Re: [PATCH] ftrace: Do not over-allocate ftrace memory

[Guenter Roeck]

On Tue, Jan 06, 2026 at 02:26:23PM -0800, Guenter Roeck wrote:
> > >  
> > >  			pages -= remaining;
> > 
> > I think pages is meaningless here, as it was set in the beginning with:
> > 
> > 	pages = DIV_ROUND_UP(count, ENTRIES_PER_PAGE);
> > 
> > Which is incorrect. I wonder if we should set it via:
> > 
> I know, I just didn't want to make more changes than absolutely
> necessary.
> 
> > 	/*
> > 	 * Use ftrace_number_of_pages to determine how many pages were
> > 	 * allocated
> > 	 */
> > 	pages = ftrace_number_of_pages;
> > 
> > 	start_pg = ftrace_allocate_pages(count);
> > 	if (!start_pg)
> > 		return -ENOMEM;
> > 
> > 	/* ftrace_allocate_pages() increments ftrace_number_of_pages */
> > 	pages = ftrace_number_of_pages - pages;
> > 
> 
> That might work, assuming that the code updating ftrace_number_of_pages
> is (mutex) protected. I don't immediately see that, and the
> "mutex_lock(&ftrace_lock);" right after the above code makes me a bit
> concerned.
> 

One way to avoid the locking problem without potentially risky code changes
would be to pass a pointer to pages to ftrace_allocate_pages() and to
ftrace_allocate_records(), and to update it from there. I tested that and
confirmed that it works.

Guenter