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From:   Jeff Moyer <jmoyer@redhat.com>
To:     Jan Kara <jack@suse.cz>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Pankaj Gupta <pagupta@redhat.com>,
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Subject: Re: [PATCH v5 1/6] libnvdimm: nd_region flush callback support
References: <20190410040826.24371-1-pagupta@redhat.com>
        <20190410040826.24371-2-pagupta@redhat.com>
        <CAPcyv4i1FjBzfcWtzeCvj-giHa9xaX0dMQ==q4LZC_L8VEqkHg@mail.gmail.com>
        <20190412083230.GA29850@quack2.suse.cz>
Date:   Fri, 12 Apr 2019 09:12:10 -0400
In-Reply-To: <20190412083230.GA29850@quack2.suse.cz> (Jan Kara's message of
        "Fri, 12 Apr 2019 10:32:30 +0200")
Message-ID: <x494l73z7th.fsf@segfault.boston.devel.redhat.com>
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Jan Kara <jack@suse.cz> writes:

> On Thu 11-04-19 07:51:48, Dan Williams wrote:
>> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta <pagupta@redhat.com> wrote:
>> > +       } else {
>> > +               if (nd_region->flush(nd_region))
>> > +                       rc = -EIO;
>> 
>> Given the common case wants to be fast and synchronous I think we
>> should try to avoid retpoline overhead by default. So something like
>> this:
>> 
>> if (nd_region->flush == generic_nvdimm_flush)
>>     rc = generic_nvdimm_flush(...);
>
> I'd either add a comment about avoiding retpoline overhead here or just
> make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't
> get confused by the code.

Isn't this premature optimization?  I really don't like adding things
like this without some numbers to show it's worth it.

-Jeff