• Hugh Dickins's avatar
    swap_info: swap count continuations · 570a335b
    Hugh Dickins authored
    Swap is duplicated (reference count incremented by one) whenever the same
    swap page is inserted into another mm (when forking finds a swap entry in
    place of a pte, or when reclaim unmaps a pte to insert the swap entry).
    
    swap_info_struct's vmalloc'ed swap_map is the array of these reference
    counts: but what happens when the unsigned short (or unsigned char since
    the preceding patch) is full? (and its high bit is kept for a cache flag)
    
    We then lose track of it, never freeing, leaving it in use until swapoff:
    at which point we _hope_ that a single pass will have found all instances,
    assume there are no more, and will lose user data if we're wrong.
    
    Swapping of KSM pages has not yet been enabled; but it is implemented,
    and makes it very easy for a user to overflow the maximum swap count:
    possible with ordinary process pages, but unlikely, even when pid_max
    has been raised from PID_MAX_DEFAULT.
    
    This patch implements swap count continuations: when the count overflows,
    a continuation page is allocated and linked to the original vmalloc'ed
    map page, and this used to hold the continuation counts for that entry
    and its neighbours.  These continuation pages are seldom referenced:
    the common paths all work on the original swap_map, only referring to
    a continuation page when the low "digit" of a count is incremented or
    decremented through SWAP_MAP_MAX.
    Signed-off-by: default avatarHugh Dickins <hugh.dickins@tiscali.co.uk>
    Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    570a335b
rmap.c 34.8 KB