linux-kernel - Re: [PATCH v3 01/17] mm: support madvise(MADV

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Date:	Fri, 13 Nov 2015 15:15:11 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Daniel Micay <danielmicay@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Evans <je@...com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>
Subject: Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

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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