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Date: Thu, 22 Mar 2018 09:51:03 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>,
        Michal Hocko <mhocko@kernel.org>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section
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On Thu, Mar 22, 2018 at 05:18:55PM +0100, Laurent Dufour wrote:
> > It's *really* rare to page-fault on a VMA which is in the middle of
> > being replaced.  Why are you trying to optimise it?
> 
> I was not trying to optimize it, but to not wait in the page fault handler.
> This could become tricky in the case the VMA is removed once mmap(MAP_FIXED) is
> done and before the waiting page fault got woken up. This means that the
> removed VMA structure will have to remain until all the waiters are woken up
> which implies ref_count or similar.

Yes, that's why we don't want an actual rwsem.  What I had in mind was
a struct completion on the stack of the caller of munmap(), and a pointer
to it from the vma.  The page fault handler grabs the VMA tree lock, walks
the VMA tree and finds a VMA.  If the VMA is marked as locked, it waits
for the completion.  Upon wakeup *it does not look at the VMA*, instead it
restarts the page fault.