Under OpenSolaris all memory reclaim is done asyncronously. Under
Linux memory reclaim is done asynchronously _and_ synchronously.
When a process allocates memory with GFP_KERNEL it explicitly allows
the kernel to do reclaim on its behalf to satify the allocation.
If that GFP_KERNEL allocation fails the kernel may take more drastic
measures to reclaim the memory such as killing user space processes.
This was observed to happen with ZFS because the ARC could consume
a large fraction of the system memory but no synchronous reclaim
could be performed on it. The result was GFP_KERNEL allocations
could fail resulting in OOM events, and only moments latter the
arc_reclaim thread would free unused memory from the ARC.
This change leaves the arc_thread in place to manage the fundamental
ARC behavior. But it adds a synchronous (direct) reclaim path for
the ARC which can be called when memory is badly needed. It also
adds an asynchronous (indirect) reclaim path which is called
much more frequently to prune the ARC slab caches.
The following useful values were missing the arcstats. This change
adds them in to provide greater visibility in to the arcs behavior.
arc_no_grow 4 0
arc_tempreserve 4 0
arc_loaned_bytes 4 0
arc_meta_used 4 624774592
arc_meta_limit 4 400785408
arc_meta_max 4 625594176
It used to be the case that all KM_SLEEP allocations were GFS_NOFS.
Unfortunately this often resulted in the kernel being unable to
reclaim the ARC, inode, and dentry caches in a timely manor.
The fix was to make KM_SLEEP a GFP_KERNEL allocation in the SPL.
However, this increases the posibility of deadlocking the system
on a zfs write thread. If a zfs write thread attempts to perform
an allocation it may trigger synchronous reclaim. This reclaim
may attempt to flush dirty data/inode to disk to free memory.
Unforunately, this write cannot finish because the write thread
which would handle it is holding the previous transaction open.
Deadlock.
To avoid this all allocations in the zfs write thread path must
use KM_PUSHPAGE which prohibits synchronous reclaim for that
thread. In this way forward progress in ensured. The risk
with this change is I missed updating an allocation for the
write threads leaving an increased posibility of deadlock. If
any deadlocks remain they will be unlikely but we'll have to
make sure they all get fixed.
The issue is that cv_timedwait() sleeps uninterruptibly to block signals
and avoid waking up early. Under Linux this counts against the load
average keeping it artificially high. This change allows the arc to
sleep interruptibly which mean it may be woken up early due to a signal.
Normally this means some extra care must be taken to handle a potential
signal. But for the arcs usage of cv_timedwait() there is no harm in
waking up before the timeout expires so no extra handling is required.
This commit fixes a sign extension bug affecting l2arc devices. Extremely
large offsets may be passed down to the low level block device driver on
reads, generating errors similar to
attempt to access beyond end of device
sdbi1: rw=14, want=36028797014862705, limit=125026959
The unwanted sign extension occurrs because the function arc_read_nolock()
stores the offset as a daddr_t, a 32-bit signed int type in the Linux kernel.
This offset is then passed to zio_read_phys() as a uint64_t argument, causing
sign extension for values of 0x80000000 or greater. To avoid this, we store
the offset in a uint64_t.
This change also changes a few daddr_t struct members to uint64_t in the libspl
headers to avoid similar bugs cropping up in the future. We also add an ASSERT
to __vdev_disk_physio() to check for invalid offsets.
Closes#66
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Remove all instances of list handling where the API is not used
and instead list data members are directly accessed. Doing this
sort of thing is bad for portability.
Additionally, ensure that list_link_init() is called on newly
created list nodes. This ensures the node is properly initialized
and does not rely on the assumption that zero'ing the list_node_t
via kmem_zalloc() is the same as proper initialization.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fix non-c90 compliant code, for the most part these changes
simply deal with where a particular variable is declared.
Under c90 it must alway be done at the very start of a block.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
