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    Subject[PATCHv6 36/36] thp: update documentation
    Date
    The patch updates Documentation/vm/transhuge.txt to reflect changes in
    THP design.

    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    ---
    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
    + can continue working on the regular pages or regular pte mappings.

    - if a hugepage allocation fails because of memory fragmentation,
    regular pages should be gracefully allocated instead and mixed in
    @@ -200,9 +200,18 @@ thp_collapse_alloc_failed is incremented if khugepaged found a range
    of pages that should be collapsed into one huge page but failed
    the allocation.

    -thp_split is incremented every time a huge page is split into base
    +thp_split_page is incremented every time a huge page is split into base
    pages. This can happen for a variety of reasons but a common
    reason is that a huge page is old and is being reclaimed.
    + This action implies splitting all PMD the page mapped with.
    +
    +thp_split_page_failed is is incremented if kernel fails to split huge
    + page. This can happen if the page was pinned by somebody.
    +
    +thp_split_pmd is incremented every time a PMD split into table of PTEs.
    + This can happen, for instance, when application calls mprotect() or
    + munmap() on part of huge page. It doesn't split huge page, only
    + page table entry.

    thp_zero_page_alloc is incremented every time a huge zero page is
    successfully allocated. It includes allocations which where
    @@ -253,10 +262,8 @@ is complete, so they won't ever notice the fact the page is huge. But
    if any driver is going to mangle over the page structure of the tail
    page (like for checking page->mapping or other bits that are relevant
    for the head page and not the tail page), it should be updated to jump
    -to check head page instead (while serializing properly against
    -split_huge_page() to avoid the head and tail pages to disappear from
    -under it, see the futex code to see an example of that, hugetlbfs also
    -needed special handling in futex code for similar reasons).
    +to check head page instead. Taking reference on any head/tail page would
    +prevent page from being split by anyone.

    NOTE: these aren't new constraints to the GUP API, and they match the
    same constrains that applies to hugetlbfs too, so any driver capable
    @@ -291,9 +298,9 @@ unaffected. libhugetlbfs will also work fine as usual.
    == Graceful fallback ==

    Code walking pagetables but unware about huge pmds can simply call
    -split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by
    +split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
    pmd_offset. It's trivial to make the code transparent hugepage aware
    -by just grepping for "pmd_offset" and adding split_huge_page_pmd where
    +by just grepping for "pmd_offset" and adding split_huge_pmd where
    missing after pmd_offset returns the pmd. Thanks to the graceful
    fallback design, with a one liner change, you can avoid to write
    hundred if not thousand of lines of complex code to make your code
    @@ -302,7 +309,8 @@ hugepage aware.
    If you're not walking pagetables but you run into a physical hugepage
    but you can't handle it natively in your code, you can split it by
    calling split_huge_page(page). This is what the Linux VM does before
    -it tries to swapout the hugepage for example.
    +it tries to swapout the hugepage for example. split_huge_page() can fail
    +if the page is pinned and you must handle this correctly.

    Example to make mremap.c transparent hugepage aware with a one liner
    change:
    @@ -314,14 +322,14 @@ diff --git a/mm/mremap.c b/mm/mremap.c
    return NULL;

    pmd = pmd_offset(pud, addr);
    -+ split_huge_page_pmd(vma, addr, pmd);
    ++ split_huge_pmd(vma, pmd, addr);
    if (pmd_none_or_clear_bad(pmd))
    return NULL;

    == Locking in hugepage aware code ==

    We want as much code as possible hugepage aware, as calling
    -split_huge_page() or split_huge_page_pmd() has a cost.
    +split_huge_page() or split_huge_pmd() has a cost.

    To make pagetable walks huge pmd aware, all you need to do is to call
    pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
    @@ -330,47 +338,53 @@ created from under you by khugepaged (khugepaged collapse_huge_page
    takes the mmap_sem in write mode in addition to the anon_vma lock). If
    pmd_trans_huge returns false, you just fallback in the old code
    paths. If instead pmd_trans_huge returns true, you have to take the
    -mm->page_table_lock and re-run pmd_trans_huge. Taking the
    -page_table_lock will prevent the huge pmd to be converted into a
    -regular pmd from under you (split_huge_page can run in parallel to the
    +page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
    +page table lock will prevent the huge pmd to be converted into a
    +regular pmd from under you (split_huge_pmd can run in parallel to the
    pagetable walk). If the second pmd_trans_huge returns false, you
    -should just drop the page_table_lock and fallback to the old code as
    -before. Otherwise you should run pmd_trans_splitting on the pmd. In
    -case pmd_trans_splitting returns true, it means split_huge_page is
    -already in the middle of splitting the page. So if pmd_trans_splitting
    -returns true it's enough to drop the page_table_lock and call
    -wait_split_huge_page and then fallback the old code paths. You are
    -guaranteed by the time wait_split_huge_page returns, the pmd isn't
    -huge anymore. If pmd_trans_splitting returns false, you can proceed to
    -process the huge pmd and the hugepage natively. Once finished you can
    -drop the page_table_lock.
    -
    -== compound_lock, get_user_pages and put_page ==
    +should just drop the page table lock and fallback to the old code as
    +before. Otherwise you can proceed to process the huge pmd and the
    +hugepage natively. Once finished you can drop the page table lock.
    +
    +== Refcounts and transparent huge pages ==
    +
    +Refcounting on THP is mostly consistent with refcounting on other compound
    +pages:
    +
    + - get_page()/put_page() and GUP operate in head page's ->_count.
    +
    + - ->_count in tail pages is always zero: get_page_unless_zero() never
    + succeed on tail pages.
    +
    + - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
    + on relevent sub-page of the compound page.
    +
    + - map/unmap of the whole compound page accounted in compound_mapcount
    + (stored in first tail page).
    +
    +PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.
    +This additional reference is required to get race-free detection of unmap of
    +subpages when we have them mapped with both PMDs and PTEs.
    +
    +This is optimization required to lower overhead of per-subpage mapcount
    +tracking. The alternative is alter ->_mapcount in all subpages on each
    +map/unmap of the whole compound page.
    +
    +We set PG_double_map when a PMD of the page got split for the first time,
    +but still have PMD mapping. The addtional references go away with last
    +compound_mapcount.

    split_huge_page internally has to distribute the refcounts in the head
    -page to the tail pages before clearing all PG_head/tail bits from the
    -page structures. It can do that easily for refcounts taken by huge pmd
    -mappings. But the GUI API as created by hugetlbfs (that returns head
    -and tail pages if running get_user_pages on an address backed by any
    -hugepage), requires the refcount to be accounted on the tail pages and
    -not only in the head pages, if we want to be able to run
    -split_huge_page while there are gup pins established on any tail
    -page. Failure to be able to run split_huge_page if there's any gup pin
    -on any tail page, would mean having to split all hugepages upfront in
    -get_user_pages which is unacceptable as too many gup users are
    -performance critical and they must work natively on hugepages like
    -they work natively on hugetlbfs already (hugetlbfs is simpler because
    -hugetlbfs pages cannot be split so there wouldn't be requirement of
    -accounting the pins on the tail pages for hugetlbfs). If we wouldn't
    -account the gup refcounts on the tail pages during gup, we won't know
    -anymore which tail page is pinned by gup and which is not while we run
    -split_huge_page. But we still have to add the gup pin to the head page
    -too, to know when we can free the compound page in case it's never
    -split during its lifetime. That requires changing not just
    -get_page, but put_page as well so that when put_page runs on a tail
    -page (and only on a tail page) it will find its respective head page,
    -and then it will decrease the head page refcount in addition to the
    -tail page refcount. To obtain a head page reliably and to decrease its
    -refcount without race conditions, put_page has to serialize against
    -__split_huge_page_refcount using a special per-page lock called
    -compound_lock.
    +page to the tail pages before clearing all PG_head/tail bits from the page
    +structures. It can be done easily for refcounts taken by page table
    +entries. But we don't have enough information on how to distribute any
    +additional pins (i.e. from get_user_pages). split_huge_page() fails any
    +requests to split pinned huge page: it expects page count to be equal to
    +sum of mapcount of all sub-pages plus one (split_huge_page caller must
    +have reference for head page).
    +
    +split_huge_page uses migration entries to stabilize page->_count and
    +page->_mapcount.
    +
    +Note that split_huge_pmd() doesn't have any limitation on refcounting:
    +pmd can be split at any point and never fails.
    --
    2.1.4


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