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    Subject[PATCH 01/30] swap over network documentation
    From: Neil Brown <neilb@suse.de>

    Document describing the problem and proposed solution

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    Documentation/network-swap.txt | 270 +++++++++++++++++++++++++++++++++++++++++
    1 file changed, 270 insertions(+)

    Index: linux-2.6/Documentation/network-swap.txt
    ===================================================================
    --- /dev/null
    +++ linux-2.6/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 on 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)
    +
    +
    --



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