Re: [RFC 7/7] mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER

[Minchan Kim <minchan@kernel.org>]

On Tue, May 28, 2019 at 04:42:47AM -0700, Daniel Colascione wrote:
> On Tue, May 28, 2019 at 4:28 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Tue 28-05-19 20:12:08, Minchan Kim wrote:
> > > On Tue, May 28, 2019 at 12:41:17PM +0200, Michal Hocko wrote:
> > > > On Tue 28-05-19 19:32:56, Minchan Kim wrote:
> > > > > On Tue, May 28, 2019 at 11:08:21AM +0200, Michal Hocko wrote:
> > > > > > On Tue 28-05-19 17:49:27, Minchan Kim wrote:
> > > > > > > On Tue, May 28, 2019 at 01:31:13AM -0700, Daniel Colascione wrote:
> > > > > > > > On Tue, May 28, 2019 at 1:14 AM Minchan Kim <minchan@kernel.org> wrote:
> > > > > > > > > if we went with the per vma fd approach then you would get this
> > > > > > > > > > feature automatically because map_files would refer to file backed
> > > > > > > > > > mappings while map_anon could refer only to anonymous mappings.
> > > > > > > > >
> > > > > > > > > The reason to add such filter option is to avoid the parsing overhead
> > > > > > > > > so map_anon wouldn't be helpful.
> > > > > > > >
> > > > > > > > Without chiming on whether the filter option is a good idea, I'd like
> > > > > > > > to suggest that providing an efficient binary interfaces for pulling
> > > > > > > > memory map information out of processes.  Some single-system-call
> > > > > > > > method for retrieving a binary snapshot of a process's address space
> > > > > > > > complete with attributes (selectable, like statx?) for each VMA would
> > > > > > > > reduce complexity and increase performance in a variety of areas,
> > > > > > > > e.g., Android memory map debugging commands.
> > > > > > >
> > > > > > > I agree it's the best we can get *generally*.
> > > > > > > Michal, any opinion?
> > > > > >
> > > > > > I am not really sure this is directly related. I think the primary
> > > > > > question that we have to sort out first is whether we want to have
> > > > > > the remote madvise call process or vma fd based. This is an important
> > > > > > distinction wrt. usability. I have only seen pid vs. pidfd discussions
> > > > > > so far unfortunately.
> > > > >
> > > > > With current usecase, it's per-process API with distinguishable anon/file
> > > > > but thought it could be easily extended later for each address range
> > > > > operation as userspace getting smarter with more information.
> > > >
> > > > Never design user API based on a single usecase, please. The "easily
> > > > extended" part is by far not clear to me TBH. As I've already mentioned
> > > > several times, the synchronization model has to be thought through
> > > > carefuly before a remote process address range operation can be
> > > > implemented.
> > >
> > > I agree with you that we shouldn't design API on single usecase but what
> > > you are concerning is actually not our usecase because we are resilient
> > > with the race since MADV_COLD|PAGEOUT is not destruptive.
> > > Actually, many hints are already racy in that the upcoming pattern would
> > > be different with the behavior you thought at the moment.
> >
> > How come they are racy wrt address ranges? You would have to be in
> > multithreaded environment and then the onus of synchronization is on
> > threads. That model is quite clear. But we are talking about separate
> > processes and some of them might be even not aware of an external entity
> > tweaking their address space.
> 
> I don't think the difference between a thread and a process matters in
> this context. Threads race on address space operations all the time
> --- in the sense that multiple threads modify a process's address
> space without synchronization. The main reasons that these races
> hasn't been a problem are: 1) threads mostly "mind their own business"
> and modify different parts of the address space or use locks to ensure
> that they don't stop on each other (e.g., the malloc heap lock), and
> 2) POSIX mmap atomic-replacement semantics make certain classes of
> operation (like "magic ring buffer" setup) safe even in the presence
> of other threads stomping over an address space.
> 
> The thing that's new in this discussion from a synchronization point
> of view isn't that the VM operation we're talking about is coming from
> outside the process, but that we want to do a read-decide-modify-ish
> thing. We want to affect (using various hints) classes of pages like
> "all file pages" or "all anonymous pages" or "some pages referring to
> graphics buffers up to 100MB" (to pick an example off the top of my
> head of a policy that might make sense). From a synchronization point
> of view, it doesn't really matter whether it's a thread within the
> target process or a thread outside the target process that does the
> address space manipulation. What's new is the inspection of the
> address space before performing an operation.
> 
> Minchan started this thread by proposing some flags that would
> implement a few of the filtering policies I used as examples above.
> Personally, instead of providing a few pre-built policies as flags,
> I'd rather push the page manipulation policy to userspace as much as
> possible and just have the kernel provide a mechanism that *in
> general* makes these read-decide-modify operations efficient and
> robust. I still think there's way to achieve this goal very
> inexpensively without compromising on flexibility.

I'm looking forward to seeing the way. ;-)