Re: [PATCH v1] kernel/trace:check the val against the available mem

[Joel Fernandes]

Hi Steve,

On Fri, Mar 30, 2018 at 12:10 PM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
[..]
>> > I wonder if I should have the ring buffer allocate groups of pages, to
>> > avoid this. Or try to allocate with NORETRY, one page at a time, and
>> > when that fails, allocate groups of pages with RETRY_MAYFAIL, and that
>> > may keep it from causing an OOM?
>> >
>>
>> I don't see immediately how that can prevent an OOM in other
>> applications here? If ftrace allocates lots of memory with
>> RETRY_MAYFAIL, then we would still OOM in other applications if memory
>> isn't available. Sorry if I missed something.
>
> Here's the idea.
>
> Allocate one page at a time with NORETRY. If that fails, then allocate
> larger amounts (higher order of pages) with RETRY_MAYFAIL. Then if it
> can't get all the memory it needs, it wont take up all memory in the
> system before it finds out that it can't have any more.
>
> Or perhaps the memory management system can provide a
> get_available_mem() function that ftrace could call before it tries to
> increase the ring buffer and take up all the memory of the system
> before it realizes that it can't get all the memory it wants.
>
> The main problem I have with Zhaoyang's patch is that
> get_available_mem() does not belong in the tracing code. It should be
> something that the mm subsystem provides.
>

Cool. Personally I like the getting of available memory solution and
use that, since its simpler.

MM already provides it through si_mem_available since the commit
"mm/page_alloc.c: calculate 'available' memory in a separate function"
(sha d02bd27b). Maybe we could just use that?

MemAvailable was initially added in commit "/proc/meminfo: provide
estimated available memory" (sha 34e431b0ae39)

thanks,

- Joel