linux-kernel - Re: [PATCH] futex: Calculate the futex key based on a tail page for file-based futexes

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Date:	Wed, 8 Jun 2016 06:05:24 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] futex: Calculate the futex key based on a tail page for
 file-based futexes

On Tue, 07 Jun 2016, Mel Gorman wrote:

>Mike Galbraith reported that the LTP test case futex_wake04 was broken
>by commit 65d8fc777f6d ("futex: Remove requirement for lock_page()
>in get_futex_key()").
>
>This test case uses futexes backed by hugetlbfs pages and so there is an
>associated inode with a futex stored on such pages. The problem is that
>the key is being calculated based on the head page index of the hugetlbfs
>page and not the tail page.
>
>Prior to the optimisation, the page lock was used to stabilise mappings and
>pin the inode is file-backed which is overkill. If the page was a compound
>page, the head page was automatically looked up as part of the page lock
>operation but the tail page index was used to calculate the futex key.
>
>After the optimisation, the compound head is looked up early and the page
>lock is only relied upon to identify truncated pages, special pages or a
>shmem page moving to swapcache. The head page is looked up because without
>the page lock, special care has to be taken to pin the inode correctly.
>However, the tail page is still required to calculate the futex key so this
>patch records the tail page.
>
>On vanilla 4.6, the output of the test case is;
>
>futex_wake04    0  TINFO  :  Hugepagesize 2097152
>futex_wake04    1  TFAIL  :  futex_wake04.c:126: Bug: wait_thread2 did not wake after 30 secs.
>
>With the patch applied
>
>futex_wake04    0  TINFO  :  Hugepagesize 2097152
>futex_wake04    1  TPASS  :  Hi hydra, thread2 awake!
>
>Reported-by: Mike Galbraith <umgwanakikbuti@...il.com>
>Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>

(and yay for maintaining performance)