lkml.org 
[lkml]   [2020]   [May]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3 3/3] vfio-pci: Invalidate mmaps and block MMIO access on disabled memory
On Mon, May 25, 2020 at 09:37:05PM -0300, Jason Gunthorpe wrote:
> On Mon, May 25, 2020 at 01:56:28PM -0700, John Hubbard wrote:
> > On 2020-05-25 09:56, Jason Gunthorpe wrote:
> > > On Mon, May 25, 2020 at 11:11:42AM -0400, Peter Xu wrote:
> > > > On Mon, May 25, 2020 at 11:46:51AM -0300, Jason Gunthorpe wrote:
> > > > > On Mon, May 25, 2020 at 10:28:06AM -0400, Peter Xu wrote:
> > > > > > On Mon, May 25, 2020 at 09:26:07AM -0300, Jason Gunthorpe wrote:
> > > > > > > On Sat, May 23, 2020 at 07:52:57PM -0400, Peter Xu wrote:
> > > > > > >
> > > > > > > > For what I understand now, IMHO we should still need all those handlings of
> > > > > > > > FAULT_FLAG_RETRY_NOWAIT like in the initial version. E.g., IIUC KVM gup will
> > > > > > > > try with FOLL_NOWAIT when async is allowed, before the complete slow path. I'm
> > > > > > > > not sure what would be the side effect of that if fault() blocked it. E.g.,
> > > > > > > > the caller could be in an atomic context.
> > > > > > >
> > > > > > > AFAICT FAULT_FLAG_RETRY_NOWAIT only impacts what happens when
> > > > > > > VM_FAULT_RETRY is returned, which this doesn't do?
> > > > > >
> > > > > > Yes, that's why I think we should still properly return VM_FAULT_RETRY if
> > > > > > needed.. because IMHO it is still possible that the caller calls with
> > > > > > FAULT_FLAG_RETRY_NOWAIT.
> > > > > >
> > > > > > My understanding is that FAULT_FLAG_RETRY_NOWAIT majorly means:
> > > > > >
> > > > > > - We cannot release the mmap_sem, and,
> > > > > > - We cannot sleep
> > > > >
> > > > > Sleeping looks fine, look at any FS implementation of fault, say,
> > > > > xfs. The first thing it does is xfs_ilock() which does down_write().
> > > >
> > > > Yeah. My wild guess is that maybe fs code will always be without
> > > > FAULT_FLAG_RETRY_NOWAIT so it's safe to sleep unconditionally (e.g., I think
> > > > the general #PF should be fine to sleep in fault(); gup should be special, but
> > > > I didn't observe any gup code called upon file systems)?
> > >
> > > get_user_pages is called on filesystem backed pages.
> > >
> > > I have no idea what FAULT_FLAG_RETRY_NOWAIT is supposed to do. Maybe
> > > John was able to guess when he reworked that stuff?
> > >
> >
> > Although I didn't end up touching that particular area, I'm sure it's going
> > to come up sometime soon, so I poked around just now, and found that
> > FAULT_FLAG_RETRY_NOWAIT was added almost exactly 9 years ago. This flag was
> > intended to make KVM and similar things behave better when doing GUP on
> > file-backed pages that might, or might not be in memory.
> >
> > The idea is described in the changelog, but not in the code comments or
> > Documentation, sigh:
> >
> > commit 318b275fbca1ab9ec0862de71420e0e92c3d1aa7
> > Author: Gleb Natapov <gleb@redhat.com>
> > Date: Tue Mar 22 16:30:51 2011 -0700
> >
> > mm: allow GUP to fail instead of waiting on a page
> >
> > GUP user may want to try to acquire a reference to a page if it is already
> > in memory, but not if IO, to bring it in, is needed. For example KVM may
> > tell vcpu to schedule another guest process if current one is trying to
> > access swapped out page. Meanwhile, the page will be swapped in and the
> > guest process, that depends on it, will be able to run again.
> >
> > This patch adds FAULT_FLAG_RETRY_NOWAIT (suggested by Linus) and
> > FOLL_NOWAIT follow_page flags. FAULT_FLAG_RETRY_NOWAIT, when used in
> > conjunction with VM_FAULT_ALLOW_RETRY, indicates to handle_mm_fault that
> > it shouldn't drop mmap_sem and wait on a page, but return VM_FAULT_RETRY
> > instead.
>
> So, from kvm's perspective it was to avoid excessively long blocking in
> common paths when it could rejoin the completed IO by somehow waiting
> on a page itself?
>
> It all seems like it should not be used unless the page is going to go
> to IO?

I think NOWAIT is used as a common flag for kvm for its initial attempt to
fault in a normal page, however... I just noticed another fact that actually
__get_user_pages() won't work with PFNMAP (check_vma_flags should fail), but
KVM just started to support fault() for PFNMAP from commit add6a0cd1c5b (2016)
using fixup_user_fault(), where nvidia seems to have a similar request to have
a fault handler on some mapped BARs.

>
> Certainly there is no reason to optimize the fringe case of vfio
> sleeping if there is and incorrect concurrnent attempt to disable the
> a BAR.

If fixup_user_fault() (which is always with ALLOW_RETRY && !RETRY_NOWAIT) is
the only path for the new fault(), then current way seems ok. Not sure whether
this would worth a WARN_ON_ONCE(RETRY_NOWAIT) in the fault() to be clear of
that fact.

Thanks,

--
Peter Xu

\
 
 \ /
  Last update: 2020-05-26 15:51    [W:0.101 / U:0.640 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site