pagecache: use boost::variant to store rmap of a cached page

rmap will usually contain only one element so using unordered_set for it
is a little bit heavy. boost::variant allow us to use direct pointer in
common case and fall back to unordered_set only when more than one
elements are added.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: assert if sys_read() fails

sys_read() error should be propagated to an application, but for now
assert here instead of silent memory corruption.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
mmu: propagate IO errors during msync() to an application

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: add traces to pagecache function

Useful for debugging cache related problems.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
mmu: separate page unmapping and freeing.

Unmap page as soon as possible instead of waiting for max_pages to
accumulate. Will allow to free pages outside of vma_list_mutex in the
feature.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: move cache management code into pagecache.cc

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: remove redundant locking

All pagecache functions run under vma_list_lock, so no additional
locking is needed.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: avoid unnecessary tlb flushes in unmap_address

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: track ARC buffers in the pagecache

Current code assumes that for the same file and same offset ZFS will
always return same ARC buffer, but this appears to be not the case.
ZFS may create new ARC buffer while an old one is undergoing writeback.
It means that we need to track mapping between file/offset and mmapped
ARC buffer by ourselves. It's exactly what this patch is about. It adds
new kind of cached page that holds pointers to an ARC buffer and stores
them in new read_cache map.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
mmu: populate pte in page_allocator

Currently page_allocator return a page to a page mapper and the later
populates a pte with it. Sometimes page allocation and pte population
needs to be appear atomic though. For instance in case of a pagecache
we want to prevent page eviction before pte is populated since page
eviction clears pte, but if allocation and mapping is not atomic pte
can be populated with stale data after eviction. With current approach
very wide scoped lock is needed to guaranty atomicity. Moving pte
population into page_allocator allows for much simpler locking.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
pagecache: change locking between mmu and ARC

Now vma_list_mutex is used to protect against races between ARC buffer
mapping by MMU and eviction by ZFS. The problem is that MMU code calls
into ZFS with vma_list_mutex held, so on that path all ZFS related locks
are taken after vma_list_mutex. An attempt to acquire vma_list_mutex
during ARC buffer eviction, while many of the same ZFS locks are already
held, causes deadlock. It was solved by using trylock() and skipping an
eviction if vma_list_mutex cannot be acquired, but it appears that some
mmapped buffers are destroyed not during eviction, but after writeback and
this destruction cannot be delayed. It calls for locking scheme redesign.

This patch introduce arc_lock that have to be held during access to
read_cache. It prevents simultaneous eviction and mapping. arc_lock should
be the most inner lock held on any code path. Code is change to adhere to
this rule. For that the patch replaces ARC_SHARED_BUF flag by new b_mmaped
field. The reason is that access to b_flags field is guarded by hash_lock
and it is impossible to guaranty same order between hash_lock and arc_lock
on all code paths. Dropping the need for hash_lock is a nice solution.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
mmu: enable pagecache again

The bug that caused it to be disabled should be fixed now.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
mmu: drop finalize() from page_allocator

No longer used.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>