linux-kernel - Re: [PATCH] userfaultfd: clear flag if remap event not enabled

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Date:   Mon, 10 Dec 2018 15:09:25 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH] userfaultfd: clear flag if remap event not enabled

Hello,

On Mon, Dec 10, 2018 at 07:51:16PM +0200, Mike Rapoport wrote:
> On Mon, Dec 10, 2018 at 02:51:21PM +0800, Peter Xu wrote:
> > When the process being tracked do mremap() without
> > UFFD_FEATURE_EVENT_REMAP on the corresponding tracking uffd file
> > handle, we should not generate the remap event, and at the same
> > time we should clear all the uffd flags on the new VMA.  Without
> > this patch, we can still have the VM_UFFD_MISSING|VM_UFFD_WP
> > flags on the new VMA even the fault handling process does not
> > even know the existance of the VMA.
> > 
> > CC: Andrea Arcangeli <aarcange@...hat.com>
> > CC: Andrew Morton <akpm@...ux-foundation.org>
> > CC: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> > CC: Kirill A. Shutemov <kirill@...temov.name>
> > CC: Hugh Dickins <hughd@...gle.com>
> > CC: Pavel Emelyanov <xemul@...tuozzo.com>
> > CC: Pravin Shedge <pravin.shedge4linux@...il.com>
> > CC: linux-mm@...ck.org
> > CC: linux-kernel@...r.kernel.org
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> >  fs/userfaultfd.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index cd58939dc977..798ae8a438ff 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -740,6 +740,9 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> >  		vm_ctx->ctx = ctx;
> >  		userfaultfd_ctx_get(ctx);
> >  		WRITE_ONCE(ctx->mmap_changing, true);
> > +	} else if (ctx) {
> > +		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> > +		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);

Great catch Peter!

> 
> My preference would be 
> 
> 	if (!ctx)
> 		return;
> 	
> 	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
> 		...
> 	} else {
> 		...
> 	}
> 
> but I don't feel strongly about it.

Yes, it'd look nicer to run a single "ctx not null" check.

> 
> I'd appreciate a comment in the code and with it 
> 
> Acked-by: Mike Rapoport <rppt@...ux.ibm.com>
> 

Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>