Matthew Dobson wrote:
While looking at some bugs related to OOM handling in 2.6, Martin Bligh
and I noticed some order 0 page allocation failures from kswapd:
kswapd0: page allocation failure. order:0, mode:0x50
[<c0147b92>] __alloc_pages+0x288/0x295
[<c0147bb7>] __get_free_pages+0x18/0x24
[<c014b2c4>] kmem_getpages+0x15/0x94
[<c014c047>] cache_grow+0x154/0x299
[<c014c399>] cache_alloc_refill+0x20d/0x23d
[<c014c622>] kmem_cache_alloc+0x46/0x4c
[<f885a4b0>] journal_alloc_journal_head+0x10/0x5d [jbd]
[<f885a523>] journal_add_journal_head+0x1a/0xe1 [jbd]
[<f8850be3>] journal_dirty_data+0x2e/0x3a5 [jbd]
[<f8883400>] ext3_journal_dirty_data+0xc/0x2a [ext3]
[<f888329a>] walk_page_buffers+0x62/0x87 [ext3]
[<f888382d>] ext3_ordered_writepage+0xea/0x136 [ext3]
[<f8883731>] journal_dirty_data_fn+0x0/0x12 [ext3]
[<c014ec31>] pageout+0x83/0xc0
[<c014ee80>] shrink_list+0x212/0x55f
[<c014de86>] __pagevec_release+0x15/0x1d
[<c014f400>] shrink_cache+0x233/0x4d5
[<c014fee2>] shrink_zone+0x91/0x9c
[<c01501e9>] balance_pgdat+0x15f/0x208
[<c0150350>] kswapd+0xbe/0xc0
[<c011e1ef>] autoremove_wake_function+0x0/0x2d
[<c0308886>] ret_from_fork+0x6/0x20
[<c011e1ef>] autoremove_wake_function+0x0/0x2d
[<c0150292>] kswapd+0x0/0xc0
[<c01041d5>] kernel_thread_helper+0x5/0xb
We decided that seemed odd, as kswapd should be able to get a page as
long as there is even one page left in the system, since being a memory
allocator task (PF_MEMALLOC) should exempt kswapd from any page
watermark restrictions. Digging into the code I found what looked like
a bug that could potentially cause this situation to be far more common.
This chunk of code from __alloc_pages() demonstrates the problem:
/* This allocation should allow future memory freeing. */
if (((p->flags & PF_MEMALLOC) ||
unlikely(test_thread_flag(TIF_MEMDIE))) && !in_interrupt()) {
/* go through the zonelist yet again, ignoring mins */
for (i = 0; (z = zones[i]) != NULL; i++) {
if (!cpuset_zone_allowed(z))
continue;
page = buffered_rmqueue(z, order, gfp_mask);
if (page)
goto got_pg;
}
goto nopage;
}
/* Atomic allocations - we can't balance anything */
if (!wait)
goto nopage;
rebalance:
cond_resched();
/* We now go into synchronous reclaim */
p->flags |= PF_MEMALLOC;
reclaim_state.reclaimed_slab = 0;
p->reclaim_state = &reclaim_state;
did_some_progress = try_to_free_pages(zones, gfp_mask, order);
p->reclaim_state = NULL;
p->flags &= ~PF_MEMALLOC;
If, while the system is under memory pressure, something attempts to
allocate a page from interrupt context while current == kswapd we will
obviously fail the !in_interrupt() check and fall through. If this
allocation request was made with __GFP_WAIT set then we'll fall through
the next !wait check. We will then set the PF_MEMALLOC flag and set
p->reclaim_state to point to __alloc_pages() local reclaim_state
structure. kswapd alread has it's own reclaim_state and already has
PF_MEMALLOC set, which would then be lost when, after
try_to_free_pages(), we unconditionally set the reclaim_state to NULL
and turn off the PF_MEMALLOC flag.
I'm not 100% sure that this potential bug is even possible (ie: can we
have an in_interrupt() page request that has __GFP_WAIT set?), or is the
cause of the 0-order page allocation failures we see, but it does seem
like potentially dangerous code. I have attatched a patch (against
2.6.11-mm4) to check whether the current task has it's own reclaim_state
or already has PF_MEMALLOC set and if so, no longer throws away this data.
I don't think in_interrupt allocations can have __GFP_WAIT set, so
this should probably be OK.
Nick
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