Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to track dirty/accessed bits

From: Andrea Arcangeli <aarcange@redhat.com>
To: Minchan Kim <minchan@kernel.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Vineet Gupta <vgupta@synopsys.com>,
	Russell King <linux@armlinux.org.uk>,
	Will Deacon <will.deacon@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Ralf Baechle <ralf@linux-mips.org>,
	"David S. Miller" <davem@davemloft.net>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	linux-arch@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to track dirty/accessed bits
Date: Fri, 16 Jun 2017 16:27:20 +0200	[thread overview]
Message-ID: <20170616142720.GH11676@redhat.com> (raw)
In-Reply-To: <20170616135209.GA29542@bbox>

Hello Minchan,

On Fri, Jun 16, 2017 at 10:52:09PM +0900, Minchan Kim wrote:
> > > > @@ -1995,8 +1984,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> > > >  			if (soft_dirty)
> > > >  				entry = pte_mksoft_dirty(entry);
> > > >  		}
> > > > -		if (dirty)
> > > > -			SetPageDirty(page + i);
> > > >  		pte = pte_offset_map(&_pmd, addr);
[..]
> 
> split_huge_page set PG_dirty to all subpages unconditionally?
> If it's true, yes, it doesn't break MADV_FREE. However, I didn't spot
> that piece of code. What I found one is just __split_huge_page_tail
> which set PG_dirty to subpage if head page is dirty. IOW, if the head
> page is not dirty, tail page will be clean, too.
> Could you point out what routine set PG_dirty to all subpages unconditionally?

On a side note the snippet deleted above was useless, as long as
there's one left hugepmd to split, the physical page has to be still
compound and huge and as long as that's the case the tail pages
PG_dirty bit is meaningless (even if set, it's going to be clobbered
during the physical split).

In short PG_dirty is only meaningful in the head as long as it's
compound. The physical split in __split_huge_page_tail transfer the
head value to the tails like you mentioned, that's all as far as I can
tell.