linux-kernel - Re: Removing page->index

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Message-ID: <cebb44b2-e258-43ff-80a5-6bd19c8edab8@redhat.com>
Date: Wed, 4 Dec 2024 16:58:52 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-s390@...r.kernel.org, Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: Removing page->index

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.

s390x gmap is 64bit only, so we have to store stuff in 8byte. gmap page 
tables are

Maybew e could simply switch from page->index to page->private? But I 
lost track if that will also be gone in the near future :)

> 
> Other than that, allmodconfig builds on x86 and I'm convinced the build
> bots will tell me about anything else I missed.
> 
> Lorenzo is working on fb_defio and fbtft will come along for the ride
> (it's a debug printk, so could just be deleted).
> 
> s390 is complicated.  I'd really appreciate some help.
> 
> The next step is to feed most of the patches through the appropriate
> subsystems.  Some have already gone into various maintainer trees
> (thanks!)
> 
> 
> There are still many more steps to go after this; eliminating memcg_data
> is closest to complete, and after that will come (in some order)
> eliminating ->lru, ->mapping, ->refcount and ->mapcount. 

Will continue working on the latter ;)

-- 
Cheers,

David / dhildenb