| Message ID | 1441273129-20185-1-git-send-email-dominique.martinet@cea.fr |
|---|---|
| State | Not Applicable, archived |
| Delegated to: | David Miller |
| Headers | show |
On Thu, Sep 3, 2015 at 4:38 AM, Dominique Martinet <dominique.martinet@cea.fr> wrote: > That code really should never be called (rc is allocated in > tag_alloc), but if it had been it couldn't have worked... > > Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr> > --- > net/9p/trans_fd.c | 3 +++ > 1 file changed, 3 insertions(+) > > To be honest, I think it might be better to just bail out if we get in > this switch (m->req->rc == NULL after p9_tag_lookup) and not try to > allocate more, because if we get there it's likely a race condition and > silently re-allocating will end up in more troubles than trying to > recover is worth. > Thoughts ? > Hmmm...trying to rattle my brain and remember why I put it in there back in 2008. It might have just been over-defensive programming -- or more likely it just pre-dated all the zero copy infrastructure which pretty much guaranteed we had an rc allocated and what is there is vestigial. I'm happy to accept a patch which makes this an assert, or perhaps just resets the connection because something has gone horribly wrong (similar to the ENOMEM path that is there now). -eric -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Eric Van Hensbergen wrote on Sat, Sep 05, 2015: > On Thu, Sep 3, 2015 at 4:38 AM, Dominique Martinet > <dominique.martinet@cea.fr> wrote: > > To be honest, I think it might be better to just bail out if we get in > > this switch (m->req->rc == NULL after p9_tag_lookup) and not try to > > allocate more, because if we get there it's likely a race condition and > > silently re-allocating will end up in more troubles than trying to > > recover is worth. > > Thoughts ? > > > > Hmmm...trying to rattle my brain and remember why I put it in there > back in 2008. > It might have just been over-defensive programming -- or more likely it just > pre-dated all the zero copy infrastructure which pretty much guaranteed we had > an rc allocated and what is there is vestigial. I'm happy to accept a > patch which > makes this an assert, or perhaps just resets the connection because something > has gone horribly wrong (similar to the ENOMEM path that is there now). Yeah, it looks like the safety comes from the zero-copy stuff that came much later. Let's go with resetting the connection then. Hmm. EIO is a bit too generic so would be good to avoid that if possible, but can't think of anything better... Speaking of zero-copy, I believe it should be fairly straight-forward to implement for trans_fd now I've actually looked at it, since we do the payload read after a p9_tag_lookup, would just need m->req to point to a zc buffer. Write is similar, if there's a zc buffer just send it after the header. The cost is a couple more pointers in req and an extra if in both workers, that seems pretty reasonable. Well, I'm not using trans_fd much here (and unfortunately zero-copy isn't possible at all given the transport protocol for RDMA, at least for recv), but if anyone cares it probably could be done without too much hassle for the fd workers.
I thought the nature of trans_fd would have prevented any sort of true
zero copy, but I suppose one less is always welcome :)
-eric
On Sun, Sep 6, 2015 at 1:55 AM, Dominique Martinet
<dominique.martinet@cea.fr> wrote:
> Eric Van Hensbergen wrote on Sat, Sep 05, 2015:
>> On Thu, Sep 3, 2015 at 4:38 AM, Dominique Martinet
>> <dominique.martinet@cea.fr> wrote:
>> > To be honest, I think it might be better to just bail out if we get in
>> > this switch (m->req->rc == NULL after p9_tag_lookup) and not try to
>> > allocate more, because if we get there it's likely a race condition and
>> > silently re-allocating will end up in more troubles than trying to
>> > recover is worth.
>> > Thoughts ?
>> >
>>
>> Hmmm...trying to rattle my brain and remember why I put it in there
>> back in 2008.
>> It might have just been over-defensive programming -- or more likely it just
>> pre-dated all the zero copy infrastructure which pretty much guaranteed we had
>> an rc allocated and what is there is vestigial. I'm happy to accept a
>> patch which
>> makes this an assert, or perhaps just resets the connection because something
>> has gone horribly wrong (similar to the ENOMEM path that is there now).
>
> Yeah, it looks like the safety comes from the zero-copy stuff that came
> much later.
> Let's go with resetting the connection then. Hmm. EIO is a bit too
> generic so would be good to avoid that if possible, but can't think of
> anything better...
>
>
> Speaking of zero-copy, I believe it should be fairly straight-forward to
> implement for trans_fd now I've actually looked at it, since we do the
> payload read after a p9_tag_lookup, would just need m->req to point to a
> zc buffer. Write is similar, if there's a zc buffer just send it after
> the header.
> The cost is a couple more pointers in req and an extra if in both
> workers, that seems pretty reasonable.
>
> Well, I'm not using trans_fd much here (and unfortunately zero-copy
> isn't possible at all given the transport protocol for RDMA, at least
> for recv), but if anyone cares it probably could be done without too
> much hassle for the fd workers.
>
> --
> Dominique
--
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diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c index a270dcc..0d9831a 100644 --- a/net/9p/trans_fd.c +++ b/net/9p/trans_fd.c @@ -363,6 +363,9 @@ static void p9_read_work(struct work_struct *work) err = -ENOMEM; goto error; } + m->req->rc.capacity = m->client->msize; + m->req->rc.sdata = (char*)m->req->rc + + sizeof(struct p9_fcall); } m->rc.sdata = (char *)m->req->rc + sizeof(struct p9_fcall); memcpy(m->rc.sdata, m->tmp_buf, m->rc.capacity);
That code really should never be called (rc is allocated in tag_alloc), but if it had been it couldn't have worked... Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr> --- net/9p/trans_fd.c | 3 +++ 1 file changed, 3 insertions(+) To be honest, I think it might be better to just bail out if we get in this switch (m->req->rc == NULL after p9_tag_lookup) and not try to allocate more, because if we get there it's likely a race condition and silently re-allocating will end up in more troubles than trying to recover is worth. Thoughts ?