linux-kernel - Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD

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Date:   Wed, 11 Sep 2019 12:57:45 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge
 PMD

On Wed, Sep 11, 2019 at 04:05:37PM +0100, Waiman Long wrote:
> To remove the unacceptable delays, we have to limit the amount of wait
> time on the mmap_sem. So the new down_write_timedlock() function is
> used to acquire the write lock on the mmap_sem with a timeout value of
> 10ms which should not cause a perceivable delay. If timeout happens,
> the task will abandon its effort to share the PMD and allocate its own
> copy instead.

If you do a v2, this is *NOT* the mmap_sem.  It's the i_mmap_rwsem
which protects a very different data structure from the mmap_sem.

> +static inline bool i_mmap_timedlock_write(struct address_space *mapping,
> +					 ktime_t timeout)
> +{
> +	return down_write_timedlock(&mapping->i_mmap_rwsem, timeout);
> +}