Re: [RFCv1 0/6] Page Detective

[Andi Kleen <ak@linux.intel.com>]

Pasha Tatashin <pasha.tatashin@soleen.com> writes:

> Page Detective is a new kernel debugging tool that provides detailed
> information about the usage and mapping of physical memory pages.
>
> It is often known that a particular page is corrupted, but it is hard to
> extract more information about such a page from live system. Examples
> are:
>
> - Checksum failure during live migration
> - Filesystem journal failure
> - dump_page warnings on the console log
> - Unexcpected segfaults
>
> Page Detective helps to extract more information from the kernel, so it
> can be used by developers to root cause the associated problem.
>
> It operates through the Linux debugfs interface, with two files: "virt"
> and "phys".
>
> The "virt" file takes a virtual address and PID and outputs information
> about the corresponding page.
>
> The "phys" file takes a physical address and outputs information about
> that page.
>
> The output is presented via kernel log messages (can be accessed with
> dmesg), and includes information such as the page's reference count,
> mapping, flags, and memory cgroup. It also shows whether the page is
> mapped in the kernel page table, and if so, how many times.

A lot of all that is already covered in /proc/kpage{flags,cgroup,count)
Also we already have /proc/pid/pagemap to resolve virtual addresses.

At a minimum you need to discuss why these existing mechanisms are not
suitable for you and how your new one is better.

If something particular is missing perhaps the existing mechanisms
can be extended?

Outputting in the dmesg seems rather clumpsy for a production mechanism.

I personally would just use live crash or live gdb on /proc/kcore to get
extra information, although I can see that might have races.

-Andi