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

[Davidlohr Bueso]

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@xxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>

(and yay for maintaining performance)