Re: RFC: named anonymous vmas

[Sam Ben]

Hi Colin,
On 06/22/2013 07:42 AM, Colin Cross wrote:
> One of the features of ashmem (drivers/staging/android/ashmem.c) that
> hasn't gotten much discussion about moving out of staging is named
> anonymous memory.
>
> In Android, ashmem is used for three different features, and most
> users of it only care about one feature at a time.  One is volatile
> ranges, which John Stultz has been implementing.  The second is
> anonymous shareable memory without having a world-writable tmpfs that
> untrusted apps could fill with files.  The third and most heavily used

How to understand "anonymous shareable memory without having a 
world-writable tmpfs that untrusted apps could fill with files"?

> feature within the Android codebase is named anonymous memory, where a
> region of anonymous memory can have a name associated with it that
> will show up in /proc/pid/maps.  The Dalvik VM likes to use this
> feature extensively, even for memory that will never be shared and
> could easily be allocated using an anonymous mmap, and even malloc has
> used it in the past.  It provides an easy way to collate memory used
> for different purposes across multiple processes, which Android uses
> for its "dumpsys meminfo" and "librank" tools to determine how much
> memory is used for java heaps, JIT caches, native mallocs, etc.
>
> I'd like to add this feature for anonymous mmap memory.  I propose
> adding an madvise2(unsigned long start, size_t len_in, int behavior,
> void *ptr, size_t size) syscall and a new MADV_NAME behavior, which
> treats ptr as a string of length size.  The string would be copied
> somewhere reusable in the kernel, or reused if it already exists, and
> the kernel address of the string would get stashed in a new field in
> struct vm_area_struct.  Adjacent vmas would only get merged if the
> name pointer matched, and naming part of a mapping would split the
> mapping.  show_map_vma would print the name only if none of the other
> existing names rules match.
>
> Any comments as I start implementing it?  Is there any reason to allow
> naming a file-backed mapping and showing it alongside the file name in
> /proc/pid/maps?
>
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