linux-kernel - [RFC] mm: a question about high-order check in __zone_watermark

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Date:   Mon, 26 Sep 2016 16:47:57 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>
CC:     LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Yisheng Xie <xieyisheng1@...wei.com>
Subject: [RFC] mm: a question about high-order check in __zone_watermark_ok()

commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
(mm, page_alloc: only enforce watermarks for order-0 allocations)
rewrite the high-order check in __zone_watermark_ok(), but I think it
quietly fix a bug. Please see the following.

Before this patch, the high-order check is this:
__zone_watermark_ok()
	...
	for (o = 0; o < order; o++) {
		/* At the next order, this order's pages become unavailable */
		free_pages -= z->free_area[o].nr_free << o;

		/* Require fewer higher order pages to be free */
		min >>= 1;

		if (free_pages <= min)
			return false;
	}
	...

If we have cma memory, and we alloc a high-order movable page, then it's right.

But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
and there are a lot of high-order cma pages, but little high-order unmovable
pages, the it is still return *true*, but we will alloc *failed* finally, because
we cannot fallback from migrate_unmovable to migrate_cma, right?

Also if we doing __alloc_pages_slowpath(), the compact will not work, because
__zone_watermark_ok() always return true, and it lead to alloc a high-order
unmovable page failed, then do direct reclaim.

Thanks,
Xishi Qiu