linux-kernel - Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages

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Message-ID: <26bf92af-a7ab-b53f-45dd-9e3d7a1340ec@nvidia.com>
Date:   Mon, 24 Aug 2020 19:13:27 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Al Viro <viro@...iv.linux.org.uk>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        <linux-xfs@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <ceph-devel@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()

On 8/24/20 7:07 PM, Al Viro wrote:
> On Tue, Aug 25, 2020 at 02:54:28AM +0100, Al Viro wrote:
>> On Fri, Aug 21, 2020 at 09:20:54PM -0700, John Hubbard wrote:
>>
>>> Direct IO behavior:
>>>
>>>      ITER_IOVEC:
>>>          pin_user_pages_fast();
>>>          break;
>>>
>>>      ITER_KVEC:    // already elevated page refcount, leave alone
>>>      ITER_BVEC:    // already elevated page refcount, leave alone
>>>      ITER_PIPE:    // just, no :)
>>
>> Why?  What's wrong with splice to O_DIRECT file?
> 
> Sorry - s/to/from/, obviously.
> 
> To spell it out: consider generic_file_splice_read() behaviour when
> the source had been opened with O_DIRECT; you will get a call of
> ->read_iter() into ITER_PIPE destination.  And it bloody well
> will hit iov_iter_get_pages() on common filesystems, to pick the
> pages we want to read into.
> 
> So... what's wrong with having that "pin" primitive making sure
> the pages are there and referenced by the pipe?
> 

(our emails crossed) OK, yes, let me hook that up. I was just unaware
of that flow, I'll go off and figure it out.

Thanks for looking at this!

thanks,
-- 
John Hubbard
NVIDIA