diff options
Diffstat (limited to 'Documentation/vm/transhuge.txt')
| -rw-r--r-- | Documentation/vm/transhuge.txt | 23 |
1 files changed, 19 insertions, 4 deletions
diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt index f734bb2a78d..6b31cfbe2a9 100644 --- a/Documentation/vm/transhuge.txt +++ b/Documentation/vm/transhuge.txt @@ -116,6 +116,13 @@ echo always >/sys/kernel/mm/transparent_hugepage/defrag echo madvise >/sys/kernel/mm/transparent_hugepage/defrag echo never >/sys/kernel/mm/transparent_hugepage/defrag +By default kernel tries to use huge zero page on read page fault. +It's possible to disable huge zero page by writing 0 or enable it +back by writing 1: + +echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page +echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page + khugepaged will be automatically started when transparent_hugepage/enabled is set to "always" or "madvise, and it'll be automatically shutdown if it's set to "never". @@ -197,6 +204,14 @@ thp_split 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. +thp_zero_page_alloc is incremented every time a huge zero page is + successfully allocated. It includes allocations which where + dropped due race with other allocation. Note, it doesn't count + every map of the huge zero page, only its allocation. + +thp_zero_page_alloc_failed is incremented if kernel fails to allocate + huge zero page and falls back to using small pages. + As the system ages, allocating huge pages may be expensive as the system uses memory compaction to copy data around memory to free a huge page for use. There are some counters in /proc/vmstat to help @@ -276,7 +291,7 @@ 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(mm, pmd) where the pmd is the one returned by +split_huge_page_pmd(vma, addr, pmd) 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 missing after pmd_offset returns the pmd. Thanks to the graceful @@ -299,7 +314,7 @@ diff --git a/mm/mremap.c b/mm/mremap.c return NULL; pmd = pmd_offset(pud, addr); -+ split_huge_page_pmd(mm, pmd); ++ split_huge_page_pmd(vma, addr, pmd); if (pmd_none_or_clear_bad(pmd)) return NULL; @@ -345,13 +360,13 @@ 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 splitted so there wouldn't be requirement of +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 -splitted during its lifetime. That requires changing not just +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 |