linux-kernel - Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per

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Message-ID: <CAHk-=wi+rSKmNvFBpaKW+Hcn8YSzd2az2Lp5ii7Q4_o0PepJ_A@mail.gmail.com>
Date: Tue, 6 Aug 2024 07:38:09 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Oliver Sang <oliver.sang@...el.com>
Cc: Jeff Xu <jeffxu@...gle.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Pedro Falcato <pedro.falcato@...il.com>, Jeff Xu <jeffxu@...omium.org>, oe-lkp@...ts.linux.dev, 
	lkp@...el.com, linux-kernel@...r.kernel.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Dave Hansen <dave.hansen@...el.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Guenter Roeck <groeck@...omium.org>, 
	Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, 
	Jorge Lucangeli Obes <jorgelo@...omium.org>, Matthew Wilcox <willy@...radead.org>, 
	Muhammad Usama Anjum <usama.anjum@...labora.com>, Stephen Röttger <sroettger@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Amer Al Shanawany <amer.shanawany@...il.com>, 
	Javier Carrasco <javier.carrasco.cruz@...il.com>, Shuah Khan <shuah@...nel.org>, 
	linux-api@...r.kernel.org, linux-mm@...ck.org, ying.huang@...el.com, 
	feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [mseal] 8be7258aad: stress-ng.pagemove.page_remaps_per_sec
 -4.4% regression

On Mon, 5 Aug 2024 at 23:05, Oliver Sang <oliver.sang@...el.com> wrote:
>
> > New version - still untested, but now I've read through it one more
> > time - attached.
>
> we tested this version by applying it directly upon 8be7258aad,  but seems it
> have little impact to performance. still similar regression if comparing to
> ff388fe5c4.

Note that that patch (and Michael's fixes for ppc on top) in itself
doesn't fix any performance issue.

But getting rid of arch_unmap() means that now the can_modify_mm() in
do_vmi_munmap() is right above the "vma_find()" (and can in fact be
moved below it and into do_vmi_align_munmap), and that means that at
least the unmap paths don't need the vma lookup of can_modify_mm() at
all, because they've done their own.

IOW, the "arch_unmap()" removal was purely preparatory and did nothing
on its own, it's only preparatory to get rid of some of the
can_modify_mm() costs.

The call to can_modify_mm() in mremap_to() is a bit harder to get rid
of. Unless we just say "mremap will unmap the destination even if the
mremap source is sealed".

            Linus