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+
+Problem:
+ When Linux needs to allocate memory it may find that there is
+ insufficient free memory so it needs to reclaim space that is in
+ use but not needed at the moment. There are several options:
+
+ 1/ Shrink a kernel cache such as the inode or dentry cache. This
+ is fairly easy but provides limited returns.
+ 2/ Discard 'clean' pages from the page cache. This is easy, and
+ works well as long as there are clean pages in the page cache.
+ Similarly clean 'anonymous' pages can be discarded - if there
+ are any.
+ 3/ Write out some dirty page-cache pages so that they become clean.
+ The VM limits the number of dirty page-cache pages to e.g. 40%
+ of available memory so that (among other reasons) a "sync" will
+ not take excessively long. So there should never be excessive
+ amounts of dirty pagecache.
+ Writing out dirty page-cache pages involves work by the
+ filesystem which may need to allocate memory itself. To avoid
+ deadlock, filesystems use GFP_NOFS when allocating memory on the
+ write-out path. When this is used, cleaning dirty page-cache
+ pages is not an option so if the filesystem finds that memory
+ is tight, another option must be found.
+ 4/ Write out dirty anonymous pages to the "Swap" partition/file.
+ This is the most interesting for a couple of reasons.
+ a/ Unlike dirty page-cache pages, there is no need to write anon
+ pages out unless we are actually short of memory. Thus they
+ tend to be left to last.
+ b/ Anon pages tend to be updated randomly and unpredictably, and
+ flushing them out of memory can have a very significant
+ performance impact on the process using them. This contrasts
+ with page-cache pages which are often written sequentially
+ and often treated as "write-once, read-many".
+ So anon pages tend to be left until last to be cleaned, and may
+ be the only cleanable pages while there are still some dirty
+ page-cache pages (which are waiting on a GFP_NOFS allocation).
+
+[I don't find the above wholly satisfying. There seems to be too much
+ hand-waving. If someone can provide better text explaining why
+ swapout is a special case, that would be great.]
+
+So we need to be able to write to the swap file/partition without
+needing to allocate any memory ... or only a small well controlled
+amount.
+
+The VM reserves a small amount of memory that can only be allocated
+for use as part of the swap-out procedure. It is only available to
+processes with the PF_MEMALLOC flag set, which is typically just the
+memory cleaner.
+
+Traditionally swap-out is performed directly to block devices (swap
+files on block-device filesystems are supported by examining the
+mapping from file offset to device offset in advance, and then using
+the device offsets to write directly to the device). Block devices
+are (required to be) written to pre-allocate any memory that might be
+needed during write-out, and to block when the pre-allocated memory is
+exhausted and no other memory is available. They can be sure not to
+block forever as the pre-allocated memory will be returned as soon as
+the data it is being used for has been written out. The primary
+mechanism for pre-allocating memory is called "mempools".
+
+This approach does not work for writing anonymous pages
+(i.e. swapping) over a network, using e.g NFS or NBD or iSCSI.
+
+
+The main reason that it does not work is that when data from an anon
+page is written to the network, we must wait for a reply to confirm
+the data is safe. Receiving that reply will consume memory and,
+significantly, we need to allocate memory to an incoming packet before
+we can tell if it is the reply we are waiting for or not.
+
+The secondary reason is that the network code is not written to use
+mempools and in most cases does not need to use them. Changing all
+allocations in the networking layer to use mempools would be quite
+intrusive, and would waste memory, and probably cause a slow-down in
+the common case of not swapping over the network.
+
+These problems are addressed by enhancing the system of memory
+reserves used by PF_MEMALLOC and requiring any in-kernel networking
+client that is used for swap-out to indicate which sockets are used
+for swapout so they can be handled specially in low memory situations.
+
+There are several major parts to this enhancement:
+
+1/ page->reserve, GFP_MEMALLOC
+
+ To handle low memory conditions we need to know when those
+ conditions exist. Having a global "low on memory" flag seems easy,
+ but its implementation is problematic. Instead we make it possible
+ to tell if a recent memory allocation required use of the emergency
+ memory pool.
+ For pages returned by alloc_page, the new page->reserve flag
+ can be tested. If this is set, then a low memory condition was
+ current when the page was allocated, so the memory should be used
+ carefully. (Because low memory conditions are transient, this
+ state is kept in an overloaded member instead of in page flags, which
+ would suggest a more permanent state.)
+
+ For memory allocated using slab/slub: If a page that is added to a
+ kmem_cache is found to have page->reserve set, then a s->reserve
+ flag is set for the whole kmem_cache. Further allocations will only
+ be returned from that page (or any other page in the cache) if they
+ are emergency allocation (i.e. PF_MEMALLOC or GFP_MEMALLOC is set).
+ Non-emergency allocations will block in alloc_page until a
+ non-reserve page is available. Once a non-reserve page has been
+ added to the cache, the s->reserve flag on the cache is removed.
+
+ Because slab objects have no individual state its hard to pass
+ reserve state along, the current code relies on a regular alloc
+ failing. There are various allocation wrappers help here.
+
+ This allows us to
+ a/ request use of the emergency pool when allocating memory
+ (GFP_MEMALLOC), and
+ b/ to find out if the emergency pool was used.
+
+2/ SK_MEMALLOC, sk_buff->emergency.
+
+ When memory from the reserve is used to store incoming network
+ packets, the memory must be freed (and the packet dropped) as soon
+ as we find out that the packet is not for a socket that is used for
+ swap-out.
+ To achieve this we have an ->emergency flag for skbs, and an
+ SK_MEMALLOC flag for sockets.
+ When memory is allocated for an skb, it is allocated with
+ GFP_MEMALLOC (if we are currently swapping over the network at
+ all). If a subsequent test shows that the emergency pool was used,
+ ->emergency is set.
+ When the skb is finally attached to its destination socket, the
+ SK_MEMALLOC flag on the socket is tested. If the skb has
+ ->emergency set, but the socket does not have SK_MEMALLOC set, then
+ the skb is immediately freed and the packet is dropped.
+ This ensures that reserve memory is never queued on a socket that is
+ not used for swapout.
+
+ Similarly, if an skb is ever queued for delivery to user-space for
+ example by netfilter, the ->emergency flag is tested and the skb is
+ released if ->emergency is set. (so obviously the storage route may
+ not pass through a userspace helper, otherwise the packets will never
+ arrive and we'll deadlock)
+
+ This ensures that memory from the emergency reserve can be used to
+ allow swapout to proceed, but will not get caught up in any other
+ network queue.
+
+
+3/ pages_emergency
+
+ The above would be sufficient if the total memory below the lowest
+ memory watermark (i.e the size of the emergency reserve) were known
+ to be enough to hold all transient allocations needed for writeout.
+ I'm a little blurry on how big the current emergency pool is, but it
+ isn't big and certainly hasn't been sized to allow network traffic
+ to consume any.
+
+ We could simply make the size of the reserve bigger. However in the
+ common case that we are not swapping over the network, that would be
+ a waste of memory.
+
+ So a new "watermark" is defined: pages_emergency. This is
+ effectively added to the current low water marks, so that pages from
+ this emergency pool can only be allocated if one of PF_MEMALLOC or
+ GFP_MEMALLOC are set.
+
+ pages_emergency can be changed dynamically based on need. When
+ swapout over the network is required, pages_emergency is increased
+ to cover the maximum expected load. When network swapout is
+ disabled, pages_emergency is decreased.
+
+ To determine how much to increase it by, we introduce reservation
+ groups....
+
+3a/ reservation groups
+
+ The memory used transiently for swapout can be in a number of
+ different places. e.g. the network route cache, the network
+ fragment cache, in transit between network card and socket, or (in
+ the case of NFS) in sunrpc data structures awaiting a reply.
+ We need to ensure each of these is limited in the amount of memory
+ they use, and that the maximum is included in the reserve.
+
+ The memory required by the network layer only needs to be reserved
+ once, even if there are multiple swapout paths using the network
+ (e.g. NFS and NDB and iSCSI, though using all three for swapout at
+ the same time would be unusual).
+
+ So we create a tree of reservation groups. The network might
+ register a collection of reservations, but not mark them as being in
+ use. NFS and sunrpc might similarly register a collection of
+ reservations, and attach it to the network reservations as it
+ depends on them.
+ When swapout over NFS is requested, the NFS/sunrpc reservations are
+ activated which implicitly activates the network reservations.
+
+ The total new reservation is added to pages_emergency.
+
+ Provided each memory usage stays beneath the registered limit (at
+ least when allocating memory from reserves), the system will never
+ run out of emergency memory, and swapout will not deadlock.
+
+ It is worth noting here that it is not critical that each usage
+ stays beneath the limit 100% of the time. Occasional excess is
+ acceptable provided that the memory will be freed again within a
+ short amount of time that does *not* require waiting for any event
+ that itself might require memory.
+ This is because, at all stages of transmit and receive, it is
+ acceptable to discard all transient memory associated with a
+ particular writeout and try again later. On transmit, the page can
+ be re-queued for later transmission. On receive, the packet can be
+ dropped assuming that the peer will resend after a timeout.
+
+ Thus allocations that are truly transient and will be freed without
+ blocking do not strictly need to be reserved for. Doing so might
+ still be a good idea to ensure forward progress doesn't take too
+ long.
+
+4/ low-mem accounting
+
+ Most places that might hold on to emergency memory (e.g. route
+ cache, fragment cache etc) already place a limit on the amount of
+ memory that they can use. This limit can simply be reserved using
+ the above mechanism and no more needs to be done.
+
+ However some memory usage might not be accounted with sufficient
+ firmness to allow an appropriate emergency reservation. The
+ in-flight skbs for incoming packets is one such example.
+
+ To support this, a low-overhead mechanism for accounting memory
+ usage against the reserves is provided. This mechanism uses the
+ same data structure that is used to store the emergency memory
+ reservations through the addition of a 'usage' field.
+
+ Before we attempt allocation from the memory reserves, we much check
+ if the resulting 'usage' is below the reservation. If so, we increase
+ the usage and attempt the allocation (which should succeed). If
+ the projected 'usage' exceeds the reservation we'll either fail the
+ allocation, or wait for 'usage' to decrease enough so that it would
+ succeed, depending on __GFP_WAIT.
+
+ When memory that was allocated for that purpose is freed, the
+ 'usage' field is checked again. If it is non-zero, then the size of
+ the freed memory is subtracted from the usage, making sure the usage
+ never becomes less than zero.
+
+ This provides adequate accounting with minimal overheads when not in
+ a low memory condition. When a low memory condition is encountered
+ it does add the cost of a spin lock necessary to serialise updates
+ to 'usage'.
+
+
+
+5/ swapon/swapoff/swap_out/swap_in
+
+ So that a filesystem (e.g. NFS) can know when to set SK_MEMALLOC on
+ any network socket that it uses, and can know when to account
+ reserve memory carefully, new address_space_operations are
+ available.
+ "swapon" requests that an address space (i.e a file) be make ready
+ for swapout. swap_out and swap_in request the actual IO. They
+ together must ensure that each swap_out request can succeed without
+ allocating more emergency memory that was reserved by swapon. swapoff
+ is used to reverse the state changes caused by swapon when we disable
+ the swap file.
+
+
+Thanks for reading this far. I hope it made sense :-)
+
+Neil Brown (with updates from Peter Zijlstra)
+
+