linux-kernel - Re: [PATCH v2] mm: add free_hot_cold_page

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Date:	Thu, 10 Nov 2011 18:31:05 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
cc:	linux-mm@...ck.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, minchan.kim@...il.com
Subject: Re: [PATCH v2] mm: add free_hot_cold_page_list helper

On Tue, 1 Nov 2011, Konstantin Khlebnikov wrote:

> This patch adds helper free_hot_cold_page_list() to free list of 0-order pages.
> It frees pages directly from the list without temporary page-vector.
> It also calls trace_mm_pagevec_free() to simulate pagevec_free() behaviour.

Sorry for not speaking up sooner, but I do like this patch very much
(and I'm content with your trace compatibility choice - whatever).

Not so much in itself, but because it then allows a further patch
(mainly to mm/vmscan.c) to remove two levels of pagevec, reducing
its deepest stack by around 240 bytes.

I have that patch, but keep putting off sending it in, because I want
to show a reclaim stack overflow that it prevents, but the new avoidance
of writeback in direct reclaim makes that harder to demonstrate.  Damn!

One question on your patch: where you have release_pages() doing
> +		list_add_tail(&page->lru, &pages_to_free);

That seems reasonable, but given that __pagevec_free() proceeds by
	while (--i >= 0) {
, starting from the far end of the pagevec (the most recently added
struct page, the most likely to be hot), wouldn't you reproduce
existing behaviour more accurately by a simple list_add()?

Or have I got that back to front?  If so, a comment on the
list_add_tail() would help me to remember why - thanks.

Hugh
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