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

[Minchan Kim]

On Tue, May 28, 2019 at 02:06:14PM +0200, Michal Hocko wrote:
> On Tue 28-05-19 20:44:36, Minchan Kim wrote:
> > On Tue, May 28, 2019 at 01:28:40PM +0200, Michal Hocko 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@xxxxxxxxxx> 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
> > 
> > Think about MADV_FREE. Allocator would think the chunk is worth to mark
> > "freeable" but soon, user of the allocator asked the chunk - ie, it's not
> > freeable any longer once user start to use it.
> 
> That is not a race in the address space, right. The underlying object
> hasn't changed. It has been declared as freeable and since that moment
> nobody can rely on the content because it might have been discarded.
> Or put simply, the content is undefined. It is responsibility of the
> madvise caller to make sure that the object is not in active use while
> it is marking it.
> 
> > My point is that kinds of *hints* are always racy so any synchronization
> > couldn't help a lot. That's why I want to restrict hints process_madvise
> > supports as such kinds of non-destruptive one at next respin.
> 
> I agree that a non-destructive operations are safer against paralel
> modifications because you just get a annoying and unexpected latency at
> worst case. But we should discuss whether this assumption is sufficient
> for further development. I am pretty sure once we open remote madvise
> people will find usecases for destructive operations or even new madvise
> modes we haven't heard of. What then?

I support Daniel's vma seq number approach for the future plan.