Re: [LSF/MM/BPF TOPIC] tracing the source of errors

From: Matthew Wilcox
Date: Thu Feb 08 2024 - 10:57:39 EST

On Thu, Feb 08, 2024 at 08:37:29AM +1100, Dave Chinner wrote:
> ftrace using the function_graph tracer will emit the return values
> of the functions if you use it with the 'funcgraph-retval' option.

OK, but that may not be fine grained enough. Why is mmap() returning

unsigned long do_mmap(struct file *file, unsigned long addr,
/* Careful about overflows.. */
len = PAGE_ALIGN(len);
if (!len)
return -ENOMEM;
/* Too many mappings? */
if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;

So it can distinguish between mmap() returning ENOMEM because
get_unmapped_area() returned ENOMEM and do_mmap() returning ENOMEM of
its own accord (right?), but it can't tell you which of the above two
cases you hit. Or can it?