1 ============================ 2 Transparent Hugepage Support 3 ============================ 4 5 This document describes design principles for Transparent Hugepage (THP) 6 support and its interaction with other parts of the memory management 7 system. 8 9 Design principles 10 ================= 11 12 - "graceful fallback": mm components which don't have transparent hugepage 13 knowledge fall back to breaking huge pmd mapping into table of ptes and, 14 if necessary, split a transparent hugepage. Therefore these components 15 can continue working on the regular pages or regular pte mappings. 16 17 - if a hugepage allocation fails because of memory fragmentation, 18 regular pages should be gracefully allocated instead and mixed in 19 the same vma without any failure or significant delay and without 20 userland noticing 21 22 - if some task quits and more hugepages become available (either 23 immediately in the buddy or through the VM), guest physical memory 24 backed by regular pages should be relocated on hugepages 25 automatically (with khugepaged) 26 27 - it doesn't require memory reservation and in turn it uses hugepages 28 whenever possible (the only possible reservation here is kernelcore= 29 to avoid unmovable pages to fragment all the memory but such a tweak 30 is not specific to transparent hugepage support and it's a generic 31 feature that applies to all dynamic high order allocations in the 32 kernel) 33 34 get_user_pages and follow_page 35 ============================== 36 37 get_user_pages and follow_page if run on a hugepage, will return the 38 head or tail pages as usual (exactly as they would do on 39 hugetlbfs). Most GUP users will only care about the actual physical 40 address of the page and its temporary pinning to release after the I/O 41 is complete, so they won't ever notice the fact the page is huge. But 42 if any driver is going to mangle over the page structure of the tail 43 page (like for checking page->mapping or other bits that are relevant 44 for the head page and not the tail page), it should be updated to jump 45 to check head page instead. Taking a reference on any head/tail page would 46 prevent the page from being split by anyone. 47 48 .. note:: 49 these aren't new constraints to the GUP API, and they match the 50 same constraints that apply to hugetlbfs too, so any driver capable 51 of handling GUP on hugetlbfs will also work fine on transparent 52 hugepage backed mappings. 53 54 Graceful fallback 55 ================= 56 57 Code walking pagetables but unaware about huge pmds can simply call 58 split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by 59 pmd_offset. It's trivial to make the code transparent hugepage aware 60 by just grepping for "pmd_offset" and adding split_huge_pmd where 61 missing after pmd_offset returns the pmd. Thanks to the graceful 62 fallback design, with a one liner change, you can avoid to write 63 hundreds if not thousands of lines of complex code to make your code 64 hugepage aware. 65 66 If you're not walking pagetables but you run into a physical hugepage 67 that you can't handle natively in your code, you can split it by 68 calling split_huge_page(page). This is what the Linux VM does before 69 it tries to swapout the hugepage for example. split_huge_page() can fail 70 if the page is pinned and you must handle this correctly. 71 72 Example to make mremap.c transparent hugepage aware with a one liner 73 change:: 74 75 diff --git a/mm/mremap.c b/mm/mremap.c 76 --- a/mm/mremap.c 77 +++ b/mm/mremap.c 78 @@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru 79 return NULL; 80 81 pmd = pmd_offset(pud, addr); 82 + split_huge_pmd(vma, pmd, addr); 83 if (pmd_none_or_clear_bad(pmd)) 84 return NULL; 85 86 Locking in hugepage aware code 87 ============================== 88 89 We want as much code as possible hugepage aware, as calling 90 split_huge_page() or split_huge_pmd() has a cost. 91 92 To make pagetable walks huge pmd aware, all you need to do is to call 93 pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the 94 mmap_lock in read (or write) mode to be sure a huge pmd cannot be 95 created from under you by khugepaged (khugepaged collapse_huge_page 96 takes the mmap_lock in write mode in addition to the anon_vma lock). If 97 pmd_trans_huge returns false, you just fallback in the old code 98 paths. If instead pmd_trans_huge returns true, you have to take the 99 page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the 100 page table lock will prevent the huge pmd being converted into a 101 regular pmd from under you (split_huge_pmd can run in parallel to the 102 pagetable walk). If the second pmd_trans_huge returns false, you 103 should just drop the page table lock and fallback to the old code as 104 before. Otherwise, you can proceed to process the huge pmd and the 105 hugepage natively. Once finished, you can drop the page table lock. 106 107 Refcounts and transparent huge pages 108 ==================================== 109 110 Refcounting on THP is mostly consistent with refcounting on other compound 111 pages: 112 113 - get_page()/put_page() and GUP operate on the folio->_refcount. 114 115 - ->_refcount in tail pages is always zero: get_page_unless_zero() never 116 succeeds on tail pages. 117 118 - map/unmap of a PMD entry for the whole THP increment/decrement 119 folio->_entire_mapcount, increment/decrement folio->_large_mapcount 120 and also increment/decrement folio->_nr_pages_mapped by ENTIRELY_MAPPED 121 when _entire_mapcount goes from -1 to 0 or 0 to -1. 122 123 - map/unmap of individual pages with PTE entry increment/decrement 124 page->_mapcount, increment/decrement folio->_large_mapcount and also 125 increment/decrement folio->_nr_pages_mapped when page->_mapcount goes 126 from -1 to 0 or 0 to -1 as this counts the number of pages mapped by PTE. 127 128 split_huge_page internally has to distribute the refcounts in the head 129 page to the tail pages before clearing all PG_head/tail bits from the page 130 structures. It can be done easily for refcounts taken by page table 131 entries, but we don't have enough information on how to distribute any 132 additional pins (i.e. from get_user_pages). split_huge_page() fails any 133 requests to split pinned huge pages: it expects page count to be equal to 134 the sum of mapcount of all sub-pages plus one (split_huge_page caller must 135 have a reference to the head page). 136 137 split_huge_page uses migration entries to stabilize page->_refcount and 138 page->_mapcount of anonymous pages. File pages just get unmapped. 139 140 We are safe against physical memory scanners too: the only legitimate way 141 a scanner can get a reference to a page is get_page_unless_zero(). 142 143 All tail pages have zero ->_refcount until atomic_add(). This prevents the 144 scanner from getting a reference to the tail page up to that point. After the 145 atomic_add() we don't care about the ->_refcount value. We already know how 146 many references should be uncharged from the head page. 147 148 For head page get_page_unless_zero() will succeed and we don't mind. It's 149 clear where references should go after split: it will stay on the head page. 150 151 Note that split_huge_pmd() doesn't have any limitations on refcounting: 152 pmd can be split at any point and never fails. 153 154 Partial unmap and deferred_split_folio() 155 ======================================== 156 157 Unmapping part of THP (with munmap() or other way) is not going to free 158 memory immediately. Instead, we detect that a subpage of THP is not in use 159 in folio_remove_rmap_*() and queue the THP for splitting if memory pressure 160 comes. Splitting will free up unused subpages. 161 162 Splitting the page right away is not an option due to locking context in 163 the place where we can detect partial unmap. It also might be 164 counterproductive since in many cases partial unmap happens during exit(2) if 165 a THP crosses a VMA boundary. 166 167 The function deferred_split_folio() is used to queue a folio for splitting. 168 The splitting itself will happen when we get memory pressure via shrinker 169 interface.
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