On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:

On Mon 26-09-16 18:17:50, Xishi Qiu wrote:

quoted

On 2016/9/26 17:43, Michal Hocko wrote:

quoted

On Mon 26-09-16 17:16:54, Xishi Qiu wrote:

quoted

On 2016/9/26 16:58, Michal Hocko wrote:

quoted

On Mon 26-09-16 16:47:57, Xishi Qiu wrote:

quoted

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?

AFAIR CMA wmark check was always tricky and the above commit has made
the situation at least a bit more clear. Anyway IIRC 

#ifdef CONFIG_CMA
	/* If allocation can't use CMA areas don't use free CMA pages */
	if (!(alloc_flags & ALLOC_CMA))
		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif

	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
		return false;

should reduce the prioblem because a lot of CMA pages should just get us
below the wmark + reserve boundary.

Hi Michal,

If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
are also enough, but they are fragment, then it will triger the problem.
If we alloc a high-order unmovable page, water mark check return *true*, but we
will alloc *failed*, right?

As Vlastimil has written. There were known issues with the wmark checks
and high order requests.

Shall we backport to stable?

I dunno, it was a part of a larger series with high atomic reserves and
changes which sound a bit intrusive for the stable kernel. Considering
that CMA was known to be problematic and there are still some issues
left I do not think this is worth the trouble/risk.

CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
removed due to Mel's high atomic reserve series.

That series is rather large and has some problems so I think that it
is not suitable for stable tree.

Thanks.

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