linux-kernel - Re: [PATCH v4 4/7] x86/tlb: fall back to flush all when meet a THP large page

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Date:	Fri, 11 May 2012 18:28:55 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	rob@...dley.net, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, arnd@...db.de, rostedt@...dmis.org,
	fweisbec@...il.com, jeremy@...p.org, gregkh@...uxfoundation.org,
	borislav.petkov@....com, riel@...hat.com, luto@....edu,
	avi@...hat.com, len.brown@...el.com, dhowells@...hat.com,
	fenghua.yu@...el.com, ak@...ux.intel.com, cpw@....com,
	steiner@....com, akpm@...ux-foundation.org, penberg@...nel.org,
	hughd@...gle.com, rientjes@...gle.com,
	kosaki.motohiro@...fujitsu.com, n-horiguchi@...jp.nec.com,
	paul.gortmaker@...driver.com, trenn@...e.de, tj@...nel.org,
	oleg@...hat.com, axboe@...nel.dk, a.p.zijlstra@...llo.nl,
	kamezawa.hiroyu@...fujitsu.com, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/7] x86/tlb: fall back to flush all when meet a THP
 large page

Hi,

On Thu, May 10, 2012 at 01:00:10PM +0800, Alex Shi wrote:
> +	for (addr = start; addr <= end; addr += HPAGE_SIZE) {
> +		pgd = pgd_offset(mm, addr);
> +		if (likely(!pgd_none(*pgd))) {
> +			pud = pud_offset(pgd, addr);
> +			if (likely(!pud_none(*pud))) {
> +				pmd = pmd_offset(pud, addr);
> +				if (likely(!pmd_none(*pmd)))
> +					if (pmd_large(*pmd))
> +						return 1;

Note with THP we've to set the pmd temporarily not present during
split_huge_page just before it returns a regular pmd (to avoid erratas
and mixing 4k and 2m tlb on the same physical page). So pmd_large
would fail in that small time window and we would do the invlpg loop
(which should still work safe so no big deal). But I believe you could
use pmd_trans_huge above, maybe gcc can drop the whole block with
CONFIG_TRANSPARENT_HUGEPAGE=n.
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