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    SubjectRe: [patch -mm 8/9 v2] oom: avoid oom killer for lowmem allocations
    On Wed, 17 Feb 2010, KAMEZAWA Hiroyuki wrote:

    > > On Wed, 17 Feb 2010, KAMEZAWA Hiroyuki wrote:
    > >
    > > > > > > I'll add this check to __alloc_pages_may_oom() for the !(gfp_mask &
    > > > > > > __GFP_NOFAIL) path since we're all content with endlessly looping.
    > > > > >
    > > > > > Thanks. Yes endlessly looping is far preferable to randomly oopsing
    > > > > > or corrupting memory.
    > > > > >
    > > > >
    > > > > Here's the new patch for your consideration.
    > > > >
    > > >
    > > > Then, can we take kdump in this endlessly looping situaton ?
    > > >
    > > > panic_on_oom=always + kdump can do that.
    > > >
    > >
    > > The endless loop is only helpful if something is going to free memory
    > > external to the current page allocation: either another task with
    > > __GFP_WAIT | __GFP_FS that invokes the oom killer, a task that frees
    > > memory, or a task that exits.
    > >
    > > The most notable endless loop in the page allocator is the one when a task
    > > has been oom killed, gets access to memory reserves, and then cannot find
    > > a page for a __GFP_NOFAIL allocation:
    > >
    > > do {
    > > page = get_page_from_freelist(gfp_mask, nodemask, order,
    > > zonelist, high_zoneidx, ALLOC_NO_WATERMARKS,
    > > preferred_zone, migratetype);
    > >
    > > if (!page && gfp_mask & __GFP_NOFAIL)
    > > congestion_wait(BLK_RW_ASYNC, HZ/50);
    > > } while (!page && (gfp_mask & __GFP_NOFAIL));
    > >
    > > We don't expect any such allocations to happen during the exit path, but
    > > we could probably find some in the fs layer.
    > >
    > > I don't want to check sysctl_panic_on_oom in the page allocator because it
    > > would start panicking the machine unnecessarily for the integrity
    > > metadata GFP_NOIO | __GFP_NOFAIL allocation, for any
    > > order > PAGE_ALLOC_COSTLY_ORDER, or for users who can't lock the zonelist
    > > for oom kill that wouldn't have panicked before.
    > >
    >
    > Then, why don't you check higzone_idx in oom_kill.c
    >

    out_of_memory() doesn't return a value to specify whether the page
    allocator should retry the allocation or just return NULL, all that policy
    is kept in mm/page_alloc.c. For highzone_idx < ZONE_NORMAL, we want to
    fail the allocation when !(gfp_mask & __GFP_NOFAIL) and call the oom
    killer when it's __GFP_NOFAIL.
    ---
    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -1696,6 +1696,9 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
    /* The OOM killer will not help higher order allocs */
    if (order > PAGE_ALLOC_COSTLY_ORDER)
    goto out;
    + /* The OOM killer does not needlessly kill tasks for lowmem */
    + if (high_zoneidx < ZONE_NORMAL)
    + goto out;
    /*
    * GFP_THISNODE contains __GFP_NORETRY and we never hit this.
    * Sanity check for bare calls of __GFP_THISNODE, not real OOM.
    @@ -1924,15 +1927,23 @@ rebalance:
    if (page)
    goto got_pg;

    - /*
    - * The OOM killer does not trigger for high-order
    - * ~__GFP_NOFAIL allocations so if no progress is being
    - * made, there are no other options and retrying is
    - * unlikely to help.
    - */
    - if (order > PAGE_ALLOC_COSTLY_ORDER &&
    - !(gfp_mask & __GFP_NOFAIL))
    - goto nopage;
    + if (!(gfp_mask & __GFP_NOFAIL)) {
    + /*
    + * The oom killer is not called for high-order
    + * allocations that may fail, so if no progress
    + * is being made, there are no other options and
    + * retrying is unlikely to help.
    + */
    + if (order > PAGE_ALLOC_COSTLY_ORDER)
    + goto nopage;
    + /*
    + * The oom killer is not called for lowmem
    + * allocations to prevent needlessly killing
    + * innocent tasks.
    + */
    + if (high_zoneidx < ZONE_NORMAL)
    + goto nopage;
    + }

    goto restart;
    }

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