linux-kernel - Re: [PATCH 05/13] mm/frame-vector: Use FOLL

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Message-ID: <20201007173323.GV5177@ziepe.ca>
Date:   Wed, 7 Oct 2020 14:33:23 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, linux-s390@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 05/13] mm/frame-vector: Use FOLL_LONGTERM

On Wed, Oct 07, 2020 at 07:12:24PM +0200, Daniel Vetter wrote:
> On Wed, Oct 7, 2020 at 6:53 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Wed, Oct 07, 2020 at 06:44:18PM +0200, Daniel Vetter wrote:
> > >
> > > -     /*
> > > -      * While get_vaddr_frames() could be used for transient (kernel
> > > -      * controlled lifetime) pinning of memory pages all current
> > > -      * users establish long term (userspace controlled lifetime)
> > > -      * page pinning. Treat get_vaddr_frames() like
> > > -      * get_user_pages_longterm() and disallow it for filesystem-dax
> > > -      * mappings.
> > > -      */
> > > -     if (vma_is_fsdax(vma)) {
> > > -             ret = -EOPNOTSUPP;
> > > -             goto out;
> > > -     }
> > > -
> > > -     if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> > > -             vec->got_ref = true;
> > > -             vec->is_pfns = false;
> > > -             ret = pin_user_pages_locked(start, nr_frames,
> > > -                     gup_flags, (struct page **)(vec->ptrs), &locked);
> > > -             goto out;
> > > -     }
> >
> > The vm_flags still need to be checked before going into the while
> > loop. If the break is taken then nothing would check vm_flags
> 
> Hm right that's a bin inconsistent. follow_pfn also checks for this,
> so I think we can just ditch this entirely both here and in the do {}
> while () check, simplifying the latter to just while (vma). Well, just
> make it a real loop with less confusing control flow probably.

It does read very poorly with the redundant check, espeically since I
keep forgetting follow_pfn does it too :\

Jason