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

Ross Zwisler
2017-07-24T15:13:11Z

On Mon, Jul 24, 2017 at 01:15:31PM +0200, Jan Kara wrote:
> On Sat 22-07-17 09:21:31, Dan Williams wrote:
> > On Fri, Jul 21, 2017 at 3:39 PM, Ross Zwisler
> > <ross.zwisler@linux.intel.com> 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@linux.intel.com>
> > > ---
> > > 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...

Ah, that does seem cleaner. I'll try that for v5.