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Date:   Wed, 11 Sep 2019 12:57:45 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge
 PMD
Message-ID: <20190911195745.GI29434@bombadil.infradead.org>
References: <20190911150537.19527-1-longman@redhat.com>
 <20190911150537.19527-6-longman@redhat.com>
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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);
> +}