Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

I also think that the kernel should commit to either zeroing the page
or leaving it unchanged in response to MADV_FREE (even if the decision
of which to do is made later on).  I think that your patch series does
this, but only after a few of the patches are applied (the swap entry
freeing), and I think that it should be a real guaranteed part of the
semantics and maybe have a test case.

This would be a good thing to test because it would be required to add
MADV_FREE_UNDO down the road. It would mean the same semantics as the
MEM_RESET and MEM_RESET_UNDO features on Windows, and there's probably
value in that for the sake of migrating existing software too.

For one example, it could be dropped into Firefox:

https://dxr.mozilla.org/mozilla-central/source/memory/volatile/VolatileBufferWindows.cpp

And in Chromium:

https://code.google.com/p/chromium/codesearch#chromium/src/base/memory/discardable_shared_memory.cc

Worth noting that both also support the API for pinning/unpinning that's
used by Android's ashmem too. Linux really needs a feature like this for
caches. Firefox simply doesn't drop the memory at all on Linux right now:

https://dxr.mozilla.org/mozilla-central/source/memory/volatile/VolatileBufferFallback.cpp

(Lock == pin, Unlock == unpin)

For reference:

https://msdn.microsoft.com/en-us/library/windows/desktop/aa366887(v=vs.85).aspx