Re: [PATCH v4 1/5] mm/page_idle: Add per-pid idle page tracking using virtual indexing

[Michal Hocko <mhocko@kernel.org>]

On Mon 05-08-19 13:04:47, Joel Fernandes (Google) wrote:
> The page_idle tracking feature currently requires looking up the pagemap
> for a process followed by interacting with /sys/kernel/mm/page_idle.
> Looking up PFN from pagemap in Android devices is not supported by
> unprivileged process and requires SYS_ADMIN and gives 0 for the PFN.
> 
> This patch adds support to directly interact with page_idle tracking at
> the PID level by introducing a /proc/<pid>/page_idle file.  It follows
> the exact same semantics as the global /sys/kernel/mm/page_idle, but now
> looking up PFN through pagemap is not needed since the interface uses
> virtual frame numbers, and at the same time also does not require
> SYS_ADMIN.
> 
> In Android, we are using this for the heap profiler (heapprofd) which
> profiles and pin points code paths which allocates and leaves memory
> idle for long periods of time. This method solves the security issue
> with userspace learning the PFN, and while at it is also shown to yield
> better results than the pagemap lookup, the theory being that the window
> where the address space can change is reduced by eliminating the
> intermediate pagemap look up stage. In virtual address indexing, the
> process's mmap_sem is held for the duration of the access.

As already mentioned in one of the previous versions. The interface
seems sane and the usecase as well. So I do not really have high level
objections.

>From a quick look at the patch I would just object to pulling swap idle
tracking into this patch because it makes the review harder and it is
essentially a dead code until a later patch. I am also not sure whether
that is really necessary and it really begs for an explicit
justification.

I will try to go through the patch more carefully later as time allows.

> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
-- 
Michal Hocko
SUSE Labs