On Fri 23-12-16 23:01:31, Minchan Kim wrote:

On Fri, Dec 23, 2016 at 12:54:21PM +0100, Michal Hocko wrote:

quoted

On Fri 23-12-16 18:53:36, Minchan Kim wrote:

[...]

quoted

quoted

stucks until VM marked the pmd dirty.

How the emulation work depends on the architecture. In case of arm64,
when it set up pte firstly, it sets pte PTE_RDONLY to get a chance to
mark the pte dirty via triggering page fault when store access happens.
Once the page fault occurs, VM marks the pte dirty and arch code for
setting pte will clear PTE_RDONLY for application to proceed.

IOW, if VM doesn't mark the pte dirty, application hangs forever by
repeated fault(i.e., store op but the pte is PTE_RDONLY).

This patch enables dirty-bit emulation for those architectures.

Yes this is helpful and much more clear, thank you. One thing that is
still not clear to me is why cannot we handle that in the arch specific
code. I mean what is the side effect of doing pmd_mkdirty for
architectures which do not need it?

For architecture which supports H/W access/dirty bit, it couldn't be
reached there code path so there is no side effect, I think.

ahh, I knew I was missing something. It definitely wasn't obvious to me
and my x86 config it simply generates code to call
huge_pmd_set_accessed.

A thing
I can think of is just increasing code size little bit. Maybe, we
could optimize away some ifdef magic but not sure worth it.

it is not
-- 
Michal Hocko
SUSE Labs