linux-kernel - Re: [PATCH 1/4] mm/gup: Add FOLL

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Date:   Fri, 24 Jun 2022 21:23:04 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

Hi, Jason,

On Fri, Jun 24, 2022 at 09:35:54PM -0300, Jason Gunthorpe wrote:
> Can you talk abit about what is required to use this new interface
> correctly?
> 
> Lots of GUP callers are in simple system call contexts (like ioctl),
> can/should they set this flag and if so what else do they need to do?

Thanks for taking a look.

IMHO the major thing required is the caller can handle the case when GUP
returned (1) less page than expected, and (2) -EINTR returns.

For the -EINTR case, IIUC ideally in an ioctl context we should better
deliver it back to user app this -EINTR (while do cleanups gracefully),
rather than returning anything else (e.g. converting it to -EFAULT or
something else).

But note that FAULT_FLAG_INTERRUPTIBLE is only used in an userfaultfd
context (aka, userfaultfd_get_blocking_state()).  For example, if we hang
at lock_page() (if not go into whether hanging at lock_page makes sense or
not at all.. it really sounds like a bug) and we receive a non-fatal
signal, we won't be able to be scheduled for that since lock_page() uses
TASK_UNINTERRUPTIBLE always.

I think it's a separate problem on whether we should extend the usage of
FAULT_FLAG_INTERRUPTIBLE to things like lock_page() (and probably not..),
and currently it does solve a major issue regarding postcopy hanging on
pages for hypervisor use case.  Hopefully that still justifies this plumber
work to enable the interruptible cap to GUP layer.

If to go back to the original question with a shorter answer: if the ioctl
context that GUP upon a page that will never be with a uffd context, then
it's probably not gonna help at all.. at least not before we use
FAULT_FLAG_INTERRUPTIBLE outside uffd page fault handling.

Thanks,

-- 
Peter Xu