Re: [dpdk-dev] [PATCH v5 4/5] vhost: eventfd_link: replace copy-pasted sys_close

[Pavel Boldin <pboldin@mirantis.com>]

Xie, All,

Please find my comments intermixed below.

On Fri, Jul 10, 2015 at 6:42 PM, Xie, Huawei <huawei.xie@intel.com> wrote:

> Don't know why previous mail get messed.
>
> On 7/10/2015 10:50 PM, Pavel Boldin wrote:
> Xie,
>
> Regarding the patches:
> 1. The replaced code in fourth patch is checked to be a copy-paste of the
> `sys_close` syscall.
>
> sys_close does extra cleanup than the replaced coe. My concern is, for
> example, sys_close will mark the fd as next-to-be-allocated fd. Will there
> be issue when we allocate a new fd, because it will be allocated starting
> from the value of next-to-be-allocted-fd? I think kernel willn't blindly
> use that value, but not sure.
>

That is what applications do when call `close' libc function -- the freed
FD is ready to be allocated again and it is OK for applications to reuse
FDs.


>
> 2. It is not uncommon for the applications to close FD making it allocated
> for a different file. In our particular case the file is closed in the
> *source* process and *added* to a target process, so matching fds should
> not be the problem.
>
> Yes, that is exactly what the old code does.
> 3. There is an implementation of the exact same thing in the SCM_RIGHTS
> [1] that can be used as the reference code.
>
> I did a rough check. Maybe i miss something. I see it calls fd_install on
> a newly allocated fd. That is exactly what i want to replace the current
> code with.
> Currently we allcoate eventfd in user space and install a new file onto it
> through fd_install. Actually we don't need to allocate the eventfd in user
> space at all, what we should do is allocate a new fd in kernel, and install
> the file onto it.
>
> new_fd = get_unused_fd_flags(...)
> fd_install(new_fd, get_file(fp[i])
>

Well, this requires changes from the user-space side so I prefer not to do
it by myself at the moment, because I'm no expert in DPDK. I can provide
with the updated patches though but I will require a lab to check that it
works indeed.

No comments below this line.

Pavel


>
> /huawei
>
> [1] https://github.com/torvalds/linux/blob/master/net/core/scm.c#L248
>
> Pavel
>
> On Fri, Jul 10, 2015 at 5:27 PM, Xie, Huawei <huawei.xie@intel.com<mailto:
> huawei.xie@intel.com>> wrote:
> On 6/17/2015 11:24 PM, Thomas Monjalon wrote:
> > 2015-05-07 06:54, Xie, Huawei:
> >> On 4/16/2015 7:48 PM, Pavel Boldin wrote:
> >>> +   /* Closing the source_fd */
> >>> +   ret = sys_close(eventfd_copy.source_fd);
> >> Pavel:
> >> Here we close the fd and re-install a new file on this fd later.
> >> sys_close does all cleanup.
> >> But, for instance, if we allocate new fd later, normally it will reuse
> >> the just freed fds by sys_close, is there issue here?
> > Pavel, Huawei,
> > Could we come to a conclusion on this patch series please?
> For the previous 3 patches, i am OK except that i don't think inline is
> needed explicitly for non-performance critical function.
> For this patch, didn't check the fs code.
>
> >
> >
>
>
>
>