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

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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 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




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