Re: [PATCHv5 00/28] THP refcounting redesign

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, May 15, 2015 at 10:55:55AM +0200, Vlastimil Babka wrote:
> On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> >Hello everybody,
> >
> >Here's reworked version of my patchset. All known issues were addressed.
> >
> >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 reviewers look on this code before-and-after to
> >justify time budget on reviewing this patchset.
> >
> >= Changelog =
> >
> >v5:
> >   - Tested-by: Sasha Levin!a?c
> >   - re-split patchset in hope to improve readability;
> >   - rebased on top of page flags and ->mapping sanitizing patchset;
> >   - uncharge compound_mapcount rather than mapcount for hugetlb pages
> >     during removing from rmap;
> >   - differentiate page_mapped() from page_mapcount() for compound pages;
> >   - rework deferred_split_huge_page() to use shrinker interface;
> >   - fix race in page_remove_rmap();
> >   - get rid of __get_page_tail();
> >   - few random bug fixes;
> >v4:
> >   - fix sizes reported in smaps;
> >   - defines instead of enum for RMAP_{EXCLUSIVE,COMPOUND};
> >   - skip THP pages on munlock_vma_pages_range(): they are never mlocked;
> >   - properly handle huge zero page on FOLL_SPLIT;
> >   - fix lock_page() slow path on tail pages;
> >   - account page_get_anon_vma() fail to THP_SPLIT_PAGE_FAILED;
> >   - fix split_huge_page() on huge page with unmapped head page;
> >   - fix transfering 'write' and 'young' from pmd to ptes on split_huge_pmd;
> >   - call page_remove_rmap() in unfreeze_page under ptl.
> >
> >= Design overview =
> >
> >The main reason why we can't map THP with 4k is how refcounting on THP
> >designed. It built around two requirements:
> >
> >   - split of huge page should never fail;
> >   - we can't change interface of get_user_page();
> >
> >To be able to split huge page at any point we have to track which tail
> >page was pinned. It leads to tricky and expensive get_page() on tail pages
> >and also occupy tail_page->_mapcount.
> >
> >Most split_huge_page*() users want PMD to be split into table of PTEs and
> >don't care whether compound page is going to be split or not.
> >
> >The plan is:
> >
> >  - allow split_huge_page() to fail if the page is pinned. It's trivial to
> >    split non-pinned page and it doesn't require tail page refcounting, so
> >    tail_page->_mapcount is free to be reused.
> >
> >  - introduce new routine -- split_huge_pmd() -- to split PMD into table of
> >    PTEs. It splits only one PMD, not touching other PMDs the page is
> >    mapped with or underlying compound page. Unlike new split_huge_page(),
> >    split_huge_pmd() never fails.
> >
> >Fortunately, we have only few places where split_huge_page() is needed:
> >swap out, memory failure, migration, KSM. And all of them can handle
> >split_huge_page() fail.
> >
> >In new scheme we use page->_mapcount is used to account how many time
> >the page is mapped with PTEs. We have separate compound_mapcount() to
> >count mappings with PMD. page_mapcount() returns sum of PTE and PMD
> >mappings of the page.
> 
> It would be very beneficial to describe the scheme in full, both before in
> after. The latter goes also for the Documentation patch, where you fixed
> what wasn't true anymore, but I think the picture wasn't complete neither
> before, nor is it now. There's the lwn article [1] which helps a lot, but we
> shouldn't rely on that exclusively.
> 
> So the full scheme should include at least:
> - where were/are pins and mapcounts stored
> - what exactly get_page()/put_page() did/does now
> - etc.
> 
> [1] https://lwn.net/Articles/619738/

Okay. Will do.
-- 
 Kirill A. Shutemov

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