Re: [PATCH v7 3/3] mm: add a field to store names for private anonymous memory

Kirill A. Shutemov
2020-09-03T13:25:44Z

On Tue, Sep 01, 2020 at 09:44:59PM +0530, Sumit Semwal wrote:
> From: Colin Cross <ccross@google.com>
>
> In many userspace applications, and especially in VM based applications
> like Android uses heavily, there are multiple different allocators in use.
> At a minimum there is libc malloc and the stack, and in many cases there
> are libc malloc, the stack, direct syscalls to mmap anonymous memory, and
> multiple VM heaps (one for small objects, one for big objects, etc.).
> Each of these layers usually has its own tools to inspect its usage;
> malloc by compiling a debug version, the VM through heap inspection tools,
> and for direct syscalls there is usually no way to track them.
>
> On Android we heavily use a set of tools that use an extended version of
> the logic covered in Documentation/vm/pagemap.txt to walk all pages mapped
> in userspace and slice their usage by process, shared (COW) vs. unique
> mappings, backing, etc. This can account for real physical memory usage
> even in cases like fork without exec (which Android uses heavily to share
> as many private COW pages as possible between processes), Kernel SamePage
> Merging, and clean zero pages. It produces a measurement of the pages
> that only exist in that process (USS, for unique), and a measurement of
> the physical memory usage of that process with the cost of shared pages
> being evenly split between processes that share them (PSS).
>
> If all anonymous memory is indistinguishable then figuring out the real
> physical memory usage (PSS) of each heap requires either a pagemap walking
> tool that can understand the heap debugging of every layer, or for every
> layer's heap debugging tools to implement the pagemap walking logic, in
> which case it is hard to get a consistent view of memory across the whole
> system.
>
> Tracking the information in userspace leads to all sorts of problems.
> It either needs to be stored inside the process, which means every
> process has to have an API to export its current heap information upon
> request, or it has to be stored externally in a filesystem that
> somebody needs to clean up on crashes. It needs to be readable while
> the process is still running, so it has to have some sort of
> synchronization with every layer of userspace. Efficiently tracking
> the ranges requires reimplementing something like the kernel vma
> trees, and linking to it from every layer of userspace. It requires
> more memory, more syscalls, more runtime cost, and more complexity to
> separately track regions that the kernel is already tracking.
>
> This patch adds a field to /proc/pid/maps and /proc/pid/smaps to show a
> userspace-provided name for anonymous vmas. The names of named anonymous
> vmas are shown in /proc/pid/maps and /proc/pid/smaps as [anon:<name>].

Hm. I guess that there might be tools that expect the field to be empty
for anonymous memory, no?

> Userspace can set the name for a region of memory by calling
> prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, (unsigned long)name);
> Setting the name to NULL clears it.
>
> The name is stored in a user pointer in the shared union in vm_area_struct
> that points to a null terminated string inside the user process. vmas
> that point to the same address and are otherwise mergeable will be merged,
> but vmas that point to equivalent strings at different addresses will not
> be merged.
>
> The idea to store a userspace pointer to reduce the complexity within mm
> (at the expense of the complexity of reading /proc/pid/mem) came from Dave
> Hansen. This results in no runtime overhead in the mm subsystem other
> than comparing the anon_name pointers when considering vma merging. The
> pointer is stored in a union with fields that are only used on file-backed
> mappings, so it does not increase memory usage.
> (Upstream changed to remove the union, so this patch adds it back as well)

IIUC, it gives userspace direct control of content of /proc/$PID/maps and
/proc/$PID/smaps. There's no verification of the given string whatsoever.
I'm sure security experts would find clever usage of the feature :P

--
Kirill A. Shutemov