Re: dax alignment problem on arm64 (and other achitectures)

[Pavel Tatashin]

On Fri, Jan 29, 2021 at 2:12 PM Pavel Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> On Fri, Jan 29, 2021 at 2:06 PM Pavel Tatashin
> <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > > > Definitely, but we should try figuring out what's going on here. I
> > > > assume on x86-64 it behaves differently?
> > >
> > > Yes, we should root cause. I highly suspect that there is somewhere
> > > alignment miscalculations happen that cause this memory waste with the
> > > offset 16M. I am also not sure why the 2M label size was increased,
> > > and  why 16M is now an alignment requirement.
> >
> > This appears to be because even if we set vmemmap to be outside of the
> > dax device, the alignment calculates the maximum size of vmemmap for
> > this device, and subtracts it from the devdax size.
> > See [1], line 795 is where this offset is calculated.
> >
> > This also explains why with 64K pages, the 16M offset worked: because
> > fewer struct pages were able to fit within 16M - label size.
> >
> > [1] https://soleen.com/source/xref/linux/drivers/nvdimm/pfn_devs.c?r=b7b3c01b&mo=18459&fi=718#795
>
> Actually, strike the previous e-mail. The extra space is when we
> reserve vmemmap from devdax. IFF we do it from mem, the extra space is
> not added. Now, this alignment makes total sense.

commit 2522afb86a8cceba0f67dbf05772d21b76d79f06
Author: Dan Williams <dan.j.williams@xxxxxxxxx>
Date:   Thu Jan 30 12:06:23 2020 -0800

    libnvdimm/region: Introduce an 'align' attribute


This is the patch that introduced the 16M alignment.

/*
 * PowerPC requires this alignment for memremap_pages(). All other archs
 * should be ok with SUBSECTION_SIZE (see memremap_compat_align()).
 */
#define MEMREMAP_COMPAT_ALIGN_MAX SZ_16M

static unsigned long default_align(struct nd_region *nd_region)
{
        unsigned long align;
        int i, mappings;
        u32 remainder;

        if (is_nd_blk(&nd_region->dev))
                align = PAGE_SIZE;
        else
                align = MEMREMAP_COMPAT_ALIGN_MAX;

Dan, is this logic correct? Why is_nd_pmem() cannot be set to
SUBSECTION_SIZE alignment?

Thank you,
Pasha

>
> Pasha