sendfile(2) fails for devices?

sendfile(2) fails for devices?

[Jeff Garzik]

sendfile(2) fails with -EINVAL every time I try to read from a device
file.

This sounds like a bug... is it?  (the man page doesn't mention such a
restriction)

I am using kernel 2.4.0-test11-pre2.  All other tests with sendfile(2)
succeed:  file->file, file->STDOUT, STDIN->file...

-- 
Jeff Garzik             |
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Re: sendfile(2) fails for devices?

[Linus Torvalds]

In article <3A0DE0C8.C700F33D@mandrakesoft.com>,
Jeff Garzik  <jgarzik@mandrakesoft.com> wrote:
>sendfile(2) fails with -EINVAL every time I try to read from a device
>file.
>
>This sounds like a bug... is it?  (the man page doesn't mention such a
>restriction)

sendfile() on purpose only works on things that use the page cache. 
EINVAL is basically sendfiles way of saying "I would fall back on doing
a read+write, so you might as well do it yourself in user space because
it might actually be more efficient that way". 

>I am using kernel 2.4.0-test11-pre2.  All other tests with sendfile(2)
>succeed:  file->file, file->STDOUT, STDIN->file...

Yes, as long as STDIN is a file ;)

sendfile() wants the source to be in the page cache, because the whole
point of sendfile() was to avoid a copy. 

The current device model does _not_ use the page cache. Now, arguably
that's a bug - it also means that you cannot mmap() a block device - but
as it could be easily documented (maybe it is, somewhere), I'll call it
a bad feature for now.

Now, if you want to add the code to do address spaces for block devices,
I wouldn't be all that unhappy.  I've wanted to see it for a while.  I'm
not likely to apply it for 2.4.x any more, but I'd love to have it early
for 2.5.x. 

		Linus
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