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SubjectRe: recvfrom/recvmsg performance and CONFIG_HARDENED_USERCOPY
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> On Dec 9, 2019, at 3:01 AM, David Laight <David.Laight@aculab.com> wrote:
>
> From: Eric Dumazet
>> Sent: 06 December 2019 14:22
> ...
>> Real question is : Do you actually need to use recvmsg() instead of recvfrom() ?
>> If recvmsg() provides additional cmsg, this is not surprising it is more expensive.
>
> Except I'm not passing in a buffer for it.
> The reason I'm looking at recvmsg is that I'd like to use recvmmsg it order to
> read out more than one message from a socket without doing an extra poll().
> Note that I don't expect there to be a second message most of the time and
> almost never a third one.
>
> Although I think that will only ever 'win' if recvmmsg() called vfs_poll() to find
> if there was more data to read before doing any of the copy_from_user() etc

I would suggest a more general improvement: add a -EAGAIN fast path to recvmsg(). If the socket is nonblocking and has no data to read, then there shouldn’t be a need to process the iovec at all.
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