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Started Oct 1999 by Kanoj Sarcar 

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_shared_lock and the kmem cache

c_spinlock spinlocks. This is okay, since code that holds i_shared_lock

never asks for memory, and 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.

swap_list_lock/swap_device_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_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.