Re: [RFC 6/7] mm: extend process_madvise syscall to support vector arrary

On Tue, May 21, 2019 at 08:24:21AM +0200, Michal Hocko wrote:

On Tue 21-05-19 11:48:20, Minchan Kim wrote:

quoted

On Mon, May 20, 2019 at 11:22:58AM +0200, Michal Hocko wrote:

quoted

[Cc linux-api]

On Mon 20-05-19 12:52:53, Minchan Kim wrote:

quoted

Currently, process_madvise syscall works for only one address range
so user should call the syscall several times to give hints to
multiple address range.

Is that a problem? How big of a problem? Any numbers?

We easily have 2000+ vma so it's not trivial overhead. I will come up
with number in the description at respin.

Does this really have to be a fast operation? I would expect the monitor
is by no means a fast path. The system call overhead is not what it used
to be, sigh, but still for something that is not a hot path it should be
tolerable, especially when the whole operation is quite expensive on its
own (wrt. the syscall entry/exit).

What's different with process_vm_[readv|writev] and vmsplice?
If the range needed to be covered is a lot, vector operation makes senese
to me.

I am not saying we do not need a multiplexing API, I am just not sure
we need it right away. Btw. there was some demand for other MM syscalls
to provide a multiplexing API (e.g. mprotect), maybe it would be better
to handle those in one go?

That's the exactly what Daniel Colascione suggested from internal
review. That would be a interesting approach if we could aggregate
all of system call in one go.

-- 
Michal Hocko
SUSE Labs