linux-kernel - Re: [PATCH 07/32] mm: Bring back vmalloc

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Message-ID: <ZFq7JhrhyrMTNfd/@moria.home.lan>
Date:   Tue, 9 May 2023 17:29:10 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

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@...il.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?

None that I see.

The background is that bcachefs generates a per btree node unpack
function, based on the packed format for that btree node, for unpacking
keys within that node. The unpack function is only ~50 bytes, and for
locality we want it to be located with the btree node's other in-memory
lookup tables so they can be prefetched all at once.

Here's the codegen:

https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/bkey.c#n727