+++ /dev/null
-From: Neil Brown <neilb@suse.de>
-Subject: swap over network documentation
-Patch-mainline: No
-References: FATE#303834
-
-Document describing the problem and proposed solution
-
-Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
-Acked-by: Neil Brown <neilb@suse.de>
-Acked-by: Suresh Jayaraman <sjayaraman@suse.de>
-
----
- Documentation/network-swap.txt | 270 +++++++++++++++++++++++++++++++++++++++++
- 1 file changed, 270 insertions(+)
-
-Index: linux-2.6.26/Documentation/network-swap.txt
-===================================================================
---- /dev/null
-+++ linux-2.6.26/Documentation/network-swap.txt
-@@ -0,0 +1,270 @@
-+
-+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)
-+
-+
projects / people / pmueller / ipfire-2.x.git / blobdiff