• Johannes Weiner's avatar
    mm/vmalloc: remove guard page from between vmap blocks · 248ac0e1
    Johannes Weiner authored
    
    
    The vmap allocator is used to, among other things, allocate per-cpu vmap
    blocks, where each vmap block is naturally aligned to its own size.
    Obviously, leaving a guard page after each vmap area forbids packing vmap
    blocks efficiently and can make the kernel run out of possible vmap blocks
    long before overall vmap space is exhausted.
    
    The new interface to map a user-supplied page array into linear vmalloc
    space (vm_map_ram) insists on allocating from a vmap block (instead of
    falling back to a custom area) when the area size is below a certain
    threshold.  With heavy users of this interface (e.g.  XFS) and limited
    vmalloc space on 32-bit, vmap block exhaustion is a real problem.
    
    Remove the guard page from the core vmap allocator.  vmalloc and the old
    vmap interface enforce a guard page on their own at a higher level.
    
    Note that without this patch, we had accidental guard pages after those
    vm_map_ram areas that happened to be at the end of a vmap block, but not
    between every area.  This patch removes this accidental guard page only.
    
    If we want guard pages after every vm_map_ram area, this should be done
    separately.  And just like with vmalloc and the old interface on a
    different level, not in the core allocator.
    
    Mel pointed out: "If necessary, the guard page could be reintroduced as a
    debugging-only option (CONFIG_DEBUG_PAGEALLOC?).  Otherwise it seems
    reasonable."
    Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Cc: Nick Piggin <npiggin@kernel.dk>
    Cc: Dave Chinner <david@fromorbit.com>
    Acked-by: default avatarMel Gorman <mel@csn.ul.ie>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    248ac0e1
vmalloc.c 64.2 KB