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Subject: Re: [PATCH 0/5] bio: Direct IO: convert to pin_user_pages_fast()
To:     Al Viro <viro@zeniv.linux.org.uk>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
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References: <20200822042059.1805541-1-jhubbard@nvidia.com>
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From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <69d09a5f-27f2-dd50-d25e-926302b10443@nvidia.com>
Date:   Mon, 24 Aug 2020 19:27:45 -0700
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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 e=
arly
>> mainly based on Christoph's comment in [1] ("ITER_PIPE is rejected =D1=
=96n the
>> direct I/O path"), but if it's supportable then I'll hook it up.
>=20
> ; cat >a.c <<'EOF'
> #define _GNU_SOURCE
> #include <fcntl.h>
> #include <unistd.h>
> #include <stdlib.h>
>=20
> int main()
> {
>          int fd =3D open("./a.out", O_DIRECT);
>          splice(fd, NULL, 1, NULL, 4096, 0);
> 	return 0;
> }
> EOF
> ; cc a.c
> ; ./a.out | wc -c
> 4096
>=20
> and you just had ->read_iter() called with ITER_PIPE destination.
>=20

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

thanks,
--=20
John Hubbard
NVIDIA