linux-kernel - Re: [PATCH RFC 03/24] mm: allow VM_FAULT

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Date:   Mon, 21 Jan 2019 10:55:36 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH RFC 03/24] mm: allow VM_FAULT_RETRY for multiple times

On Mon, Jan 21, 2019 at 03:57:01PM +0800, Peter Xu wrote:
> The idea comes from a discussion between Linus and Andrea [1].
> 
> Before this patch we only allow a page fault to retry once.  We achieved
> this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
> handle_mm_fault() the second time.  This was majorly used to avoid
> unexpected starvation of the system by looping over forever to handle
> the page fault on a single page.  However that should hardly happen, and
> after all for each code path to return a VM_FAULT_RETRY we'll first wait
> for a condition (during which time we should possibly yield the cpu) to
> happen before VM_FAULT_RETRY is really returned.
> 
> This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY
> flag when we receive VM_FAULT_RETRY.  It means that the page fault
> handler now can retry the page fault for multiple times if necessary
> without the need to generate another page fault event. Meanwhile we
> still keep the FAULT_FLAG_TRIED flag so page fault handler can still
> identify whether a page fault is the first attempt or not.

So there is nothing protecting starvation after this patch ? AFAICT.
Do we sufficient proof that we never have a scenario where one process
might starve fault another ?

For instance some page locking could starve one process.


> 
> GUP code is not touched yet and will be covered in follow up patch.
> 
> This will be a nice enhancement for current code at the same time a
> supporting material for the future userfaultfd-writeprotect work since
> in that work there will always be an explicit userfault writeprotect
> retry for protected pages, and if that cannot resolve the page
> fault (e.g., when userfaultfd-writeprotect is used in conjunction with
> shared memory) then we'll possibly need a 3rd retry of the page fault.
> It might also benefit other potential users who will have similar
> requirement like userfault write-protection.
> 
> Please read the thread below for more information.
> 
> [1] https://lkml.org/lkml/2017/11/2/833
> 
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Suggested-by: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---