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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 12 May 2026 13:29:52 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 05/11, Rosen Penev wrote:
> >
> >  struct xol_area {
> >  	wait_queue_head_t		wq;		/* if all slots are busy */
> > -	unsigned long			*bitmap;	/* 0 = free slot */
> >
> >  	struct page			*page;
> >  	/*
> > @@ -117,6 +116,7 @@ struct xol_area {
> >  	 * the vma go away, and we must handle that reasonably gracefully.
> >  	 */
> >  	unsigned long			vaddr;		/* Page(s) of instruction slots */
> > +	unsigned long			bitmap[];	/* 0 = free slot */
> >  };
> >
> >  static void uprobe_warn(struct task_struct *t, const char *msg)
> > @@ -1755,18 +1755,13 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
> >  	struct xol_area *area;
> >  	void *insns;
> >
> > -	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 ?

Thank you,

> 
> But technically the patch looks correct so I won't argue.
> 
> Oleg.
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>