linux-kernel - Re: [PATCH RFC 00/13] mm/treewide: Remove pXd

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Message-ID: <f9b786bf-27d9-4e16-b8d2-2a2666d917df@csgroup.eu>
Date: Mon, 11 Mar 2024 09:58:47 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "peterx@...hat.com" <peterx@...hat.com>, "linux-mm@...ck.org"
	<linux-mm@...ck.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, Andrew
 Morton <akpm@...ux-foundation.org>, Muchun Song <muchun.song@...ux.dev>,
	Jason Gunthorpe <jgg@...dia.com>, Matthew Wilcox <willy@...radead.org>, Mike
 Rapoport <rppt@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
	"sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC 00/13] mm/treewide: Remove pXd_huge() API



Le 06/03/2024 à 11:41, peterx@...hat.com a écrit :
> From: Peter Xu <peterx@...hat.com>
> 
> [based on akpm/mm-unstable latest commit a7f399ae964e]
> 
> In previous work [1], we removed the pXd_large() API, which is arch
> specific.  This patchset further removes the hugetlb pXd_huge() API.
> 
> Hugetlb was never special on creating huge mappings when compared with
> other huge mappings.  Having a standalone API just to detect such pgtable
> entries is more or less redundant, especially after the pXd_leaf() API set
> is introduced with/without CONFIG_HUGETLB_PAGE.
> 
> When looking at this problem, a few issues are also exposed that we don't
> have a clear definition of the *_huge() variance API.  This patchset
> started by cleaning these issues first, then replace all *_huge() users to
> use *_leaf(), then drop all *_huge() code.
> 
> On x86/sparc, swap entries will be reported "true" in pXd_huge(), while for
> all the rest archs they're reported "false" instead.  This part is done in
> patch 1-5, in which I suspect patch 1 can be seen as a bug fix, but I'll
> leave that to hmm experts to decide.
> 
> Besides, there are three archs (arm, arm64, powerpc) that have slightly
> different definitions between the *_huge() v.s. *_leaf() variances.  I
> tackled them separately so that it'll be easier for arch experts to chim in
> when necessary.  This part is done in patch 6-9.
> 
> The final patches 10-13 do the rest on the final removal, since *_leaf()
> will be the ultimate API in the future, and we seem to have quite some
> confusions on how *_huge() APIs can be defined, provide a rich comment for
> *_leaf() API set to define them properly to avoid future misuse, and
> hopefully that'll also help new archs to start support huge mappings and
> avoid traps (like either swap entries, or PROT_NONE entry checks).
> 
> The whole series is only lightly tested on x86, while as usual I don't have
> the capability to test all archs that it touches.
> 
> Marking this series RFC as of now.
> 
> [1] https://lore.kernel.org/r/20240305043750.93762-1-peterx@redhat.com
> 

Hi Peter, and nice job you are doing in cleaning up things around _huge 
stuff.

One thing that might be worth looking at also at some point is the mess 
around pmd_clear_huge() and pud_clear_huge().

I tried to clean things up with commit c742199a014d ("mm/pgtable: add 
stubs for {pmd/pub}_{set/clear}_huge") but it was reverted because of 
arm64 by commit d8a719059b9d ("Revert "mm/pgtable: add stubs for 
{pmd/pub}_{set/clear}_huge"")

So now powerpc/8xx has to implement pmd_clear_huge() and 
pud_clear_huge() allthough 8xx page hierarchy only has 2 levels.

Christophe