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

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Date:   Fri, 16 Jun 2017 23:53:33 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 3/3] mm: Use updated pmdp_invalidate() inteface to
 track dirty/accessed bits

Hi Andrea,

On Fri, Jun 16, 2017 at 04:27:20PM +0200, Andrea Arcangeli wrote:
> 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).

I got it during reviewing this patch. That's why I didn't argue
this patch would break MADV_FREE by deleting routine which propagate
dirty to pte of subpages. However, although it's useless, I prefer
not removing the transfer of dirty bit. Because it would help MADV_FREE
users who want to use smaps to know how many of pages are not freeable
(i.e, dirtied) since MADV_FREE although it is not 100% correct.

> 
> 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.

Thanks for the comment. Then, this patch is to fix MADV_FREE's bug
which has lost dirty bit by transferring dirty bit too early.

Thanks.