cvs commit: src/sys/vm swap_pager.c

[David Schultz]

das         2004-11-05 05:36:56 UTC

  FreeBSD src repository

  Modified files:
    sys/vm               swap_pager.c 
  Log:
  Close a race in swapoff().  Here are the gory details:
  
    In order to avoid livelock, swapoff() skips over objects with a
    nonzero pip count and makes another pass if necessary.  Since it is
    impossible to know which objects we care about, it would choose an
    arbitrary object with a nonzero pip count and wait for it before
    making another pass, the theory being that this object would finish
    paging about as quickly as the ones we care about.  Unfortunately,
    we may have slept since we acquired a reference to this object.
    Hack around this problem by tsleep()ing on the pointer anyway, but
    timeout after a fixed interval.  More elegant solutions are possible,
    but the ones I considered unnecessarily complicate this rare case.
  
  Also, kill some nits that seem to have crept into the swapoff() code
  in the last 75 revisions or so:
  
  - Don't pass both sp and sp->sw_used to swap_pager_swapoff(), since
    the latter can be derived from the former.
  
  - Replace swp_pager_find_dev() with something simpler.  There's no
    need to iterate over the entire list of swap devices just to determine
    if a given block is assigned to the one we're interested in.
  
  - Expand the scope of the swhash_mtx in a couple of places so that it
    isn't released and reacquired once for every hash bucket.
  
  - Don't drop the swhash_mtx while holding a reference to an object.
    We need to lock the object first.  Unfortunately, doing so would
    violate the established lock order, so use VM_OBJECT_TRYLOCK() and
    try again on a subsequent pass if the object is already locked.
  
  - Refactor swp_pager_force_pagein() and swap_pager_swapoff() a bit.
  
  Revision  Changes    Path
  1.263     +53 -70    src/sys/vm/swap_pager.c