linux-kernel - Re: [PATCH v2 07/18] infiniband: set FOLL_PIN, FOLL_LONGTERM via pin_longterm

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Message-ID: <20191105023235.GA11093@ziepe.ca>
Date:   Mon, 4 Nov 2019 22:32:35 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.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>,
        Jérôme Glisse <jglisse@...hat.com>,
        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 07/18] infiniband: set FOLL_PIN, FOLL_LONGTERM via
 pin_longterm_pages*()

On Mon, Nov 04, 2019 at 02:03:43PM -0800, John Hubbard wrote:
> On 11/4/19 12:57 PM, Jason Gunthorpe wrote:
> > On Mon, Nov 04, 2019 at 12:48:13PM -0800, John Hubbard wrote:
> >> On 11/4/19 12:33 PM, Jason Gunthorpe wrote:
> >> ...
> >>>> diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> >>>> index 24244a2f68cc..c5a78d3e674b 100644
> >>>> +++ b/drivers/infiniband/core/umem.c
> >>>> @@ -272,11 +272,10 @@ struct ib_umem *ib_umem_get(struct ib_udata *udata, unsigned long addr,
> >>>>  
> >>>>  	while (npages) {
> >>>>  		down_read(&mm->mmap_sem);
> >>>> -		ret = get_user_pages(cur_base,
> >>>> +		ret = pin_longterm_pages(cur_base,
> >>>>  				     min_t(unsigned long, npages,
> >>>>  					   PAGE_SIZE / sizeof (struct page *)),
> >>>> -				     gup_flags | FOLL_LONGTERM,
> >>>> -				     page_list, NULL);
> >>>> +				     gup_flags, page_list, NULL);
> >>>
> >>> FWIW, this one should be converted to fast as well, I think we finally
> >>> got rid of all the blockers for that?
> >>>
> >>
> >> I'm not aware of any blockers on the gup.c end, anyway. The only broken thing we
> >> have there is "gup remote + FOLL_LONGTERM". But we can do "gup fast + LONGTERM". 
> > 
> > I mean the use of the mmap_sem here is finally in a way where we can
> > just delete the mmap_sem and use _fast
> >  
> > ie, AFAIK there is no need for the mmap_sem to be held during
> > ib_umem_add_sg_table()
> > 
> > This should probably be a standalone patch however
> > 
> 
> Yes. Oh, actually I guess the patch flow should be: change to 
> get_user_pages_fast() and remove the mmap_sem calls, as one patch. And then change 
> to pin_longterm_pages_fast() as the next patch. Otherwise, the internal fallback
> from _fast to slow gup would attempt to take the mmap_sem (again) in the same
> thread, which is not good. :)
> 
> Or just defer the change until after this series. Either way is fine, let me
> know if you prefer one over the other.
> 
> The patch itself is trivial, but runtime testing to gain confidence that
> it's solid is much harder. Is there a stress test you would recommend for that?
> (I'm not promising I can quickly run it yet--my local IB setup is still nascent 
> at best.)

If you make a patch we can probably get it tested, it is something
we should do I keep forgetting about.

Jason