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Date:   Sat, 27 Aug 2022 17:46:25 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>, Jan Kara <jack@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] NFS: direct-io: convert to FOLL_PIN pages

On 8/27/22 17:39, Al Viro wrote:
> On Sun, Aug 28, 2022 at 01:38:57AM +0100, Al Viro wrote:
>> On Sat, Aug 27, 2022 at 04:55:18PM -0700, John Hubbard wrote:
>>> On 8/27/22 15:48, Al Viro wrote:
>>>> On Sat, Aug 27, 2022 at 01:36:06AM -0700, John Hubbard wrote:
>>>>> Convert the NFS Direct IO layer to use pin_user_pages_fast() and
>>>>> unpin_user_page(), instead of get_user_pages_fast() and put_page().
>>>>
>>>> Again, this stuff can be hit with ITER_BVEC iterators
>>>>
>>>>> -		result = iov_iter_get_pages_alloc2(iter, &pagevec,
>>>>> +		result = dio_w_iov_iter_pin_pages_alloc(iter, &pagevec,
>>>>>  						  rsize, &pgbase);
>>>>
>>>> and this will break on those.
>>>
>>> If anyone has an example handy, of a user space program that leads
>>> to this situation (O_DIRECT with ITER_BVEC), it would really help
>>> me reach enlightenment a lot quicker in this area. :)
>>
>> Er...  splice(2) to O_DIRECT-opened file on e.g. ext4?  Or
>> sendfile(2) to the same, for that matter...
> 
> s/ext4/nfs/ to hit this particular codepath, obviously.

aha, thanks. I do remember that you alerted me earlier to splice(2)
problems, and I thought I'd neatly sidestepped those this time, by
staying with user_backed_iter(i) cases. 

But I see that it's going to take something more, after all.



thanks,

-- 
John Hubbard
NVIDIA

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