Subject: [PATCH] mm: pmd dirty emulation in page fault handler

From: [email-masked] (Minchan Kim)
Date: Fri, 23 Dec 2016 18:53:36 +0900

Hi,

On Fri, Dec 23, 2016 at 10:17:25AM +0100, Michal Hocko wrote:
> On Thu 22-12-16 23:52:03, Minchan Kim wrote:
> [...]
> > >From b3ec95c0df91ad113525968a4a6b53030fd0b48d Mon Sep 17 00:00:00 2001
> > From: Minchan Kim <[email-masked]>
> > Date: Thu, 22 Dec 2016 23:43:49 +0900
> > Subject: [PATCH v2] mm: pmd dirty emulation in page fault handler
> > 
> > Andreas reported [1] made a test in jemalloc hang in THP mode in arm64.
> > http://lkml.kernel.org/r/mvmmvfy37g1.fsf at hawking.suse.de
> > 
> > The problem is page fault handler supports only accessed flag emulation
> > for THP page of SW-dirty/accessed architecture.
> > 
> > This patch enables dirty-bit emulation for those architectures.
> > Without it, MADV_FREE makes application hang by repeated fault forever.
> 
> The changelog is rather terse and considering the issue is rather subtle
> and it aims the stable tree I think it could see more information. How
> do we end up looping in the page fault and why the dirty pmd stops it.
> Could you update the changelog to be more verbose, please? I am still
> digesting this patch but I believe it is correct fwiw...
> 

How about this? Feel free to suggest better wording.

Andreas reported [1] made a test in jemalloc hang in THP mode in arm64.
http://lkml.kernel.org/r/mvmmvfy37g1.fsf at hawking.suse.de

The problem is currently page fault handler doesn't supports dirty bit
emulation of pte for non-HW dirty-bit architecture so that application
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.