folios
belong to different files, they must be ordered by folio->mapping, the
mapping lowest in memory first.  You can see this locking rule embedded
in vfs_lock_two_folios() in fs/remap_range.c.

I don't know what the locking rules are for two folios which are not file
folios, or for two folios when one is anonymous and the other belongs
to a file.  Maybe it's the same; you can lock them ordered by ->mapping
first, then by ->index.

Or you can just trylock multiple folios and skip the ones that don't work.