linux-kernel - Re: [PATCH] hugetlbfs: Take read_lock on i

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Date:   Thu, 7 Nov 2019 11:54:41 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing

On Thu, Nov 07, 2019 at 02:06:28PM -0500, Waiman Long wrote:
> A customer with large SMP systems (up to 16 sockets) with application
> that uses large amount of static hugepages (~500-1500GB) are experiencing
> random multisecond delays. These delays was caused by the long time it
> took to scan the VMA interval tree with mmap_sem held.
> 
> The sharing of huge PMD does not require changes to the i_mmap at all.
> As a result, we can just take the read lock and let other threads
> searching for the right VMA to share in parallel. Once the right
> VMA is found, either the PMD lock (2M huge page for x86-64) or the
> mm->page_table_lock will be acquired to perform the actual PMD sharing.
> 
> Lock contention, if present, will happen in the spinlock. That is much
> better than contention in the rwsem where the time needed to scan the
> the interval tree is indeterminate.

I don't think this description really explains the contention argument
well.  There are _more_ PMD locks than there are i_mmap_sem locks, so
processes accessing different parts of the same file can work in parallel.

Are there other current users of the write lock that could use a read lock?
At first blush, it would seem that unmap_ref_private() also only needs
a read lock on the i_mmap tree.  I don't think hugetlb_change_protection()
needs the write lock either.  Nor retract_page_tables().