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From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     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
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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?

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