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

[Minchan Kim <minchan@kernel.org>]

On Thu, Nov 12, 2015 at 12:21:30AM -0500, Daniel Micay wrote:
> > 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.

So, do you mean that we could implement MADV_FREE_UNDO with "read"
opearation("just access bit marking) easily in future?

If so, it would be good reason to change MADV_FREE from dirty bit to
access bit. Okay, I will look at that.

> 
> 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
> 


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