Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Oct 12, 2021 at 12:37 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 11-10-21 18:20:25, Suren Baghdasaryan wrote:
> > On Mon, Oct 11, 2021 at 6:18 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > On Mon, Oct 11, 2021 at 1:36 AM Michal Hocko <mhocko@suse.com> wrote:
> > > >
> > > > On Fri 08-10-21 13:58:01, Kees Cook wrote:
> > > > > - Strings for "anon" specifically have no required format (this is good)
> > > > >   it's informational like the task_struct::comm and can (roughly)
> > > > >   anything. There's no naming convention for memfds, AF_UNIX, etc. Why
> > > > >   is one needed here? That seems like a completely unreasonable
> > > > >   requirement.
> > > >
> > > > I might be misreading the justification for the feature. Patch 2 is
> > > > talking about tools that need to understand memeory usage to make
> > > > further actions. Also Suren was suggesting "numbering convetion" as an
> > > > argument against.
> > > >
> > > > So can we get a clear example how is this being used actually? If this
> > > > is just to be used to debug by humans than I can see an argument for
> > > > human readable form. If this is, however, meant to be used by tools to
> > > > make some actions then the argument for strings is much weaker.
> > >
> > > The simplest usecase is when we notice that a process consumes more
> > > memory than usual and we do "cat /proc/$(pidof my_process)/maps" to
> > > check which area is contributing to this growth. The names we assign
> > > to anonymous areas are descriptive enough for a developer to get an
> > > idea where the increased consumption is coming from and how to proceed
> > > with their investigation.
> > > There are of course cases when tools are involved, but the end-user is
> > > always a human and the final report should contain easily
> > > understandable data.
>
> OK, it would have been much more preferable to be explicit about this
> main use case from the very beginning. Just to make sure we are at the
> same page. Is the primary usecase usage and bug reporting?

Sorry, I should have spent more time on patch #2 description. Yes,
debugging memory issues is the primary usecase. In fact that's the
only usecase in Android AFAIK.

>
> My initial understanding was that at userspace managed memory management
> could make an educated guess about targeted reclaim (e.g. MADV_{FREE,COLD,PAGEOUT}
> for cached data in memory like uncompressed images/data). Such a usecase
> would clearly require a standardized id/naming convention to be
> application neutral.

Ah, now I understand your angle. Our prior work on process_madvise()
probably helped in leading your thoughts in this direction :) Sorry
about the confusion.

>
> > > IIUC, the main argument here is whether the userspace can provide
> > > tools to perform the translations between ids and names, with the
> > > kernel accepting and reporting ids instead of strings. Technically
> > > it's possible, but to be practical that conversion should be fast
> > > because we will need to make name->id conversion potentially for each
> > > mmap. On the consumer side the performance is not as critical, but the
> > > fact that instead of dumping /proc/$pid/maps we will have to parse the
> > > file, do id->name conversion and replace all [anon:id] with
> > > [anon:name] would be an issue when we do that in bulk, for example
> > > when collecting system-wide data for a bugreport.
>
> Whether you use ids or human readable strings you still have to
> understand the underlying meaning to make any educated guess. Let me
> give you an example. Say I have an application with a memory leak. Right
> now I can only tell that it is anonymous memory growing but it is not
> clear who uses that anonymous. You are adding a means to tell different
> users appart. That is really helpful. Now I know this is an anon
> user 1234 or MySuperAnonMemory. Neither of the will not tell me more
> without a id/naming convention or reading the code. A convention can be
> useful for the most common users (e.g. a specific allocator) but I am
> rather dubious there are many more that would be _generally_ recognized
> without some understanding of the said application.

I guess an example would be better to clarify this. Here are some vma
names from Google maps app:

[anon:dalvik-main space (region space)]
[anon:dalvik-/apex/com.android.art/javalib/boot.art]
[anon:dalvik-/apex/com.android.art/javalib/boot-apache-xml.art]
[anon:.bss]
[anon:dalvik-zygote space]
[anon:dalvik-non moving space]
[anon:dalvik-free list large object space]
[anon:dalvik-/product/app/Maps/oat/arm64/Maps.art]
[anon:stack_and_tls:20792]
[anon:stack_and_tls:20791]
[anon:dalvik-LinearAlloc]
[anon:dalvik-CompilerMetadata]
[anon:dalvik-indirect ref table]
[anon:dalvik-live stack]
[anon:dalvik-allocation stack]
[anon:dalvik-large object free list space allocation info map]
[anon:scudo:primary]
[anon:scudo:secondary]
[anon:bionic_alloc_small_objects]

Most of them have names standard for Android and can be recognized by
developers and even Android framework (example where "anon:dalvik-main
space" and other standard names are being parsed:
https://cs.android.com/android/platform/superproject/+/master:frameworks/base/core/jni/android_os_Debug.cpp;l=340).
Names like "anon:dalvik-/apex/com.android.art/javalib/boot.art" help
the developer to recognize the component responsible for the memory.
Names like "anon:stack_and_tls:20792" include the TID of the thread
which uses this memory. All this information can help in narrowing
down memory consumption investigation. Hopefully these examples
clarify the usage a bit better?

>
> Maybe the situation in Android is different because the runtime is more
> coupled but is it reasonable to expect any common naming conventions for
> general Linux platforms?

Well, to be useful the system would have to agree to *some* convention I guess.

>
> I am slightly worried that we have spent way too much time talking
> specifics about id->name translation rather than the actual usability
> of the token.

Agree. I'll try to avoid further confusions.
Thanks!

> --
> Michal Hocko
> SUSE Labs