|Started Oct 1999 by Kanoj Sarcar <email@example.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
|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_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
|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.
|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.