linux-kernel - why not add __GFP_HIGHMEM directly in alloc_migrate

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Date:	Tue, 12 Jul 2016 21:16:07 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: why not add __GFP_HIGHMEM directly in alloc_migrate_target()?

alloc_migrate_target() is called from migrate_pages(),
so the page is always from user space, so why not add
__GFP_HIGHMEM directly, instead of the following code.

	if (PageHighMem(page))  // it always return false in x86_64
		gfp_mask |= __GFP_HIGHMEM;

Another question, when we do migration, why should split THP
first?
e.g. 2M(512*4kb) should flush 512 times TLB, and 2M(2M*1) only need one. 
I find flush TLB takes a lot of time, especially multithreaded app.

Thanks,
Xishi Qiu