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

From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Jason Evans <je@fb.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	"[4.5+]" <stable@vger.kernel.org>,
	Andreas Schwab <schwab@suse.de>
Subject: Re: [PATCH] mm: pmd dirty emulation in page fault handler
Date: Fri, 23 Dec 2016 15:53:05 +0100	[thread overview]
Message-ID: <20161223145305.GF23109@dhcp22.suse.cz> (raw)
In-Reply-To: <20161223140131.GA5724@bbox>

On Fri 23-12-16 23:01:31, Minchan Kim wrote:
> On Fri, Dec 23, 2016 at 12:54:21PM +0100, Michal Hocko wrote:
> > On Fri 23-12-16 18:53:36, Minchan Kim wrote:
[...]
> > > 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

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