linux-kernel - Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

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Message-ID: <20180322165102.GF28468@bombadil.infradead.org>
Date:   Thu, 22 Mar 2018 09:51:03 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        Michal Hocko <mhocko@...nel.org>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

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.