linux-ext4 - Re: [PATCH v2 16/18] mm/memremap_pages: Support initializing pages to a zero reference count

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Message-ID: <87wn9qlklh.fsf@nvdebian.thelocal>
Date:   Mon, 26 Sep 2022 16:17:18 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, akpm@...ux-foundation.org,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 16/18] mm/memremap_pages: Support initializing pages
 to a zero reference count


Dan Williams <dan.j.williams@...el.com> writes:

[...]

>> >> > > How on earth can a free'd page have both a 0 and 1 refcount??
>> >> >
>> >> > This is residual wonkiness from memremap_pages() handing out pages with
>> >> > elevated reference counts at the outset.
>> >>
>> >> I think the answer to my question is the above troubled code where we
>> >> still set the page refcount back to 1 even in the page_free path, so
>> >> there is some consistency "a freed paged may have a refcount of 1" for
>> >> the driver.
>> >>
>> >> So, I guess this patch makes sense but I would put more noise around
>> >> INIT_PAGEMAP_BUSY (eg annotate every driver that is using it with the
>> >> explicit constant) and alert people that they need to fix their stuff
>> >> to get rid of it.
>> >
>> > Sounds reasonable.
>> >
>> >> We should definately try to fix hmm_test as well so people have a good
>> >> reference code to follow in fixing the other drivers :(
>> >
>> > Oh, that's a good idea. I can probably fix that up and leave it to the
>> > GPU driver folks to catch up with that example so we can kill off
>> > INIT_PAGEMAP_BUSY.
>>
>> I'm hoping to send my series that fixes up all drivers using device
>> coherent/private later this week or early next. So you could also just
>> wait for that and remove INIT_PAGEMAP_BUSY entirely.
>
> Oh, perfect, thanks!

See
https://lore.kernel.org/linux-mm/3d74bb439723c7e46cbe47d1711795308aee4ae3.1664171943.git-series.apopple@nvidia.com/

I already had this in a series because the change was motivated by a
later patch there, but it's a standalone change and there's no reason it
couldn't be split out into it's own patch if that's better for you.

 - Alistair