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

[Daniel Colascione]

On Tue, May 28, 2019 at 4:44 AM Minchan Kim <minchan@xxxxxxxxxx> 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.
>
> 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 think it's more natural for process_madvise to be a superset of
regular madvise. What's the harm? There are no security implications,
since anyone who could process_madvise could just ptrace anyway. I
also don't think limiting the hinting to non-destructive operations
guarantees safety (in a broad sense) either, since operating on the
wrong memory range can still cause unexpected system performance
issues even if there's no data loss.

More broadly, what I want to see is a family of process_* APIs that
provide a superset of the functionality that the existing intraprocess
APIs provide. I think this approach is elegant and generalizes easily.
I'm worried about prematurely limiting the interprocess memory APIs
and creating limitations that will last a long time in order to avoid
having to consider issues like VMA synchronization.