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

[Matthew Wilcox <willy@infradead.org>]

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
-ENOMEM?

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?