linux-kernel - Re: Removing page->index

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Message-ID: <023d1c53-783e-4d6d-a5e9-d15b9e068986@redhat.com>
Date: Tue, 10 Dec 2024 12:05:25 +0100
From: David Hildenbrand <david@...hat.com>
To: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-s390@...r.kernel.org
Subject: Re: Removing page->index

On 09.12.24 18:36, Claudio Imbrenda wrote:
> On Wed, 4 Dec 2024 16:58:52 +0100
> David Hildenbrand <david@...hat.com> wrote:
> 
>> On 03.12.24 20:51, Matthew Wilcox wrote:
>>> I've pushed out a new tree to
>>> git://git.infradead.org/users/willy/pagecache.git shrunk-page
>>> aka
>>> http://git.infradead.org/?p=users/willy/pagecache.git;a=shortlog;h=refs/heads/shrunk-page
>>>
>>> The observant will notice that it doesn't actually shrink struct page
>>> yet.  However, we're getting close.  What it does do is rename
>>> page->index to page->__folio_index to prevent new users of page->index
>>> from showing up.
>>
>> BTW, I was wondering how often we convert a page to a folio to then
>> access folio->index / folio->mapping and not actually having a folio (in
>> the future).
>>
>> I suspect this will need quite some changes to get it right, and I would
>> count that as "less obvious".
>>
>> Calling PageAnon() on anything mapped into user space page tables might
>> be one such case, for example.
>>
>>>
>>> There are (I believe) three build failures in that tree:
>>>
>>>    - fb_defio
>>>    - fbtft
>>>    - s390's gmap (and vsie?  is that the same thing?)
>>
>> Not completely (vsie (nested VMs) uses shadow gmap, ordinary VMs use
>> ordinary gmap) , but they are very related (-> KVM implementation on s390x).
>>
>> I know that Claudio is working on some changes, but not sure how that
>> would affect gmap's usage of page->index.
> 
> After I'm done, we won't use page->index anymore.
> 
> The changes I'm working on are massive, it's very impractical to push
> everything at once, so I'm refactoring and splitting smaller and more
> manageable (and reviewable) series.
> 
> This means that it will take some time before I'm done (I'm *hoping*
> to be done for 6.15)

Thanks for the information. So for the time being, we could likely 
switch to page->private.

One question may be whether these (not-user-space) page tables should at 
some point deserve a dedicated memdesc. But likely the question is what 
it will all look like after your rework.

-- 
Cheers,

David / dhildenb