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

On Tue 28-05-19 02:39:03, Daniel Colascione wrote:

On Tue, May 28, 2019 at 2:08 AM Michal Hocko [off-list ref] wrote:

quoted

On Tue 28-05-19 17:49:27, Minchan Kim wrote:

quoted

On Tue, May 28, 2019 at 01:31:13AM -0700, Daniel Colascione wrote:

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On Tue, May 28, 2019 at 1:14 AM Minchan Kim [off-list ref] wrote:

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if we went with the per vma fd approach then you would get this

quoted

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?

[...]

quoted

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