On Mon 20-01-20 17:21:32, Kirill A. Shutemov wrote:

On Mon, Jan 20, 2020 at 02:24:05PM +0100, Michal Hocko wrote:

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On Mon 20-01-20 15:39:35, Kirill A. Shutemov wrote:

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On Mon, Jan 20, 2020 at 12:27:22PM +0100, Michal Hocko wrote:

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On Mon 20-01-20 13:24:35, Kirill Tkhai wrote:

[...]

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Even two threads on common memory need a synchronization
to manage mappings in a sane way. Managing memory from two processes
is the same in principle, and the only difference is that another level
of synchronization is required.

Well, not really. The operation might simply attempt to perform an
operation on a specific memory area and get a failure if it doesn't
reference the same object anymore. What I think we need is some form of
a handle to operate on. In the past we have discussed several
directions. I was proposing /proc/self/map_anon/ (analogous to
map_files) where you could inspect anonymous memory and get a file
handle for it. madvise would then operate on the fd and then there
shouldn't be a real problem to revalidate that the object is still
valid. But there was no general enthusiasm about that approach. There
are likely some land mines on the way.

Converting anon memory to file-backed is bad idea and going to backfire.

I didn't mean to convert. I meant to expose that information via proc
the same way we do for file backed mappings. That shouldn't really
require to re-design the way how anonymous vma work IMO. But I haven't
tried that so there might be many gotchas there.

There are obvious things to think about though. Such fd cannot be sent
to other processes (SCM stuff), mmap of the file would have to be
disallowed and many others I am not aware of. I am not even pushing this
direction because I am not convinced about how viable it is myself. But
it would sound like a nice extension of the existing mechanism we have
and a file based madvise sounds attractive to me as well because we
already have that.

If the fd cannot be passed around or mmaped what does it represent?

Because it is a cookie maintained by the kernel.

And how is it different from plain address?

Kernel can revalidate that the given fd is denoting the vma it was
created for and simply fail with ENOENT or whatever suits if somebody
did unmap and mmap to the same address.
-- 
Michal Hocko
SUSE Labs