linux-kernel - Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

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Message-ID: <b001adf2-238d-1708-673d-6f512a53e1e9@redhat.com>
Date:   Fri, 11 Aug 2023 17:32:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

On 11.08.23 17:18, Peter Xu wrote:
> On Fri, Aug 11, 2023 at 12:27:13AM +0200, David Hildenbrand wrote:
>> On 10.08.23 23:48, Matthew Wilcox wrote:
>>> On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
>>>> AFAICS if that patch was all correct (while I'm not yet sure..), you can
>>>> actually fit your new total mapcount field into page 1 so even avoid the
>>>> extra cacheline access.  You can have a look: the trick is refcount for
>>>> tail page 1 is still seems to be free on 32 bits (if that was your worry
>>>> before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.
>>>
>>> No, refcount must be 0 on all tail pages.  We rely on this in many places
>>> in the MM.
>>
>> Very right.
> 
> Obviously I could have missed this in the past.. can I ask for an example
> explaining why refcount will be referenced before knowing it's a head?

I think the issue is, when coming from a PFN walker (or GUP-fast), you 
might see "oh, this is a folio, let's lookup the head page". And you do 
that.

Then, you try taking a reference on that head page. (see try_get_folio()).

But as you didn't hold a reference on the folio yet, it can happily get 
freed + repurposed in the meantime, so maybe it's not a head page anymore.

So if the field would get reused for something else, grabbing a 
reference would corrupt whatever is now stored in there.

-- 
Cheers,

David / dhildenb