Re: [WIP 0/3] Memory model and atomic API in Rust

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	"Philipp Stanner" <pstanner@redhat.com>,
	"Kent Overstreet" <kent.overstreet@linux.dev>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org, llvm@lists.linux.dev,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Gary Guo" <gary@garyguo.net>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Alan Stern" <stern@rowland.harvard.edu>,
	"Andrea Parri" <parri.andrea@gmail.com>,
	"Will Deacon" <will@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"David Howells" <dhowells@redhat.com>,
	"Jade Alglave" <j.alglave@ucl.ac.uk>,
	"Luc Maranget" <luc.maranget@inria.fr>,
	"Akira Yokosawa" <akiyks@gmail.com>,
	"Daniel Lustig" <dlustig@nvidia.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	kent.overstreet@gmail.com,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	elver@google.com, "Mark Rutland" <mark.rutland@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
Date: Mon, 8 Apr 2024 11:47:39 -0700	[thread overview]
Message-ID: <c3e507cb-26bb-4fdc-b1dc-b512c7bfc093@paulmck-laptop> (raw)
In-Reply-To: <ZhQjT4xdS3h-GbtC@casper.infradead.org>

On Mon, Apr 08, 2024 at 06:03:11PM +0100, Matthew Wilcox wrote:
> On Mon, Apr 08, 2024 at 09:55:23AM -0700, Paul E. McKenney wrote:
> > On Mon, Apr 08, 2024 at 05:02:37PM +0100, Matthew Wilcox wrote:
> > > In my ideal world, the compiler would turn this into:
> > > 
> > > 	newfolio->flags |= folio->flags & MIGRATE_MASK;
> > 
> > Why not accumulate the changes in a mask, and then apply the mask the
> > one time?  (In situations where __folio_set_foo() need not apply.)
> 
> Yes, absolutely, we can, should and probably eventually will do this
> when it gets to the top of somebody's todo list.  But it irks me that
> we can't tell the compiler this is a safe transformation for it to make.
> There are a number of places where similar things happen.
> 
> $ git grep folio_test.*folio_test
> 
> will find you 82 of them (where they happen to be on the same line)
> 
>                 if (folio_test_dirty(folio) || folio_test_locked(folio) ||
>                                 folio_test_writeback(folio))
>                         break;
> 
> turns into:
> 
>     1f41:       48 8b 29                mov    (%rcx),%rbp
>     1f44:       48 c1 ed 04             shr    $0x4,%rbp
>     1f48:       83 e5 01                and    $0x1,%ebp
>     1f4b:       0f 85 d5 00 00 00       jne    2026 <filemap_range_has_writeback+0x1a6>
>     1f51:       48 8b 29                mov    (%rcx),%rbp
>     1f54:       83 e5 01                and    $0x1,%ebp
>     1f57:       0f 85 c9 00 00 00       jne    2026 <filemap_range_has_writeback+0x1a6>
>     1f5d:       48 8b 29                mov    (%rcx),%rbp
>     1f60:       48 d1 ed                shr    $1,%rbp
>     1f63:       83 e5 01                and    $0x1,%ebp
>     1f66:       0f 85 ba 00 00 00       jne    2026 <filemap_range_has_writeback+0x1a6>
> 
> rather than _one_ load from rcx and a test against a mask.

Agreed, it would be nice if we could convince the compiler to do this
for us, preferably without breaking anything.

> > If it turns out that we really do need a not-quite-volatile, what exactly
> > does it do?  You clearly want it to be able to be optimized so as to merge
> > similar accesses.  Is there a limit to the number of accesses that can
> > be merged or to the region of code over which such merging is permitted?
> > Either way, how is the compiler informed of these limits?
> 
> Right, like I said, it's not going to be easy to define exactly what we
> want.

Or to convince the usual suspects that any definition we might come up
with is useful/implementable/teacheable/...  :-/

							Thanx, Paul

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