• Johannes Weiner's avatar
    mm: fix false-positive OVERCOMMIT_GUESS failures · 8c7829b0
    Johannes Weiner authored
    With the default overcommit==guess we occasionally run into mmap
    rejections despite plenty of memory that would get dropped under
    pressure but just isn't accounted reclaimable. One example of this is
    dying cgroups pinned by some page cache. A previous case was auxiliary
    path name memory associated with dentries; we have since annotated
    those allocations to avoid overcommit failures (see d79f7aa4 ("mm:
    treat indirectly reclaimable memory as free in overcommit logic")).
    
    But trying to classify all allocated memory reliably as reclaimable
    and unreclaimable is a bit of a fool's errand. There could be a myriad
    of dependencies that constantly change with kernel versions.
    
    It becomes even more questionable of an effort when considering how
    this estimate of available memory is used: it's not compared to the
    system-wide allocated virtual memory in any way. It's not even
    compared to the allocating process's address space. It's compared to
    the single allocation request at hand!
    
    So we have an elaborate left-hand side of the equation that tries to
    assess the exact breathing room the system has available down to a
    page - and then compare it to an isolated allocation request with no
    additional context. We could fail an allocation of N bytes, but for
    two allocations of N/2 bytes we'd do this elaborate dance twice in a
    row and then still let N bytes of virtual memory through. This doesn't
    make a whole lot of sense.
    
    Let's take a step back and look at the actual goal of the
    heuristic. From the documentation:
    
       Heuristic overcommit handling. Obvious overcommits of address
       space are refused. Used for a typical system. It ensures a
       seriously wild allocation fails while allowing overcommit to
       reduce swap usage.  root is allowed to allocate slightly more
       memory in this mode. This is the default.
    
    If all we want to do is catch clearly bogus allocation requests
    irrespective of the general virtual memory situation, the physical
    memory counter-part doesn't need to be that complicated, either.
    
    When in GUESS mode, catch wild allocations by comparing their request
    size to total amount of ram and swap in the system.
    
    Link: http://lkml.kernel.org/r/20190412191418.26333-1-hannes@cmpxchg.org
    
    Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Acked-by: default avatarRoman Gushchin <guro@fb.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8c7829b0
util.c 18.9 KB