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-Started Oct 1999 by Kanoj Sarcar <kanojsarcar@yahoo.com>
-
-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
-page_table_lock.
-
-
-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
-the list:
-1. expand_stack
-2. mprotect
-3. mlock
-4. mremap
-
-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_mutex 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
-held.
-
-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
-truncate) 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.
-
-swap_lock
---------------
-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_lock.
-
-The swap_lock also protects all the device reference counts on the
-corresponding swaphandles, maintained in the "swap_map" array, and the
-"highest_bit" and "lowest_bit" fields.
-
-The swap_lock is a spinlock, and is never acquired from intr level.
-
-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.

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