On 06/22/2015 03:18 PM, Kirill A. Shutemov wrote:
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.
Thanks.
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?
I think so.
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.
I guess that works, but it's rather non-obvious thus worth documenting
somewhere.
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