Re: [patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE

From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	patrice.chotard@foss.st.com,
	Mikulas Patocka <mpatocka@redhat.com>,
	markhemm@googlemail.com, Lukas Czerner <lczerner@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Hugh Dickins <hughd@google.com>,
	patches@lists.linux.dev, Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org
Subject: Re: [patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE
Date: Sat, 16 Apr 2022 08:36:56 +0200	[thread overview]
Message-ID: <Ylpj9of+CP4ipDtm@zn.tnic> (raw)
In-Reply-To: <CAHk-=wh=Mu_EYhtOmPn6AxoQZyEh-4fo2Zx3G7rBv1g7vwoKiw@mail.gmail.com>

On Fri, Apr 15, 2022 at 03:10:51PM -0700, Linus Torvalds wrote:
> Adding PeterZ and Borislav (who seem to be the last ones to have
> worked on the copy and clear_page stuff respectively) and the x86
> maintainers in case somebody gets the urge to just fix this.

I guess if enough people ask and keep asking, some people at least try
to move...

> Because memory clearing should be faster than copying, and the thing
> that makes copying fast is that FSRM and ERMS logic (the whole
> "manually unrolled copy" is hopefully mostly a thing of the past and
> we can consider it legacy)

So I did give it a look and it seems to me, if we want to do the
alternatives thing here, it will have to look something like
arch/x86/lib/copy_user_64.S.

I.e., the current __clear_user() will have to become the "handle_tail"
thing there which deals with uncopied rest-bytes at the end and the new
fsrm/erms/rep_good variants will then be alternative_call_2 or _3.

The fsrm thing will have only the handle_tail thing at the end when size
!= 0.

The others - erms and rep_good - will have to check for sizes smaller
than, say a cacheline, and for those call the handle_tail thing directly
instead of going into a REP loop. The current __clear_user() is still a
lot better than that copy_user_generic_unrolled() abomination. And it's
not like old CPUs would get any perf penalty - they'll simply use the
same code.

And then you need the labels for _ASM_EXTABLE_UA() exception handling.

Anyway, something along those lines.

And then we'll need to benchmark this on a bunch of current machines to
make sure there's no funny surprises, perf-wise.

I can get cracking on this but I would advise people not to hold their
breaths. :)

Unless someone has a better idea or is itching to get hands dirty
her-/himself.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette