Re: [PATCH v4 1/5] mm: add mkwrite param to vm_insert_mixed()

[Jan Kara]

On Sat 22-07-17 09:21:31, Dan Williams wrote:
> On Fri, Jul 21, 2017 at 3:39 PM, Ross Zwisler
> <ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> > To be able to use the common 4k zero page in DAX we need to have our PTE
> > fault path look more like our PMD fault path where a PTE entry can be
> > marked as dirty and writeable as it is first inserted, rather than waiting
> > for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call.
> >
> > Right now we can rely on having a dax_pfn_mkwrite() call because we can
> > distinguish between these two cases in do_wp_page():
> >
> >         case 1: 4k zero page => writable DAX storage
> >         case 2: read-only DAX storage => writeable DAX storage
> >
> > This distinction is made by via vm_normal_page().  vm_normal_page() returns
> > false for the common 4k zero page, though, just as it does for DAX ptes.
> > Instead of special casing the DAX + 4k zero page case, we will simplify our
> > DAX PTE page fault sequence so that it matches our DAX PMD sequence, and
> > get rid of the dax_pfn_mkwrite() helper.  We will instead use
> > dax_iomap_fault() to handle write-protection faults.
> >
> > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd()
> > and allow us to pass in a 'mkwrite' flag.  If 'mkwrite' is set insert_pfn()
> > will do the work that was previously done by wp_page_reuse() as part of the
> > dax_pfn_mkwrite() call path.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/dax/device.c                    |  2 +-
> >  drivers/gpu/drm/exynos/exynos_drm_gem.c |  3 ++-
> >  drivers/gpu/drm/gma500/framebuffer.c    |  2 +-
> >  drivers/gpu/drm/msm/msm_gem.c           |  3 ++-
> >  drivers/gpu/drm/omapdrm/omap_gem.c      |  6 ++++--
> >  drivers/gpu/drm/ttm/ttm_bo_vm.c         |  2 +-
> >  fs/dax.c                                |  2 +-
> >  include/linux/mm.h                      |  2 +-
> >  mm/memory.c                             | 27 +++++++++++++++++++++------
> >  9 files changed, 34 insertions(+), 15 deletions(-)
> >
> > diff --git a/drivers/dax/device.c b/drivers/dax/device.c
> > index e9f3b3e..3973521 100644
> > --- a/drivers/dax/device.c
> > +++ b/drivers/dax/device.c
> > @@ -273,7 +273,7 @@ static int __dev_dax_pte_fault(struct dev_dax *dev_dax, struct vm_fault *vmf)
> >
> >         pfn = phys_to_pfn_t(phys, dax_region->pfn_flags);
> >
> > -       rc = vm_insert_mixed(vmf->vma, vmf->address, pfn);
> > +       rc = vm_insert_mixed(vmf->vma, vmf->address, pfn, false);
> 
> Ugh, I generally find bool flags unreadable. They place a tax on
> jumping to function definition to recall what true and false mean. If
> we want to go this 'add an argument' route can we at least add an enum
> like:
> 
> enum {
>     PTE_MKDIRTY,
>     PTE_MKCLEAN,
> };
> 
> ...to differentiate the two cases?

So how I usually deal with this is that I create e.g.:

__vm_insert_mixed() that takes the bool argument, make vm_insert_mixed()
pass false, and vm_insert_mixed_mkwrite() pass true. That way there's no
code duplication, old call sites can stay unchanged, the naming clearly
says what's going on...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR