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