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

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Message-ID: <ZGFyHY6pH9CU4fzf@moria.home.lan>
Date:   Sun, 14 May 2023 19:43:25 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-bcachefs@...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" <linux-mm@...ck.org>
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

On Sun, May 14, 2023 at 06:39:00PM +0000, Christophe Leroy wrote:
> I addition to that, I still don't understand why you bring back 
> vmalloc_exec() instead of using module_alloc().
> 
> As reminded in a previous response, some architectures like powerpc/32s 
> cannot allocate exec memory in vmalloc space. On powerpc this is because 
> exec protection is performed on 256Mbytes segments and vmalloc space is 
> flagged non-exec. Some other architectures have a constraint on distance 
> between kernel core text and other text.
> 
> Today you have for instance kprobes in the kernel that need dynamic exec 
> memory. It uses module_alloc() to get it. On some architectures you also 
> have ftrace that gets some exec memory with module_alloc().
> 
> So, I still don't understand why you cannot use module_alloc() and need 
> vmalloc_exec() instead.

Because I didn't know about it :)

Looks like that is indeed the appropriate interface (if a bit poorly
named), I'll switch to using that, thanks.

It'll still need to be exported, but it looks like the W|X attribute
discussion is not really germane here since it's what other in kernel
users are using, and there's nothing particularly special about how
bcachefs is using it compared to them.