Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 9 May 2023 17:54:26 -0400
From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-bcachefs@vger.kernel.org,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Uladzislau Rezki <urezki@gmail.com>, linux-mm@kvack.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec
Message-ID: <ZFrBEsjrfseCUzqV@moria.home.lan>
References: <20230509165657.1735798-1-kent.overstreet@linux.dev>
 <20230509165657.1735798-8-kent.overstreet@linux.dev>
 <ZFqxEWqD19eHe353@infradead.org>
 <ZFq3SdSBJ_LWsOgd@murray>
 <20230509214319.GA858791@frogsfrogsfrogs>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230509214319.GA858791@frogsfrogsfrogs>
Precedence: bulk

On Tue, May 09, 2023 at 02:43:19PM -0700, Darrick J. Wong wrote:
> On Tue, May 09, 2023 at 02:12:41PM -0700, Lorenzo Stoakes wrote:
> > On Tue, May 09, 2023 at 01:46:09PM -0700, Christoph Hellwig wrote:
> > > On Tue, May 09, 2023 at 12:56:32PM -0400, Kent Overstreet wrote:
> > > > From: Kent Overstreet <kent.overstreet@gmail.com>
> > > >
> > > > This is needed for bcachefs, which dynamically generates per-btree node
> > > > unpack functions.
> > >
> > > No, we will never add back a way for random code allocating executable
> > > memory in kernel space.
> > 
> > Yeah I think I glossed over this aspect a bit as it looks ostensibly like simply
> > reinstating a helper function because the code is now used in more than one
> > place (at lsf/mm so a little distracted :)
> > 
> > But it being exported is a problem. Perhaps there's another way of acheving the
> > same aim without having to do so?
> 
> I already trolled Kent with this on IRC, but for the parts of bcachefs
> that want better assembly code than whatever gcc generates from the C
> source, could you compile code to BPF and then let the BPF JIT engines
> turn that into machine code for you?

It's an intriguing idea, but it'd be a _lot_ of work and this is old
code that's never had a single bug - I'm not in a hurry to rewrite it.

And there would still be the issue that we've still got lots of little
unpack functions that go with other tables; we can't just burn a full
page per unpack function, that would waste way too much memory, and if
we put them together then we're stuck writing a whole nother allocator
- nope, and then we're also mucking with the memory layout of the data
structures used in the very hottest paths in the filesystem - I'm very
wary of introducing performance regressions there.

I think it'd be much more practical to find some way of making
vmalloc_exec() more palatable. What are the exact concerns?