linux-kernel - Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL

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Date:   Mon, 4 Nov 2019 12:57:59 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Jerome Glisse <jglisse@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>, Jonathan Corbet <corbet@....net>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN

On 11/4/19 12:37 PM, Jason Gunthorpe wrote:
> On Mon, Nov 04, 2019 at 03:31:53PM -0500, Jerome Glisse wrote:
>>> Note for Jason: the (a) or (b) items are talking about the vfio case, which is
>>> one of the two call sites that now use pin_longterm_pages_remote(), and the
>>> other one is infiniband:
>>>
>>> drivers/infiniband/core/umem_odp.c:646:         npages = pin_longterm_pages_remote(owning_process, owning_mm,
>>> drivers/vfio/vfio_iommu_type1.c:353:            ret = pin_longterm_pages_remote(NULL, mm, vaddr, 1,
>>
>> vfio should be reverted until it can be properly implemented.
>> The issue is that when you fix the implementation you might
>> break vfio existing user and thus regress the kernel from user
>> point of view. So i rather have the change to vfio reverted,
>> i believe it was not well understood when it got upstream,
>> between in my 5.4 tree it is still gup_remote not longterm.
> 
> It is clearly a bug, vfio must use LONGTERM, and does right above this
> remote call:
> 
>         if (mm == current->mm) {
>                 ret = get_user_pages(vaddr, 1, flags | FOLL_LONGTERM, page,
>                                      vmas);
>         } else {
>                 ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags, page,
>                                             vmas, NULL);
> 
> 
> I'm not even sure that it really makes any sense to build a 'if' like
> that, surely just always call remote??
> 


Right, and I thought about this when converting, and realized that the above 
code is working around the current gup.c limitations, which are "cannot support
gup remote with FOLL_LONGTERM".

Given that observation, the code is getting itself some FOLL_LONGTERM support
for the non-remote case, and only hitting the limitation if the mm really is
non-current.

And if you look at my patch, it keeps the same behavior, while adding in the
new wrapper calls.

So...thoughts, preferences?


thanks,

John Hubbard
NVIDIA