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

[Michal Hocko <mhocko@kernel.org>]

On Tue 28-05-19 02:39:03, Daniel Colascione wrote:
> On Tue, May 28, 2019 at 2:08 AM Michal Hocko <mhocko@kernel.org> 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.
> 
> I don't think the vma fd approach is viable. We have some processes
> with a *lot* of VMAs --- system_server had 4204 when I checked just
> now (and that's typical) --- and an FD operation per VMA would be
> excessive.

What do you mean by excessive here? Do you expect the process to have
them open all at once?

> VMAs also come and go pretty easily depending on changes in
> protections and various faults.

Is this really too much different from /proc/<pid>/map_files?

[...]

> > An interface to query address range information is a separate but
> > although a related topic. We have /proc/<pid>/[s]maps for that right
> > now and I understand it is not a general win for all usecases because
> > it tends to be slow for some. I can see how /proc/<pid>/map_anons could
> > provide per vma information in a binary form via a fd based interface.
> > But I would rather not conflate those two discussions much - well except
> > if it could give one of the approaches more justification but let's
> > focus on the madvise part first.
> 
> I don't think it's a good idea to focus on one feature in a
> multi-feature change when the interactions between features can be
> very important for overall design of the multi-feature system and the
> design of each feature.
> 
> Here's my thinking on the high-level design:
> 
> I'm imagining an address-range system that would work like this: we'd
> create some kind of process_vm_getinfo(2) system call [1] that would
> accept a statx-like attribute map and a pid/fd parameter as input and
> return, on output, two things: 1) an array [2] of VMA descriptors
> containing the requested information, and 2) a VMA configuration
> sequence number. We'd then have process_madvise() and other
> cross-process VM interfaces accept both address ranges and this
> sequence number; they'd succeed only if the VMA configuration sequence
> number is still current, i.e., the target process hasn't changed its
> VMA configuration (implicitly or explicitly) since the call to
> process_vm_getinfo().

The sequence number is essentially a cookie that is transparent to the
userspace right? If yes, how does it differ from a fd (returned from
/proc/<pid>/map_{anons,files}/range) which is a cookie itself and it can
be used to revalidate when the operation is requested and fail if
something has changed. Moreover we already do have a fd based madvise
syscall so there shouldn't be really a large need to add a new set of
syscalls.

[...]

> Or maybe the whole sequence number thing is overkill and we don't need
> atomicity? But if there's a concern  that A shouldn't operate on B's
> memory without knowing what it's operating on, then the scheme I've
> proposed above solves this knowledge problem in a pretty lightweight
> way.

This is the main question here. Do we really want to enforce an external
synchronization between the two processes to make sure that they are
both operating on the same range - aka protect from the range going away
and being reused for a different purpose. Right now it wouldn't be fatal
because both operations are non destructive but I can imagine that there
will be more madvise operations to follow (including those that are
destructive) because people will simply find usecases for that. This
should be reflected in the proposed API.
-- 
Michal Hocko
SUSE Labs