linux-kernel - Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and

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Date:   Tue, 27 Sep 2022 18:44:53 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     Yang Shi <shy828301@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap()
 and _move()

On 9/27/22 18:41, Huang, Ying wrote:
>>>> I also agree that we cannot make any rules such as "do not lock > 1 page
>>>> at the same time, elsewhere in the kernel", because it is already
>>>> happening, for example in page-writeback.c, which locks PAGEVEC_SIZE
>>>> (15) pages per batch [1].
>>
>> That's not really the case though. The inner loop of write_cache_page()
>> only ever locks one page at a time, either directly via the
>> unlock_page() on L2338 (those goto's are amazing) or indirectly via
>> (*writepage)() on L2359.
>>
>> So there's no deadlock potential there because unlocking any previously
>> locked page(s) doesn't depend on obtaining the lock for another page.
>> Unless I've missed something?
> 
> Yes.  This is my understanding too after checking ext4_writepage().
> 

Yes, I missed the ".writepage() shall unlock the page" design point. Now
it seems much more reasonable and safer. :)

thanks,

-- 
John Hubbard
NVIDIA