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+Started Oct 1999 by Kanoj Sarcar <firstname.lastname@example.org>
+The intent of this file is to have an uptodate, running commentary
+from different people about how locking and synchronization is done
+in the Linux vm code.
+page_table_lock & mmap_sem
+Page stealers pick processes out of the process pool and scan for
+the best process to steal pages from. To guarantee the existence
+of the victim mm, a mm_count inc and a mmdrop are done in swap_out().
+Page stealers hold kernel_lock to protect against a bunch of races.
+The vma list of the victim mm is also scanned by the stealer,
+and the page_table_lock is used to preserve list sanity against the
+process adding/deleting to the list. This also guarantees existence
+of the vma. Vma existence is not guaranteed once try_to_swap_out()
+drops the page_table_lock. To guarantee the existence of the underlying
+file structure, a get_file is done before the swapout() method is
+invoked. The page passed into swapout() is guaranteed not to be reused
+for a different purpose because the page reference count due to being
+present in the user's pte is not released till after swapout() returns.
+Any code that modifies the vmlist, or the vm_start/vm_end/
+vm_flags:VM_LOCKED/vm_next of any vma *in the list* must prevent
+kswapd from looking at the chain.
+The rules are:
+1. To scan the vmlist (look but don't touch) you must hold the
+ mmap_sem with read bias, i.e. down_read(&mm->mmap_sem)
+2. To modify the vmlist you need to hold the mmap_sem with
+ read&write bias, i.e. down_write(&mm->mmap_sem) *AND*
+ you need to take the page_table_lock.
+3. The swapper takes _just_ the page_table_lock, this is done
+ because the mmap_sem can be an extremely long lived lock
+ and the swapper just cannot sleep on that.
+4. The exception to this rule is expand_stack, which just
+ takes the read lock and the page_table_lock, this is ok
+ because it doesn't really modify fields anybody relies on.
+5. You must be able to guarantee that while holding page_table_lock
+ or page_table_lock of mm A, you will not try to get either lock
+ for mm B.
+The caveats are:
+1. find_vma() makes use of, and updates, the mmap_cache pointer hint.
+The update of mmap_cache is racy (page stealer can race with other code
+that invokes find_vma with mmap_sem held), but that is okay, since it
+is a hint. This can be fixed, if desired, by having find_vma grab the
+Code that add/delete elements from the vmlist chain are
+1. callers of insert_vm_struct
+2. callers of merge_segments
+3. callers of avl_remove
+Code that changes vm_start/vm_end/vm_flags:VM_LOCKED of vma's on
+It is advisable that changes to vm_start/vm_end be protected, although
+in some cases it is not really needed. Eg, vm_start is modified by
+expand_stack(), it is hard to come up with a destructive scenario without
+having the vmlist protection in this case.
+The page_table_lock nests with the inode i_mmap_lock and the kmem cache
+c_spinlock spinlocks. This is okay, since the kmem code asks for pages after
+dropping c_spinlock. The page_table_lock also nests with pagecache_lock and
+pagemap_lru_lock spinlocks, and no code asks for memory with these locks
+The page_table_lock is grabbed while holding the kernel_lock spinning monitor.
+The page_table_lock is a spin lock.
+Note: PTL can also be used to guarantee that no new clones using the
+mm start up ... this is a loose form of stability on mm_users. For
+example, it is used in copy_mm to protect against a racing tlb_gather_mmu
+single address space optimization, so that the zap_page_range (from
+vmtruncate) does not lose sending ipi's to cloned threads that might
+be spawned underneath it and go to user mode to drag in pte's into tlbs.
+The swap devices are chained in priority order from the "swap_list" header.
+The "swap_list" is used for the round-robin swaphandle allocation strategy.
+The #free swaphandles is maintained in "nr_swap_pages". These two together
+are protected by the swap_list_lock.
+The swap_device_lock, which is per swap device, protects the reference
+counts on the corresponding swaphandles, maintained in the "swap_map"
+array, and the "highest_bit" and "lowest_bit" fields.
+Both of these are spinlocks, and are never acquired from intr level. The
+locking hierarchy is swap_list_lock -> swap_device_lock.
+To prevent races between swap space deletion or async readahead swapins
+deciding whether a swap handle is being used, ie worthy of being read in
+from disk, and an unmap -> swap_free making the handle unused, the swap
+delete and readahead code grabs a temp reference on the swaphandle to
+prevent warning messages from swap_duplicate <- read_swap_cache_async.
+Swap cache locking
+Pages are added into the swap cache with kernel_lock held, to make sure
+that multiple pages are not being added (and hence lost) by associating
+all of them with the same swaphandle.
+Pages are guaranteed not to be removed from the scache if the page is
+"shared": ie, other processes hold reference on the page or the associated
+swap handle. The only code that does not follow this rule is shrink_mmap,
+which deletes pages from the swap cache if no process has a reference on
+the page (multiple processes might have references on the corresponding
+swap handle though). lookup_swap_cache() races with shrink_mmap, when
+establishing a reference on a scache page, so, it must check whether the
+page it located is still in the swapcache, or shrink_mmap deleted it.
+(This race is due to the fact that shrink_mmap looks at the page ref
+count with pagecache_lock, but then drops pagecache_lock before deleting
+the page from the scache).
+do_wp_page and do_swap_page have MP races in them while trying to figure
+out whether a page is "shared", by looking at the page_count + swap_count.
+To preserve the sum of the counts, the page lock _must_ be acquired before
+calling is_page_shared (else processes might switch their swap_count refs
+to the page count refs, after the page count ref has been snapshotted).
+Swap device deletion code currently breaks all the scache assumptions,
+since it grabs neither mmap_sem nor page_table_lock.