Re: [PATCH RFC 03/24] mm: allow VM_FAULT_RETRY for multiple times

Jerome Glisse
2019-01-21T15:55:48Z

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@linux-foundation.org>
> Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---