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

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Date: Mon, 8 Apr 2024 11:47:39 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Philipp Stanner <pstanner@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Boqun Feng <boqun.feng@...il.com>, rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,
	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
	Akira Yokosawa <akiyks@...il.com>,
	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

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