Re: [PATCHv2] uprobes: Use flexible array for xol_area bitmap

[Oleg Nesterov]

On 05/12, Masami Hiramatsu wrote:
>
> On Tue, 12 May 2026 13:29:52 +0200
> Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> > >
> > > -	area = kzalloc_obj(*area);
> > > +	area = kzalloc_flex(*area, bitmap, BITS_TO_LONGS(UINSNS_PER_PAGE));
> >
> > The downside is that kmalloc will use kmem_cache with ->object_size = PAGE_SIZE * 2,
> > almost half of the allocated memory won't be used...
>
> Hmm, is the bitmap so big?
>
> #define UINSNS_PER_PAGE			(PAGE_SIZE/UPROBE_XOL_SLOT_BYTES)
>
> And even on arm64,
>
> #define UPROBE_XOL_SLOT_BYTES	AARCH64_INSN_SIZE
>
> So if PAGE_SIZE is 4k, UINSNS_PER_PAGE is 1k, its BITS_TO_LONGS will
> be 1024/64 = 16. So 128 bytes. So the object is allocated from
> object_size = 256 ?

Indeed you are right.

Sorry for the noise and thanks for correcting me! I can't even explain how can
I came to conclusion that object_size can be greater than PAGE_SIZE with this
change ;)

So I think the patch from Rosen is fine.

Thanks,

Oleg.