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

On Tue, May 28, 2019 at 12:41:17PM +0200, Michal Hocko wrote:

On Tue 28-05-19 19:32:56, Minchan Kim wrote:

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On Tue, May 28, 2019 at 11:08:21AM +0200, Michal Hocko 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.

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.

If you are still concerning of address range synchronization, how about
moving such hints to per-process level like prctl?
Does it make sense to you?