1. 14 Jul, 2016 1 commit
    • Kirill Smelkov's avatar
      bigfile/virtmem: usleep() needs unistd.h · d9d6409f
      Kirill Smelkov authored
      The following started to appear after recent gcc upgrade on my host:
      
      bigfile/virtmem.c: In function `vma_on_pagefault':
      bigfile/virtmem.c:696:9: warning: implicit declaration of function `usleep' [-Wimplicit-function-declaration]
               usleep(10000);  // XXX with 1000 uslepp still busywaits
      d9d6409f
  2. 15 Dec, 2015 3 commits
    • Kirill Smelkov's avatar
      bigfile/virtmem: Do loadblk() with virtmem lock released · f49c11a3
      Kirill Smelkov authored
      loadblk() calls are potentially slow and external code that serve the cal can
      take other locks in addition to virtmem lock taken by virtmem subsystem. If
      that "other locks" are also taken before external code calls e.g.
      fileh_invalidate_page() in different codepath a deadlock can happen, e.g.
      
            T1                  T2
      
            page-access         invalidation-from-server received
            V -> loadblk
                                Z   <- ClientStorage.invalidateTransaction()
            Z -> zeo.load
                                V   <- fileh_invalidate_page
      
      The solution to avoid deadlock is to call loadblk() with virtmem lock released
      and upon loadblk() completion recheck virtmem data structures carefully.
      
      To make that happen:
      
      - new page state is introduces:
      
          PAGE_LOADING                (file content loading is  in progress)
      
      - virtmem releases virt_lock before calling loadblk() when serving pagefault
      
      - because loading is now done with virtmem lock released, now:
      
      1. After loading completes we need to recheck fileh/vma data structures
      
         The recheck is done in full - vma_on_pagefault() just asks its driver (see
         VM_RETRY and VM_HANDLED codes) to retry handling the fault completely. This
         should work as the freshly loaded page was just inserted into fileh->pagemap
         and should be found there in the cache on next lookup.
      
         On the other hand this also works correctly, if there was concurrent change
         - e.g. vma was unmapped while we were loading the data - in that case the
         fault will be also processed correctly - but loaded data will stay in
         fileh->pagemap (and if not used will be evicted as not-needed
         eventually by RAM reclaim).
      
      2. Similar to retrying mechanism is used for cases when two threads
         concurrently access the same page and would both try to load corresponding
         block - only one thread issues the actual loadblk() and another waits for load
         to complete with polling and VM_RETRY.
      
      3. To correctly invalidate loading-in-progress pages another new page state
         is introduced:
      
          PAGE_LOADING_INVALIDATED    (file content loading was in progress
                                       while request to invalidate the page came in)
      
         which fileh_invalidate_page() uses to propagate invalidation message to
         loadblk() caller.
      
      4. Blocks loading can now happen in parallel with other block loading and
         other virtmem operations - e.g. invalidation. For such cases tests are added
         to test_thread.py
      
      5. virtmem lock now becomes just regular lock, instead of being previously
         recursive.
      
         For virtmem lock to be recursive was needed for cases, when code under
         loadblk() could trigger other virtmem calls, e.g. due to GC and calling
         another VMA dtor that would want to lock virtmem, but virtmem lock was
         already held.
      
         This is no longer needed.
      
      6. To catch double faults we now cannot use just on static variable
         in_on_pagefault. That variable thus becomes thread-local.
      
      7. Old test in test_thread to "test that access vs access don't overlap" no
         longer holds true - and is thus removed.
      
      /cc @Tyagov, @klaus
      f49c11a3
    • Kirill Smelkov's avatar
      bigfile/virtmem: Factor functionality to unlock/retake GIL into own functions · 0231a65d
      Kirill Smelkov authored
      Previously we were doing virt_lock() / virt_unlock() which automatically
      were making sure to unlock GIL before locking virtmem, and to restore
      GIL state to previous after virtmem lock happened. virt_unlock() was
      unlocking just the virtmem lock without touching GIL at all - that works
      because the running code would eventually release GIL as python
      regularly does so to allowing multiple threads to run.
      
      In the next patch however, we'll need to wait for in-progress-loading
      page to complete, and that wait has to be done with GIL released (so
      other python threads could run), and for doing so we'll need
      functionality to make sure GIL is unlocked and retake it back, not tied
      to virt_lock().
      
      So factor it out.
      0231a65d
    • Kirill Smelkov's avatar
      bigfile/virtmem: Remove obsolete XXX about locking · 81bf620c
      Kirill Smelkov authored
      Both comments are from the beginning - from 9a293c2d (bigfile/virtmem:
      Userspace Virtual Memory Manager) - but d53271b9 patch (bigfile/virtmem:
      Big Virtmem lock) missed to update them.
      81bf620c
  3. 17 Aug, 2015 1 commit
    • Kirill Smelkov's avatar
      bigfile/virtmem: Client API to invalidate a fileh page · cb779c7b
      Kirill Smelkov authored
      FileH is a handle representing snapshot of a file. If, for a pgoffset,
      fileh already has loaded page, but we know the content of the file has
      changed externally after loading has been done, we need to propagate to
      fileh that such-and-such page should be invalidated (and reloaded on
      next access).
      
      This patch introduces
      
          fileh_invalidate_page(fileh, pgoffset)
      
      to do just that.
      
      In the next patch we'll use this facility to propagate invalidations of
      ZBlk ZODB objects to virtmem subsystem.
      
      NOTE
      
      Since invalidation removes "dirtiness" from a page state, several
      subsequent invalidations can make a fileh completely non-dirty
      (invalidating all dirty page). Previously fileh->dirty was just a one
      bit, so we needed to improve how we track dirtiness.
      
      One way would be to have a dirty list for fileh pages and operate on
      that. This has advantage to even optimize dirty pages processing like
      fileh_dirty_writeout() where we currently scan through all fileh pages
      just to write only PAGE_DIRTY ones.
      
      Another simpler way is to make fileh->dirty a counter and maintain that.
      
      Since we are going to move virtmem subsystem back into the kernel, here,
      a simpler less-intrusive approach is used.
      cb779c7b
  4. 06 Aug, 2015 3 commits
    • Kirill Smelkov's avatar
      bigfile/virtmem: Big Virtmem lock · d53271b9
      Kirill Smelkov authored
      At present several threads running can corrupt internal virtmem
      datastructures (e.g. ram->lru_list, fileh->pagemap, etc).
      
      This can happen even if we have zope instances only with 1 worker thread
      - because there are other "system" thread, and python garbage collection
      can trigger at any thread, so if a virtmem object, e.g. VMA or FileH was
      there sitting at GC queue to be collected, their collection, and thus
      e.g. vma_unmap() and fileh_close() will be called from
      different-from-worker thread.
      
      Because of that virtmem just has to be aware of threads not to allow
      internal datastructure corruption.
      
      On the other hand, the idea of introducing userspace virtual memory
      manager turned out to be not so good from performance and complexity
      point of view, and thus the plan is to try to move it back into the
      kernel. This way it does not make sense to do a well-optimised locking
      implementation for userspace version.
      
      So we do just a simple single "protect-all" big lock for virtmem.
      
      Of a particular note is interaction with Python's GIL - any long-lived
      lock has to be taken with GIL released, because else it can deadlock:
      
          t1  t2
      
          G
          V   G
         !G   V
          G
      
      so we introduce helpers to make sure the GIL is not taken, and to retake
      it back if we were holding it initially.
      
      Those helpers (py_gil_ensure_unlocked / py_gil_retake_if_waslocked) are
      symmetrical opposites to what Python provides to make sure the GIL is
      locked (via PyGILState_Ensure / PyGILState_Release).
      
      Otherwise, the patch is more-or-less straightforward application for
      one-big-lock to protect everything idea.
      d53271b9
    • Kirill Smelkov's avatar
      bigfile/virtmem: When restoring SIGSEGV, don't change procmask for other signals · d7c33cd7
      Kirill Smelkov authored
      We factored out SIGSEGV block/restore from fileh_dirty_writeout() to all
      functions in cb7a7055 (bigfile/virtmem: Block/restore SIGSEGV in
      non-pagefault-handling function). The restoration however just sets
      whole thread sigmask.
      
      It could be possible that between block/restore calls procmask for other
      signals could be changed, and this way - setting procmask directly - we
      will overwrite them.
      
      So be careful, and when restoring SIGSEGV mask, touch mask bit for only
      that signal.
      
      ( we need xsigismember helper to get this done, which is also introduced
        in this patch )
      d7c33cd7
    • Kirill Smelkov's avatar
      bigfile/virtmem: Block/restore SIGSEGV in non-pagefault-handling function · cb7a7055
      Kirill Smelkov authored
      Non on-pagefault code should not access any not-mmapped memory.
      
      Here we just refactor the code we already had to block/restore
      SIGSEGV from fileh_dirty_writeout() and use it in all functions called
      from non-pagefaulting context, as promised.
      
      This way, if there is an error in virtmem implementation which
      incorrectly accesses prepared for BigFile maps memory, we'll just die
      with coredump instead of trying to incorrectly handle the pagefault.
      cb7a7055
  5. 03 Apr, 2015 2 commits
    • Kirill Smelkov's avatar
      bigfile/virtmem: Userspace Virtual Memory Manager · 9a293c2d
      Kirill Smelkov authored
      Does similar things to what kernel does - users can mmap file parts into
      address space and access them read/write. The manager will be getting
      invoked by hardware/OS kernel for cases when there is no page loaded for
      read, or when a previousle read-only page is being written to.
      
      Additionally to features provided in kernel, it support to be used to
      store back changes in transactional way (see fileh_dirty_writeout()) and
      potentially use huge pages for mappings (though this is currently TODO)
      9a293c2d
    • Kirill Smelkov's avatar
      bigfile: Stub for virtmem · 77d61533
      Kirill Smelkov authored
      This will be the core of virtual memory subsystem. For now we just
      define a structure to describe pages of memory and add utility to
      allocate address space from OS.
      77d61533