Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.
1 Started Oct 1999 by Kanoj Sarcar <email@example.com> 2 3 The intent of this file is to have an uptodate, running commentary 4 from different people about how locking and synchronization is done 5 in the Linux vm code. 6 7 page_table_lock & mmap_sem 8 -------------------------------------- 9 10 Page stealers pick processes out of the process pool and scan for 11 the best process to steal pages from. To guarantee the existence 12 of the victim mm, a mm_count inc and a mmdrop are done in swap_out(). 13 Page stealers hold kernel_lock to protect against a bunch of races. 14 The vma list of the victim mm is also scanned by the stealer, 15 and the page_table_lock is used to preserve list sanity against the 16 process adding/deleting to the list. This also guarantees existence 17 of the vma. Vma existence is not guaranteed once try_to_swap_out() 18 drops the page_table_lock. To guarantee the existence of the underlying 19 file structure, a get_file is done before the swapout() method is 20 invoked. The page passed into swapout() is guaranteed not to be reused 21 for a different purpose because the page reference count due to being 22 present in the user's pte is not released till after swapout() returns. 23 24 Any code that modifies the vmlist, or the vm_start/vm_end/ 25 vm_flags:VM_LOCKED/vm_next of any vma *in the list* must prevent 26 kswapd from looking at the chain. 27 28 The rules are: 29 1. To scan the vmlist (look but don't touch) you must hold the 30 mmap_sem with read bias, i.e. down_read(&mm->mmap_sem) 31 2. To modify the vmlist you need to hold the mmap_sem with 32 read&write bias, i.e. down_write(&mm->mmap_sem) *AND* 33 you need to take the page_table_lock. 34 3. The swapper takes _just_ the page_table_lock, this is done 35 because the mmap_sem can be an extremely long lived lock 36 and the swapper just cannot sleep on that. 37 4. The exception to this rule is expand_stack, which just 38 takes the read lock and the page_table_lock, this is ok 39 because it doesn't really modify fields anybody relies on. 40 5. You must be able to guarantee that while holding page_table_lock 41 or page_table_lock of mm A, you will not try to get either lock 42 for mm B. 43 44 The caveats are: 45 1. find_vma() makes use of, and updates, the mmap_cache pointer hint. 46 The update of mmap_cache is racy (page stealer can race with other code 47 that invokes find_vma with mmap_sem held), but that is okay, since it 48 is a hint. This can be fixed, if desired, by having find_vma grab the 49 page_table_lock. 50 51 52 Code that add/delete elements from the vmlist chain are 53 1. callers of insert_vm_struct 54 2. callers of merge_segments 55 3. callers of avl_remove 56 57 Code that changes vm_start/vm_end/vm_flags:VM_LOCKED of vma's on 58 the list: 59 1. expand_stack 60 2. mprotect 61 3. mlock 62 4. mremap 63 64 It is advisable that changes to vm_start/vm_end be protected, although 65 in some cases it is not really needed. Eg, vm_start is modified by 66 expand_stack(), it is hard to come up with a destructive scenario without 67 having the vmlist protection in this case. 68 69 The page_table_lock nests with the inode i_mmap_mutex and the kmem cache 70 c_spinlock spinlocks. This is okay, since the kmem code asks for pages after 71 dropping c_spinlock. The page_table_lock also nests with pagecache_lock and 72 pagemap_lru_lock spinlocks, and no code asks for memory with these locks 73 held. 74 75 The page_table_lock is grabbed while holding the kernel_lock spinning monitor. 76 77 The page_table_lock is a spin lock. 78 79 Note: PTL can also be used to guarantee that no new clones using the 80 mm start up ... this is a loose form of stability on mm_users. For 81 example, it is used in copy_mm to protect against a racing tlb_gather_mmu 82 single address space optimization, so that the zap_page_range (from 83 truncate) does not lose sending ipi's to cloned threads that might 84 be spawned underneath it and go to user mode to drag in pte's into tlbs. 85 86 swap_lock 87 -------------- 88 The swap devices are chained in priority order from the "swap_list" header. 89 The "swap_list" is used for the round-robin swaphandle allocation strategy. 90 The #free swaphandles is maintained in "nr_swap_pages". These two together 91 are protected by the swap_lock. 92 93 The swap_lock also protects all the device reference counts on the 94 corresponding swaphandles, maintained in the "swap_map" array, and the 95 "highest_bit" and "lowest_bit" fields. 96 97 The swap_lock is a spinlock, and is never acquired from intr level. 98 99 To prevent races between swap space deletion or async readahead swapins 100 deciding whether a swap handle is being used, ie worthy of being read in 101 from disk, and an unmap -> swap_free making the handle unused, the swap 102 delete and readahead code grabs a temp reference on the swaphandle to 103 prevent warning messages from swap_duplicate <- read_swap_cache_async. 104 105 Swap cache locking 106 ------------------ 107 Pages are added into the swap cache with kernel_lock held, to make sure 108 that multiple pages are not being added (and hence lost) by associating 109 all of them with the same swaphandle. 110 111 Pages are guaranteed not to be removed from the scache if the page is 112 "shared": ie, other processes hold reference on the page or the associated 113 swap handle. The only code that does not follow this rule is shrink_mmap, 114 which deletes pages from the swap cache if no process has a reference on 115 the page (multiple processes might have references on the corresponding 116 swap handle though). lookup_swap_cache() races with shrink_mmap, when 117 establishing a reference on a scache page, so, it must check whether the 118 page it located is still in the swapcache, or shrink_mmap deleted it. 119 (This race is due to the fact that shrink_mmap looks at the page ref 120 count with pagecache_lock, but then drops pagecache_lock before deleting 121 the page from the scache). 122 123 do_wp_page and do_swap_page have MP races in them while trying to figure 124 out whether a page is "shared", by looking at the page_count + swap_count. 125 To preserve the sum of the counts, the page lock _must_ be acquired before 126 calling is_page_shared (else processes might switch their swap_count refs 127 to the page count refs, after the page count ref has been snapshotted). 128 129 Swap device deletion code currently breaks all the scache assumptions, 130 since it grabs neither mmap_sem nor page_table_lock.