Re: [PATCHv6 36/36] thp: update documentation

[Kirill A. Shutemov]

On Thu, Jun 11, 2015 at 02:30:15PM +0200, Vlastimil Babka wrote:
> On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> >The patch updates Documentation/vm/transhuge.txt to reflect changes in
> >THP design.
> 
> One thing I'm missing is info about the deferred splitting.

Okay, I'll add this.

> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >---
> >  Documentation/vm/transhuge.txt | 124 +++++++++++++++++++++++------------------
> >  1 file changed, 69 insertions(+), 55 deletions(-)
> >
> >diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
> >index 6b31cfbe2a9a..2352b12cae93 100644
> >--- a/Documentation/vm/transhuge.txt
> >+++ b/Documentation/vm/transhuge.txt
> >@@ -35,10 +35,10 @@ miss is going to run faster.
> >
> >  == Design ==
> >
> >-- "graceful fallback": mm components which don't have transparent
> >-  hugepage knowledge fall back to breaking a transparent hugepage and
> >-  working on the regular pages and their respective regular pmd/pte
> >-  mappings
> >+- "graceful fallback": mm components which don't have transparent hugepage
> >+  knowledge fall back to breaking huge pmd mapping into table of ptes and,
> >+  if nesessary, split a transparent hugepage. Therefore these components
> 
>         necessary
> >+
> >+split_huge_page uses migration entries to stabilize page->_count and
> >+page->_mapcount.
> 
> Hm, what if there's some physical memory scanner taking page->_count pins? I
> think compaction shouldn't be an issue, but maybe some others?

The only legitimate way scanner can get reference to a page is
get_page_unless_zero(), right?

All tail pages has zero ->_count until atomic_add() in
__split_huge_page_tail() -- get_page_unless_zero() will fail.
After the atomic_add() we don't care about ->_count value.
We already known how many references with should uncharge from 
head page.

For head page get_page_unless_zero() will succeed and we don't
mind. It's clear where reference should go after split: it will
stay on head page.

-- 
 Kirill A. Shutemov
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