Re: [PATCHv6 00/36] THP refcounting redesign

Kirill A. Shutemov
2015-06-22T13:39:51Z

On Mon, Jun 22, 2015 at 03:32:54PM +0200, Jerome Marchand wrote:
> On 06/22/2015 03:21 PM, Kirill A. Shutemov wrote:
> > On Tue, Jun 16, 2015 at 03:17:13PM +0200, Jerome Marchand wrote:
> >> On 06/03/2015 07:05 PM, Kirill A. Shutemov wrote:
> >>> Hello everybody,
> >>>
> >>> Here's new revision of refcounting patchset. Please review and consider
> >>> applying.
> >>>
> >>> The goal of patchset is to make refcounting on THP pages cheaper with
> >>> simpler semantics and allow the same THP compound page to be mapped with
> >>> PMD and PTEs. This is required to get reasonable THP-pagecache
> >>> implementation.
> >>>
> >>> With the new refcounting design it's much easier to protect against
> >>> split_huge_page(): simple reference on a page will make you the deal.
> >>> It makes gup_fast() implementation simpler and doesn't require
> >>> special-case in futex code to handle tail THP pages.
> >>>
> >>> It should improve THP utilization over the system since splitting THP in
> >>> one process doesn't necessary lead to splitting the page in all other
> >>> processes have the page mapped.
> >>>
> >>> The patchset drastically lower complexity of get_page()/put_page()
> >>> codepaths. I encourage people look on this code before-and-after to
> >>> justify time budget on reviewing this patchset.
> >>>
> >>> = Changelog =
> >>>
> >>> v6:
> >>> - rebase to since-4.0;
> >>> - optimize mapcount handling: significantely reduce overhead for most
> >>> common cases.
> >>> - split pages on migrate_pages();
> >>> - remove infrastructure for handling splitting PMDs on all architectures;
> >>> - fix page_mapcount() for hugetlb pages;
> >>>
> >>
> >> Hi Kirill,
> >>
> >> I ran some LTP mm tests and hugemmap tests trigger the following:
> >>
> >> [ 438.749457] page:ffffea0000df8000 count:2 mapcount:0 mapping: (null) index:0x0 compound_mapcount: 0
> >> [ 438.750089] flags: 0x3ffc0000004001(locked|head)
> >> [ 438.750089] page dumped because: VM_BUG_ON_PAGE(page_mapped(page))
> >
> > Did you run with original or updated version of patch 27/36?
> > In original post of v6 there was bug: page_mapped() always returned true.
> >
>
> Indeed! I'll try again with the corrected patch.

I'm going to post updated patchset soon. Probably tomorrow.

--
Kirill A. Shutemov
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