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From: Balbir Singh on 23 Feb 2010 06:30
* David Rientjes <rientjes(a)google.com> [2010-02-16 16:21:11]:

> 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;

I am not sure if this is a good idea, ZONE_DMA could have a lot of
memory on some architectures. IIUC, we return NULL for allocations
from ZONE_DMA? What is the reason for the heuristic?

> /*
> * 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;
> }

--
Three Cheers,
Balbir
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Prev: X25: Fix x25_create errors for bad protocol and ENOBUFS
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