linux-kernel - Re: [PATCH v2 01/13] mm: support madvise(MADV

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Date:	Wed, 4 Nov 2015 13:43:35 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Shaohua Li <shli@...nel.org>
Cc:	Minchan Kim <minchan@...nel.org>,
	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>, Daniel Micay <danielmicay@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Michal Hocko <mhocko@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>,
	Ben Maurer <bmaurer@...com>
Subject: Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

On Wed, Nov 4, 2015 at 12:00 PM, Shaohua Li <shli@...nel.org> wrote:
>
> The new proposal tries to fix the TLB issue. We introduce two madvise verbs:
>
> MARK_FREE. Userspace notifies kernel the memory range can be discarded. Kernel
> just records the range in current stage. Should memory pressure happen, page
> reclaim can free the memory directly regardless the pte state.
>
> MARK_NOFREE. Userspace notifies kernel the memory range will be reused soon.
> Kernel deletes the record and prevents page reclaim discards the memory. If the
> memory isn't reclaimed, userspace will access the old memory, otherwise do
> normal page fault handling.
>
> The point is to let userspace notify kernel if memory can be discarded, instead
> of depending on pte dirty bit used by MADV_FREE. With these, no TLB flush is
> required till page reclaim actually frees the memory (page reclaim need do the
> TLB flush for MADV_FREE too). It still preserves the lazy memory free merit of
> MADV_FREE.
>
> Compared to MADV_FREE, reusing memory with the new proposal isn't transparent,
> eg must call MARK_NOFREE. But it's easy to utilize the new API in jemalloc.
>

I can't speak to the usefulness of this or to other arches, but on x86
(unless you have nohz_full or similar enabled), a pair of syscalls
should be *much* faster than an IPI or a page fault.

I don't know how expensive it is to write to a clean page or to access
an unaccessed page on x86.  I'm sure it's not free (there's memory
bandwidth if nothing else), but it could be very cheap.

--Andy
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