Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()

From: John Hubbard <jhubbard@nvidia.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	Ilya Dryomov <idryomov@gmail.com>, Jens Axboe <axboe@kernel.dk>,
	Jeff Layton <jlayton@kernel.org>, <linux-xfs@vger.kernel.org>,
	<linux-fsdevel@vger.kernel.org>, <linux-block@vger.kernel.org>,
	<ceph-devel@vger.kernel.org>, <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()
Date: Mon, 24 Aug 2020 19:27:45 -0700	[thread overview]
Message-ID: <69d09a5f-27f2-dd50-d25e-926302b10443@nvidia.com> (raw)
In-Reply-To: <20200825022219.GW1236603@ZenIV.linux.org.uk>

On 8/24/20 7:22 PM, Al Viro wrote:
> On Mon, Aug 24, 2020 at 07:07:02PM -0700, John Hubbard wrote:
>> On 8/24/20 6:54 PM, 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?
>>>
>>
>> Oh! I'll take a look. Is this the fs/splice.c stuff?  I ruled this out early
>> mainly based on Christoph's comment in [1] ("ITER_PIPE is rejected іn the
>> direct I/O path"), but if it's supportable then I'll hook it up.
> 
> ; cat >a.c <<'EOF'
> #define _GNU_SOURCE
> #include <fcntl.h>
> #include <unistd.h>
> #include <stdlib.h>
> 
> int main()
> {
>          int fd = open("./a.out", O_DIRECT);
>          splice(fd, NULL, 1, NULL, 4096, 0);
> 	return 0;
> }
> EOF
> ; cc a.c
> ; ./a.out | wc -c
> 4096
> 
> and you just had ->read_iter() called with ITER_PIPE destination.
> 

That example saves me a lot of time!  Much appreciated.

thanks,
-- 
John Hubbard
NVIDIA