linux-kernel - Re: [PATCH net-next 1/3] net: devmem: support single IOV with sendmsg

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Message-ID: <dd139cb5-3554-4b65-b886-fe648f2413d3@gmail.com>
Date: Thu, 22 May 2025 09:31:04 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Stanislav Fomichev <stfomichev@...il.com>,
 Mina Almasry <almasrymina@...gle.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, viro@...iv.linux.org.uk,
 horms@...nel.org, andrew+netdev@...n.ch, shuah@...nel.org, sagi@...mberg.me,
 willemb@...gle.com, jdamato@...tly.com, kaiyuanz@...gle.com,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] net: devmem: support single IOV with sendmsg

On 5/21/25 18:33, Stanislav Fomichev wrote:
> On 05/21, Mina Almasry wrote:
>> On Tue, May 20, 2025 at 1:30 PM Stanislav Fomichev <stfomichev@...il.com> wrote:
>>>
>>> sendmsg() with a single iov becomes ITER_UBUF, sendmsg() with multiple
>>> iovs becomes ITER_IOVEC. iter_iov_len does not return correct
>>> value for UBUF, so teach to treat UBUF differently.
>>>
>>> Cc: Al Viro <viro@...iv.linux.org.uk>
>>> Cc: Pavel Begunkov <asml.silence@...il.com>
>>> Cc: Mina Almasry <almasrymina@...gle.com>
>>> Fixes: bd61848900bf ("net: devmem: Implement TX path")
>>> Signed-off-by: Stanislav Fomichev <stfomichev@...il.com>
>>> ---
>>>   include/linux/uio.h | 8 +++++++-
>>>   net/core/datagram.c | 3 ++-
>>>   2 files changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/uio.h b/include/linux/uio.h
>>> index 49ece9e1888f..393d0622cc28 100644
>>> --- a/include/linux/uio.h
>>> +++ b/include/linux/uio.h
>>> @@ -99,7 +99,13 @@ static inline const struct iovec *iter_iov(const struct iov_iter *iter)
>>>   }
>>>
>>>   #define iter_iov_addr(iter)    (iter_iov(iter)->iov_base + (iter)->iov_offset)
>>> -#define iter_iov_len(iter)     (iter_iov(iter)->iov_len - (iter)->iov_offset)
>>> +
>>> +static inline size_t iter_iov_len(const struct iov_iter *i)
>>> +{
>>> +       if (i->iter_type == ITER_UBUF)
>>> +               return i->count;
>>> +       return iter_iov(i)->iov_len - i->iov_offset;
>>> +}
>>>
>>
>> This change looks good to me from devmem perspective, but aren't you
>> potentially breaking all these existing callers to iter_iov_len?
>>
>> ackc -i iter_iov_len
>> fs/read_write.c
>> 846:                                            iter_iov_len(iter), ppos);
>> 849:                                            iter_iov_len(iter), ppos);
>> 858:            if (nr != iter_iov_len(iter))
>>
>> mm/madvise.c
>> 1808:           size_t len_in = iter_iov_len(iter);
>> 1838:           iov_iter_advance(iter, iter_iov_len(iter));
>>
>> io_uring/rw.c
>> 710:                    len = iter_iov_len(iter);
>>
>> Or are you confident this change is compatible with these callers for
>> some reason?
>   
> Pavel did go over all callers, see:
> https://lore.kernel.org/netdev/7f06216e-1e66-433e-a247-2445dac22498@gmail.com/

Yes, the patch should work

Reviewed-by: Pavel Begunkov <asml.silence@...il.com>

> 
>> Maybe better to handle this locally in zerocopy_fill_skb_from_devmem,
>> and then follow up with a more ambitious change that streamlines how
>> all the iters behave.
> 
> Yes, I can definitely do that, but it seems a bit strange that the
> callers need to distinguish between IOVEC and UBUF (which is a 1-entry
> IOVEC), so having working iter_iov_len seems a bit cleaner.

It might be a good idea to rename it at some point to highlight that
it also works with ubufs (but not as a part of this fix).

-- 
Pavel Begunkov