linux-kernel - Re: [PATCH v3] libnvdimm, region: sysfs trigger for nvdimm

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Message-ID: <x49d1bxsngb.fsf@segfault.boston.devel.redhat.com>
Date:   Thu, 27 Apr 2017 14:41:08 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "linux-nvdimm\@lists.01.org" <linux-nvdimm@...ts.01.org>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] libnvdimm, region: sysfs trigger for nvdimm_flush()

Dan Williams <dan.j.williams@...el.com> writes:

>> The sentiment is that programs shouldn't have to grovel around in sysfs
>> to do stuff related to an open file descriptor or mapping.  I don't take
>> issue with the name.  I do worry that something like 'wpq_drain' may be
>> too platform specific, though.  The NVM Programming Model specification
>> is going to call this "deep flush", so maybe that will give you
>> some inspiration if you do want to change the name.
>
> I'll change to "deep_flush", and I quibble that this is related to a
> single open file descriptor or mapping. It really is a "region flush"
> for giving extra protection for global metadata, but the persistence
> of individual fds or mappings is handled by ADR. I think an ioctl
> might give the false impression that every time you flush a cacheline
> to persistence you need to call the ioctl.

fsync, for example, may affect more than one fd--all data in the drive
write cache will be flushed.  I don't see how this is so different.  I
think a sysfs file is awkward because it requires an application to
chase down the correct file in the sysfs hierarchy.  If the application
already has an open fd or a mapping, it should be able to operate on
that.

As for confusion on when to use the interface, I think that's inevitable
no matter how it's implemented.  We're introducing a flush type that has
never been exposed before, and we're not giving any information on how
likely an ADR failure is, or how expensive this flush will be.

Cheers,
Jeff