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

On Tue, May 28, 2019 at 3:33 AM Michal Hocko [off-list ref] wrote:

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

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

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

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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

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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?

Minchan's already done timing. More broadly, in an era with various
speculative execution mitigations, making a system call is pretty
expensive. If we have two options for remote VMA manipulation, one
that requires thousands of system calls (with the count proportional
to the address space size of the process) and one that requires only a
few system calls no matter how large the target process is, the latter
ought to start off with more points than the former under any kind of
design scoring.

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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?

It's very different. See below.

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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

If you want to operate on N VMAs simultaneously under an FD-per-VMA
model, you'd need to have those N FDs all open at the same time *and*
add some kind of system call that accepted those N FDs and an
operation to perform. The sequence number I'm proposing also applies
to the whole address space, not just one VMA. Even if you did have
these N FDs open all at once and supplied them all to some batch
operation, you couldn't guarantee via the FD mechanism that some *new*
VMA didn't appear in the address range you want to manipulate. A
global sequence number would catch this case. I still think supplying
a list of address ranges (like we already do for scatter-gather IO) is
less error-prone, less resource-intensive, more consistent with
existing practice, and equally flexible, especially if we start
supporting destructive cross-process memory operations, which may be
useful for things like checkpointing and optimizing process startup.

Besides: process_vm_readv and process_vm_writev already work on
address ranges. Why should other cross-process memory APIs use a very
different model for naming memory regions?

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.

We have various system calls that provide hints for open files, but
the memory operations are distinct. Modeling anonymous memory as a
kind of file-backed memory for purposes of VMA manipulation would also
be a departure from existing practice. Can you help me understand why
you seem to favor the FD-per-VMA approach so heavily? I don't see any
arguments *for* an FD-per-VMA model for remove memory manipulation and
I see a lot of arguments against it. Is there some compelling
advantage I'm missing?

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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.

A sequence number gives us this synchronization at very low cost and
adds safety. It's also a general-purpose mechanism that would
safeguard *any* cross-process VM operation, not just the VM operations
we're discussing right now.