On Fri, Apr 28, 2017 at 1:59 AM, Dan Williams <[email protected]> wrote:
> On Thu, Apr 27, 2017 at 2:15 AM, Oliver O'Halloran <[email protected]> wrote:
>> Adds two new sysfs attributes for pfn (and dax) devices:
>> supported_alignements and default_alignment. These advertise to
>> userspace what alignments this kernel supports, and provides a nominal
>> default alignment to use.
>>
>> Signed-off-by: Oliver O'Halloran <[email protected]>
>> ---
>> I'm not sure it makes sense to provide these for pfn devices. In the dax
>> case we have hard restrictions because of how fault handling works, but
>> I'm not convinced this makes sense for the pfn case since it's going to
>> be used with fs-dax.
> We still want this for fs-dax so we can make sure that the namespace
> is aligned to allow for opportunistic large mappings. We have pmd
> support for fs-dax currently shipping, and looking to expand that to
> pud support.
Sure, but whether we can use a PUD for userspace mappings mostly
depends on the allocation decisions of the filesystem rather than the
alignment of the namespace. The reservations for the PFN superblock,
altmap and dax labels mean the namespace is always going to be
unaligned so forcing a PUD alignment will result in a lot of wasted
space for dubious benefits. I suppose there's no reason not to provide
the functionality, but I don't see it buying us much.
>> ---
>> drivers/nvdimm/pfn_devs.c | 26 ++++++++++++++++++++++++++
>> 1 file changed, 26 insertions(+)
>>
>> diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
>> index 6c033c9a2f06..5157e7d89f0b 100644
>> --- a/drivers/nvdimm/pfn_devs.c
>> +++ b/drivers/nvdimm/pfn_devs.c
>> @@ -260,6 +260,30 @@ static ssize_t size_show(struct device *dev,
>> }
>> static DEVICE_ATTR_RO(size);
>>
>> +static ssize_t supported_alignments_show(struct device *dev,
>> + struct device_attribute *attr, char *buf)
>> +{
>> + /* Fun fact: These aren't always constants! */
>> + unsigned long supported_alignments[] = {
>> + PAGE_SIZE,
>> + HPAGE_PMD_SIZE,
>> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
>> + HPAGE_PUD_SIZE,
>> +#endif
>> + 0,
>> + };
>> +
>> + return nd_sector_size_show(0, supported_alignments, buf);
>> +}
>> +DEVICE_ATTR_RO(supported_alignments);
>> +
>> +static ssize_t default_alignment_show(struct device *dev,
>> + struct device_attribute *attr, char *buf)
>> +{
>> + return sprintf(buf, "%ld\n", HPAGE_PMD_SIZE);
>> +}
>> +DEVICE_ATTR_RO(default_alignment);
>> +
>> static struct attribute *nd_pfn_attributes[] = {
>> &dev_attr_mode.attr,
>> &dev_attr_namespace.attr,
>> @@ -267,6 +291,8 @@ static struct attribute *nd_pfn_attributes[] = {
>> &dev_attr_align.attr,
>> &dev_attr_resource.attr,
>> &dev_attr_size.attr,
>> + &dev_attr_supported_alignments.attr,
>> + &dev_attr_default_alignment.attr,
>> NULL,
>
> So, we don't need DEVICE_ATTR_RO(default_alignment), that can be
> reflected by setting nd_pfn->align to HPAGE_PMD_SIZE by default.
Hmm true, if we do this then we can use the alignment of the seed as
the default rather than having a separate attribute.
> passing nd_pfn->align to nd_sector_size_show(). Should probably rename
> nd_sector_size_show() to nd_size_select_show().
I agree. I figured another respin would be required so I kept the
changes to a minimum.
> The other concern is that the current DEVICE_ATTR_RW(align) can be
> made redundant by this new interface if you make it writable. I wonder
> if we can avoid breaking old ndctl versions by making the current
> align setting the first one in the output? Worse comes to worse we can
> live with two attributes 'align' and 'aligns', but I'd like to see if
> can add this to the existing attribute.
I'd rather have a small amount of redundancy and keep the the
attribute consistent with the the btt sector size attribute. We could
always remove align some time down the track since I imagine ndctl is
the only thing that consumes that part of the interface and ndctl
already handles align being missing.
Oliver
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