Re: [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing

[Matthew Wilcox <willy@infradead.org>]

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().