Revision fb59e9f1e9786635ea12e12bf6adbb132e10f979 authored by Hugh Dickins on 04 March 2008, 22:29:16 UTC, committed by Linus Torvalds on 05 March 2008, 00:35:15 UTC
While testing force_empty, during an exit_mmap, __mem_cgroup_remove_list called from mem_cgroup_uncharge_page oopsed on a NULL pointer in the lru list. I couldn't see what racing tasks on other cpus were doing, but surmise that another must have been in mem_cgroup_charge_common on the same page, between its unlock_page_cgroup and spin_lock_irqsave near done (thanks to that kzalloc which I'd almost changed to a kmalloc). Normally such a race cannot happen, the ref_cnt prevents it, the final uncharge cannot race with the initial charge. But force_empty buggers the ref_cnt, that's what it's all about; and thereafter forced pages are vulnerable to races such as this (just think of a shared page also mapped into an mm of another mem_cgroup than that just emptied). And remain vulnerable until they're freed indefinitely later. This patch just fixes the oops by moving the unlock_page_cgroups down below adding to and removing from the list (only possible given the previous patch); and while we're at it, we might as well make it an invariant that page->page_cgroup is always set while pc is on lru. But this behaviour of force_empty seems highly unsatisfactory to me: why have a ref_cnt if we always have to cope with it being violated (as in the earlier page migration patch). We may prefer force_empty to move pages to an orphan mem_cgroup (could be the root, but better not), from which other cgroups could recover them; we might need to reverse the locking again; but no time now for such concerns. Signed-off-by: Hugh Dickins <hugh@veritas.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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File | Mode | Size |
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Kconfig | -rw-r--r-- | 4.8 KB |
Makefile | -rw-r--r-- | 392 bytes |
TODO | -rw-r--r-- | 944 bytes |
af_ax25.c | -rw-r--r-- | 44.3 KB |
ax25_addr.c | -rw-r--r-- | 6.1 KB |
ax25_dev.c | -rw-r--r-- | 4.8 KB |
ax25_ds_in.c | -rw-r--r-- | 7.1 KB |
ax25_ds_subr.c | -rw-r--r-- | 5.1 KB |
ax25_ds_timer.c | -rw-r--r-- | 5.8 KB |
ax25_iface.c | -rw-r--r-- | 5.1 KB |
ax25_in.c | -rw-r--r-- | 10.8 KB |
ax25_ip.c | -rw-r--r-- | 5.3 KB |
ax25_out.c | -rw-r--r-- | 8.7 KB |
ax25_route.c | -rw-r--r-- | 11.2 KB |
ax25_std_in.c | -rw-r--r-- | 11.1 KB |
ax25_std_subr.c | -rw-r--r-- | 2.3 KB |
ax25_std_timer.c | -rw-r--r-- | 4.2 KB |
ax25_subr.c | -rw-r--r-- | 7.1 KB |
ax25_timer.c | -rw-r--r-- | 5.1 KB |
ax25_uid.c | -rw-r--r-- | 4.8 KB |
sysctl_net_ax25.c | -rw-r--r-- | 6.3 KB |
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