linux-kernel - Re: [PATCH 4/7] mm/gup: introduce the FOLL_SAME

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Message-ID: <7f9c3ec7-ea79-4f2f-bc23-acc4c11c9620@lucifer.local>
Date:   Mon, 17 Apr 2023 14:25:54 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 4/7] mm/gup: introduce the FOLL_SAME_FILE GUP flag

On Mon, Apr 17, 2023 at 10:14:02AM -0300, Jason Gunthorpe wrote:
> On Sat, Apr 15, 2023 at 12:27:40AM +0100, Lorenzo Stoakes wrote:
> > This flag causes GUP to assert that all VMAs within the input range possess
> > the same vma->vm_file. If not, the operation fails.
> >
> > This is part of a patch series which eliminates the vmas parameter from the
> > GUP API, implementing the one remaining assertion within the entire kernel
> > that requires access to the VMAs associated with a GUP range.
> >
> > Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> > ---
> >  include/linux/mm_types.h |  2 ++
> >  mm/gup.c                 | 16 ++++++++++++----
> >  2 files changed, 14 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3fc9e680f174..84d1aec9dbab 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1185,6 +1185,8 @@ enum {
> >  	FOLL_PCI_P2PDMA = 1 << 10,
> >  	/* allow interrupts from generic signals */
> >  	FOLL_INTERRUPTIBLE = 1 << 11,
> > +	/* assert that the range spans VMAs with the same vma->vm_file */
> > +	FOLL_SAME_FILE = 1 << 12,
>
> I hope we don't add this flag, but it needs to be rejected in
> internal_get_user_pages_fast()
>

intenal_get_user_pages_fast() checks against the complement of accepted
masks, therefore it will reject this as-is unless I'm missing something.

As for not adding the flag (an entirely understandable sentiment), it would
be good to get an insight into the semantics of what you feel would be more
suitable!

> Jason