Re: [PATCH] libnvdimm, nfit: treat volatile virtual CD region as read-only pmem

[Dan Williams]

On Sat, Jun 4, 2016 at 4:01 AM, joeyli <jlee@xxxxxxxx> wrote:
> Hi Dan,
>
> Thanks for your review.
>
> On Fri, Jun 03, 2016 at 12:27:34PM -0700, Dan Williams wrote:
>> On Fri, Jun 3, 2016 at 12:13 AM, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx> wrote:
>> > This patch adds codes to treat a volatile virtual CD region as a
>> > read-only pmem region, then read-only /dev/pmem* device can be mounted
>> > with iso9660.
>> >
>> > It's useful to work with the httpboot in EFI firmware to pull a remote
>> > ISO file to the local memory region for booting and installation.
>> >
>> > Wiki page of UEFI HTTPBoot with OVMF:
>> >         https://en.opensuse.org/UEFI_HTTPBoot_with_OVMF
>> >
>> > Signed-off-by: Lee, Chun-Yi <jlee@xxxxxxxx>
>> > Cc: Gary Lin <GLin@xxxxxxxx>
>> > Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
>> > Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
>> > ---
>> >  drivers/acpi/nfit.c          |  8 +++++++-
>> >  drivers/nvdimm/region_devs.c | 26 +++++++++++++++++++++++++-
>> >  include/linux/libnvdimm.h    |  2 ++
>> >  3 files changed, 34 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/acpi/nfit.c b/drivers/acpi/nfit.c
>> > index 2215fc8..b100a17 100644
>> > --- a/drivers/acpi/nfit.c
>> > +++ b/drivers/acpi/nfit.c
>> > @@ -1949,6 +1949,7 @@ static int acpi_nfit_init_mapping(struct acpi_nfit_desc *acpi_desc,
>> >         switch (nfit_spa_type(spa)) {
>> >         case NFIT_SPA_PM:
>> >         case NFIT_SPA_VOLATILE:
>> > +       case NFIT_SPA_VCD:
>> >                 nd_mapping->start = memdev->address;
>> >                 nd_mapping->size = memdev->region_size;
>> >                 break;
>>
>> Why do we need to distinguish NFIT_SPA_VOLATILE vs NFIT_SPA_VCD, i.e.
>> what happens if something writes to a VCD device?
>
> Actually I didn't try to write SPA-VCD device before. Every time I mount it
> that the system responses read-only:
>
> # mount /dev/pmem0 /mnt/
> mount: /dev/pmem0 is write-protected, mounting read-only
>
> If it can be written, then I think there have no difference between
> NFIT_SPA_VOLATILE with NFIT_SPA_VCD region.
>
> I implemented this patch to treat VCD region as read-only pmem because the
> pmem region generates /dev/pmem* device that it can be mounted.
>
> Maybe I missed. Does NFIT_SPA_VOLATILE region also generate a device in /dev
> that it can be mounted with filesystem? Then I think treat the VCD region as
> a read-only VOLATILE region that's also a solution.

My question is why does it need to be read-only?  If it's a volatile
region, does it matter if we allow writes at the block device level?
Especially if it is formatted as iso9660, it won't be writable through
the filesystem anyway.