Re: [PATCH bpf-next 06/13] uprobes/x86: Add uprobe syscall to speed up uprobe

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Dec 13, 2024 at 04:12:46PM +0100, Thomas Weißschuh wrote:

SNIP

> > > > +static int __init arch_uprobes_init(void)
> > > > +{
> > > > +	unsigned long size = uprobe_trampoline_end - uprobe_trampoline_entry;
> > > > +	static struct page *pages[2];
> > > > +	struct page *page;
> > > > +
> > > > +	page = alloc_page(GFP_HIGHUSER);
> > > 
> > > That page could be in static memory, removing the need for the explicit
> > > allocation. It could also be __ro_after_init.
> > > Then tramp_mapping itself can be const.
> > 
> > hum, how would that look like? I think that to get proper page object
> > you have to call alloc_page or some other page alloc family function..
> > what do I miss?
> 
> static u8 trampoline_page[PAGE_SIZE] __ro_after_init __aligned(PAGE_SIZE);
> static struct page *tramp_mapping_pages[2] __ro_after_init;
> 
> static const struct vm_special_mapping tramp_mapping = {
> 	.name   = "[uprobes-trampoline]",
> 	.pages  = tramp_mapping_pages,
> 	.mremap = tramp_mremap,
> };
> 
> static int __init arch_uprobes_init(void)
> {
> 	...
> 	trampoline_pages[0] = virt_to_page(trampoline_page);
> 	...
> }
> 
> Untested, but it's similar to the stuff the vDSO implementations are
> doing which I am working with at the moment.

nice idea, better than allocating the page, will do that

> 
> > > 
> > > Also this seems to waste the page on 32bit kernels.
> > 
> > it's inside CONFIG_X86_64 ifdef
> > 
> > > 
> > > > +	if (!page)
> > > > +		return -ENOMEM;
> > > > +	pages[0] = page;
> > > > +	tramp_mapping.pages = (struct page **) &pages;
> > > 
> > > tramp_mapping.pages = pages; ?
> > 
> > I think the compiler will cry about *pages[2] vs **pages types mismatch,
> > but I'll double check that
> 
> It compiles for me.

ok

thanks,
jirka