Re: efivarfs and writev() support

From: Marcel Holtmann
Date: Thu Mar 12 2015 - 10:58:48 EST

Hi Al,

>>>> it seems that efivarfs only supports readv(), but when it comes to
>>>> writev(), I am getting an error. Is there any reason to not support
>>>> vectored write on this filesystem? Especially with the uint32 header
>>>> for each file, I think it would make perfect sense to support it.
>>> What error are you seeing? I thought that the vfs fell back to a looped
>>> write if the file system doesn't support .write_iter()?
>> that seems to work for readv(), but not for writev().
>>> But yes, we definitely should support writev().
>> I just get an EIO error and have not traced this down any further.
> What arguments are you feeding to it? Note that the thing is sensitive to
> range boundaries; it's not as if series of write() to it would be equivalent
> to single write() from concatenation. And writev() is equivalent to
> series of write().

I did something really simple and from my point obvious. I took the uint32 header that every file needs and put that in iov[0] pointer and then the rest in iov[1] pointer. The reason was that I didn't want to copy the actual file content around to just add a uint32 header in front of it.

> If you want behaviour a-la UDP sockets (syscall boundaries matter,
> boundaries between vector elements do not), we can certainly do that,
> but this is different from the current semantics. AFAICS, said
> semantics makes little sense, but it's a user-visible change...

I do not know about the specific semantics of efivarfs and frankly I have not tried every single combination. However it sounds to me that currently it requires that the whole file content is provided with a single write(). I have no idea if this is true or not. I do not know enough about the internals here.

Maybe efivarfs just needs to implemented .write_iter properly to actually support writev() and can not rely on a fallback of multiple write() calls.



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