Re: [PATCH v2 1/3] mm: add vm_insert_mixed_mkwrite()

From: Jan Kara
Date: Fri Jun 23 2017 - 11:25:41 EST

On Fri 16-06-17 22:09:26, Ross Zwisler wrote:
> On Thu, Jun 15, 2017 at 04:42:04PM +0200, Jan Kara wrote:
> > On Wed 14-06-17 11:22:09, Ross Zwisler 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 dax_pfn_mkwrite() completely.
> > >
> > > 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>
> >
> > So I agree that getting rid of dax_pfn_mkwrite() and using fault handler in
> > that case is a way to go. However I somewhat dislike the
> > vm_insert_mixed_mkwrite() thing - it looks like a hack - and I'm aware that
> > we have a similar thing for PMD which is ugly as well. Besides being ugly
> > I'm also concerned that when 'mkwrite' is set, we just silently overwrite
> > whatever PTE was installed at that position. Not that I'd see how that
> > could screw us for DAX but still a concern that e.g. some PTE flag could
> > get discarded by this is there... In fact, for !HAVE_PTE_SPECIAL
> > architectures, you will leak zero page references by just overwriting the
> > PTE - for those archs you really need to unmap zero page before replacing
> > PTE (and the same for PMD I suppose).
> >
> > So how about some vmf_insert_pfn(vmf, pe_size, pfn) helper that would
> > properly detect PTE / PMD case, read / write case etc., check that PTE did
> > not change from orig_pte, and handle all the nasty details instead of
> > messing with insert_pfn?
> I played around with this some today, and I wasn't super happy with the
> results. Here were some issues I encountered:
> 1) The pte_mkyoung(), maybe_mkwrite() and pte_mkdirty() calls need to happen
> with the PTE locked, and I'm currently able to piggy-back on the locking done
> in insert_pfn(). If I keep those steps out of insert_pfn() I either have to
> essentially duplicate all the work done by insert_pfn() into another function
> so I can do everything I need under one lock, or I have to insert the PFN via
> insert_pfn() (which as you point out, will just leave the pfn alone if it's
> already present), then for writes I have to re-grab the PTE lock and set do
> the mkwrite steps.
> Either of these work, but they both also seem kind of gross...
> 2) Combining the PTE and PMD cases into a common function will require
> mm/memory.c to call vmf_insert_pfn_pmd(), which depends on
> CONFIG_TRANSPARENT_HUGEPAGE being defined. This works, it just means some
> more #ifdef CONFIG_TRANSPARENT_HUGEPAGE hackery in mm/memory.c.
> I agree that unconditionally overwriting the PTE when mkwrite is set is
> undesireable, and should be fixed. My implementation of the wrapper just
> didn't seem that natural, which usually tells me I'm headed down the wrong
> path. Maybe I'm just not fully understanding what you intended?
> In any case, my current favorite soultion for this issue is still what I had
> in v1:
> with perhaps the removal of the new vm_insert_mixed_mkwrite() symbol, and just
> adding a 'write' flag to vm_insert_mixed() and updating all the call sites,
> and fixing the flow where mkwrite unconditionally overwrites the PTE?
> If not, can you help me understand what you think is ugly about the 'write'
> flag to vm_insert_mixed() and vmf_insert_pfn_pmd()?

Yeah, so write flag is probably OK. I just dislike the implicit "replace"
side-effect of the write flag. If 'write' would just mean whether PTE is
created writeable, that is fine with me.

Jan Kara <jack@xxxxxxxx>