Subject: Re: [PATCH 00/28] Swap over NFS -v16

From: Peter Zijlstra <[email-masked]>
Date: Fri, 07 Mar 2008 12:17:55 +0100

Hi Neil,

I'm so glad you are working with me on this and writing this in human
English. It seems to be my eternal short-comming to communicate my ideas
clearly :-/. Thanks for your effort!

On Fri, 2008-03-07 at 14:33 +1100, Neil Brown wrote:
> On Tuesday March 4, [email-masked] wrote:
> > 
> > On Tue, 2008-03-04 at 10:41 +1100, Neil Brown wrote:
> > > 
> > > Those skbs we allocated - they are either sitting in the fragment
> > > cache, or have been attached to a SK_MEMALLOC socket, or have been
> > > freed - correct?  If so, then there is already a limit to how much
> > > memory they can consume.
> > 
> > Not really, there is no natural limit to the amount of packets that can
> > be in transit between RX and socket demux. So we need the extra (skb)
> > accounting to impose that.
> 
> Isn't there?  A brief look at the code suggests that (except for
> fragment handling) there is a fairly straight path from
> network-receive to socket demux.  No queues along the way.
> That suggests the number of in-transit skbs should be limited by the
> number of CPUs.  Did I miss something?  Or is the number of CPUs
> potentially too large to be a suitable limit (seems unlikely).

That would be so if the whole path from RX to socket demux would have
hard-irqs disabled. However I didn't see that. Moreover I think the
whole purpose of the NetPoll interface is to allow some RX queueing to
cut down on softirq overhead.

> While looking at the code it also occurred to me that:
>   1/ tcpdump could be sent incoming packets.  Is there a limit
>      to the number of packets that can be in-kernel waiting for
>      tcpdump to collect them?  Should this limit be added to the base
>      reserve?

We could indeed do something like that, building a cache there that
re-cycles the oldest waiting skbs, allowing taps to continue working
under light pressure seems like a reasonable thing.

Good suggestion for future work.

>   2/ If the host is routing network packets, then incoming packets
>      might go on an outbound queue.  Is this space limited?  and
>      included in the reserve?

Not sure, somewhere along the routing code I lost it again. Constructive
input from someone versed in that part of the kernel would be most
welcome.

> Not major points, but I thought I would mention them.
> 
> > > I also don't see the value of tracking pages to see if they are
> > > 'reserve' pages or not.  The decision to drop an skb that is not for
> > > an SK_MEMALLOC socket should be based on whether we are currently
> > > short on memory.  Not whether we were short on memory when the skb was
> > > allocated.
> > 
> > That comes from accounting, once you need to account data you need to
> > know when to start accounting, and keep state so that you can properly
> > un-account.
> > 
> 
> skbs in the main (only?) thing you do accounting on, so focusing on
> those:
> 
> Suppose that every time you allocate memory for an skb, you check
> if the allocation had to dip into emergency reserves, and account the
> memory if so - releasing the memory and dropping the packet if we are
> over the limit.
> And any time you free memory associated with an skb, you check if the
> accounts currently say '0', and if not subtract the size of the
> allocation from the accounts.
> 
> Then you have quite workable accounting that doesn't need to tag every
> piece of memory with its 'reserve' status, and only pays the
> accounting cost (presumably a spinlock) when running out of memory, or
> just recovering.

Quite so, that has been the intent.

> This more relaxed approach to accounting reserved vs non-reserved
> memory has a strong parallel in your slub code (which I now
> understand).  When sl[au]b first gets a ->reserve page, it sets the
> ->reserve flag on the memcache and leaves it set until it sometime
> later gets a non-"->reserve" page.  Any memory freed in the mean time
> (whether originally reserved or not) is treated as reserve memory in
> that it will only be returned for ALLOC_NO_WATERMARKS allocations.
> I think this is a good way of working with reserved memory.  It isn't
> precise, but it is low-cost and good enough to get you through the
> difficult patch.

Agreed.

> Your netvm-skbuff-reserve.patch has some code to make sure that all
> the allocations in an skb have the same 'reserve' status.   I don't
> think that is needed and just makes the code messy - plus it requires
> the 'overcommit' flag to mem_reserve_kmalloc_charge which is a bit of
> a wart on the interface.

It is indeed.

> I would suggest getting rid of that.  Just flag the whole skb if any
> part gets a 'reserve' allocation, and use that flag to decide to drop
> packets arriving at non-SK_MEMALLOC sockets.

OK, I'll look into doing that. I must have been in pedantic mode when I
wrote that code.

> So: I think I now really understand what your code is doing, so I will
> try to explain it in terms that even I understand... This text in
> explicitly available under GPLv2 in case you want it.

Great work! Thanks!

> It actually describes something a bit different to what your code
> currently does, but I think it is very close to the spirit.  Some
> differences follow from my observations above.  Others the way that
> seemed to make sense while describing the problem and solution
> differed slightly from what I saw the code doing.  Obviously the code
> and the description should be aligned one way or another before being
> finalised.  
> The description is a bit long ... sorry about that.  But I wanted to
> make sure I included motivation and various assumptions.  Some of my
> understanding may well be wrong, but I present it here anyway.  It is
> easier for you to correct if it is clearly visible:-)
> 
> 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.]

Anonymous pages are dirty by definition (except the zero page, but I
think we recently ditched it). So shrinking of the anonymous pool will
require swapping.

It is indeed the last refuge for those with GFP_NOFS. Allong with the
strict limit on the amount of dirty file pages it also ensures writing
those out will never deadlock the machine as there are always clean file
pages and or anonymous pages to launder.

Your observation about the difference in swap vs file disk patterns is
the motivation (one of the, at least) for Rik van Riel's split VM
series, it is currently not of any consequence.

Swap is indeed special in that it requires 'atomic' writeout. This
requirement comes from the cyclic dependency you outlined and we have to
break: we're out of memory, but need memory to write out pages to free
memory. So we must make it appear as if swap writes are indeed atomic.

> 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/ PG_emergency, 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 flag PG_emergency
>   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.
> 
>   For memory allocated using slab/slub: If a page that is added to a
>   kmem_cache is found to have PG_emergency set, then a  ->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 ->reserve flag on the cache is removed.
>   When memory is returned by slab/slub, PG_emergency is set on the page
>   holding the memory to match the ->reserve flag on that cache.
> 
>   After memory has been returned by kmem_cache_alloc or kmalloc, the
>   page's PG_emergency flag can be tested.  If it is set, then the most
>   recent allocation from that cache required reserve memory, so this
>   allocation should be used with care.
> 
>   It is not safe to test the cache's ->reserve flag immediately after
>   an allocation as that flag is in per-cpu data, and the process could
>   have be rescheduled to a different cpu if preemption is enabled.
>   Thus the use of PG_emergency to carry this information.
> 
>   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.

Right. I've had a long conversation on PG_emergency with Pekka. And I
think the conclusion was that PG_emergency will create more head-aches
than it solves. I probably have the conversation in my IRC logs and
could email it if you're interested (and Pekka doesn't object).

> 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 deliver to user-space for
>   example by netfilter, the ->emergency flag is tested and the skb is
>   released if ->emergency is set.
> 
>   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/ lo-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 (claimed to be) 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.
> 
>   When memory allocation for a particular purpose succeeds, the memory
>   is checked to see if it is 'reserve' memory.  If it is, the size of
>   the allocation is added to the 'usage'.  If this exceeds the
>   reservation, the usage is reduced again and the memory that was
>   allocated is free.
> 
>   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'.

Agreed, minimizing the overhead for the common !net_swap case has been
my goal.

> 5/ swapfile/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.
>   "swapfile" 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 swapfile.

Miklos kindly provided code to slightly out-date this piece, but not in
a conceptual manner.

I've already heard interest from other people to use these hooks to
provide swap on other non-block filesystems such as jffs2, logfs and the
like.

> Thanks for reading this far.  I hope it made sense :-)

It does, and I hope it does so for more people.