linux-kernel - Re: [PATCH] mm/memory.c: Convert to use vmf

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Date:   Fri, 6 Sep 2019 17:54:21 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Ralph Campbell <rcampbell@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, airlied@...hat.com,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memory.c: Convert to use vmf_error()

On Fri, Sep 6, 2019 at 12:29 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Sep 06, 2019 at 12:13:00AM +0530, Souptick Joarder wrote:
> > +++ b/mm/memory.c
> > @@ -1750,13 +1750,10 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
> >       } else {
> >               return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
> >       }
> > -
> > -     if (err == -ENOMEM)
> > -             return VM_FAULT_OOM;
> > -     if (err < 0 && err != -EBUSY)
> > -             return VM_FAULT_SIGBUS;
> > -
> > -     return VM_FAULT_NOPAGE;
> > +     if (!err || err == -EBUSY)
> > +             return VM_FAULT_NOPAGE;
> > +     else
> > +             return vmf_error(err);
> >  }
>
> My plan is to convert insert_page() to return a VM_FAULT error code like
> insert_pfn() does.  Need to finish off the vm_insert_page() conversions
> first ;-)

Previously we have problem while converting vm_insert_page() to return
vm_fault_t.

vm_insert_page() is called from different drivers. Some of them are
already converted
to use vm_map_pages()/ vm_map_pages_zero(). But still we left with few users.

drivers/media/usb/usbvision/usbvision-video.c#L1045
mm/vmalloc.c#L2969

These 2 can be converted with something like vm_map_vmalloc_pages().
I am working on it. Will post it in sometime.

drivers/android/binder_alloc.c#L259 (have objection)
drivers/infiniband/hw/efa/efa_verbs.c#L1701
drivers/infiniband/hw/mlx5/main.c#L2085 (have objection as using
vm_map_pages_zero() doesn't make sense)
drivers/xen/gntalloc.c#L548 (have limitation)
kernel/kcov.c#L297 (have objection)
net/ipv4/tcp.c#L1799
net/packet/af_packet.c#L4453

But these are the places where replacing vm_insert_page() is bit
difficult/ have objection.
In some cases, maintainers/ reviewers will not agree to replace
vm_insert_page().

In  other scenario, if we change return type of vm_insert_page() to vm_fault_t,
we end up with adding few lines of conversion code from vm_fault_t to errno
in drivers which is not a correct way to go with.

Any suggestion, how to solve this ?