Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <50ba595b-d992-cadf-9064-6b99347fa348@redhat.com>
Date:   Fri, 11 Aug 2023 17:17:02 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Content-Language: en-US
To:     Peter Xu <peterx@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-doc@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Hugh Dickins <hughd@google.com>,
        Ryan Roberts <ryan.roberts@arm.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yang Shi <shy828301@gmail.com>, Zi Yan <ziy@nvidia.com>
References: <20230809083256.699513-1-david@redhat.com> <ZNQD4pxo8svpGmvX@x1n>
 <ZNRYx8GhYftE4Xeb@casper.infradead.org>
 <7e31254d-8889-7e79-50e1-2630bd493d59@redhat.com> <ZNVbIyHcqeKUDuSg@x1n>
 <ZNVcfdH8+N5Q83J/@casper.infradead.org>
 <73d6d29f-9947-9b50-3b94-77f1ee547387@redhat.com> <ZNZNuooaFH9P4raS@x1n>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios
In-Reply-To: <ZNZNuooaFH9P4raS@x1n>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 11.08.23 17:03, Peter Xu wrote:
> On Thu, Aug 10, 2023 at 11:59:25PM +0200, David Hildenbrand wrote:
>> On 10.08.23 23:54, Matthew Wilcox wrote:
>>> On Thu, Aug 10, 2023 at 05:48:19PM -0400, Peter Xu wrote:
>>>>> Yes, that comment from Hugh primarily discusses how we could possibly
>>>>> optimize the loop, and if relying on folio_nr_pages_mapped() to reduce the
>>>>> iterations would be racy. As far as I can see, there are cases where "it
>>>>> would be certainly a bad idea" :)
>>>>
>>>> Is the race described about mapcount being changed right after it's read?
>>>> Are you aware of anything specific that will be broken, and will be fixed
>>>> with this patch?
>>>
>>> The problem is that people check the mapcount while holding no locks;
>>> not the PTL, not the page lock.  So it's an unfixable race.
>>>
>>>> Having a total mapcount does sound helpful if partial folio is common
>>>> indeed.
>>>>
>>>> I'm curious whether that'll be so common after the large anon folio work -
>>>> isn't it be sad if partial folio will be a norm?  It sounds to me that's
>>>> the case when small page sizes should be used.. and it's prone to waste?
>>>
>>> The problem is that entire_mapcount isn't really entire_mapcount.
>>> It's pmd_mapcount.  I have had thoughts about using it as entire_mapcount,
>>> but it gets gnarly when people do partial unmaps.  So the _usual_ case
>>> ends up touching every struct page.  Which sucks.  Also it's one of the
>>> things which stands in the way of shrinking struct page.
>>
>> Right, so one current idea is to have a single total_mapcount and look into
>> removing the subpage mapcounts (which will require first removing
>> _nr_pages_mapped, because that's still one of the important users).
>>
>> Until we get there, also rmap code has to do eventually "more tracking" and
>> might, unfortunately, end up slower.
>>
>>>
>>> But it's kind of annoying to explain all of this to you individually.
>>> There have been hundreds of emails about it over the last months on
>>> this mailing list.  It would be nice if you could catch up instead of
>>> jumping in.
>>
>> To be fair, a lot of the details are not readily available and in the heads
>> of selected people :)
>>
>> Peter, if you're interested, we can discuss the current plans, issues and
>> ideas offline!
> 
> Thanks for offering help, David.
> 
> Personally I still am unclear yet on why entire_mapcount cannot be used as
> full-folio mapcounts, and why "partial unmap" can happen a lot (I don't
> expect), but yeah I can try to catch up to educate myself first.

Using fork() is the easiest way. mremap(), MADV_DONTNEED, munmap, ...

You might end up having to scan page tables and/or the rmap to figure 
out which mapcount to adjust, which we should absolutely avoid.


> The only issue regarding an offline sync-up is that even if David will help
> Peter on catching up the bits, it'll not scale when another Peter2 had the
> same question..  So David, rather than I waste your time on helping one
> person, let me try to catch up with the public threads - I'm not sure how
> far I can go myself;

Sure. But note that it's a moving target, and some discussions have been 
going on for a long time. I recall there were various discussions, 
including LSF/MM, mm biweekly meeting, and more. So even if you scan 
through all that, you might either get outdated or incomplete information.

-- 
Cheers,

David / dhildenb