On Thu, 25 Oct 2012, Ni zhan Chen wrote: > On 10/25/2012 12:36 PM, Hugh Dickins wrote: > > On Wed, 24 Oct 2012, Dave Jones wrote: > > > > > Machine under significant load (4gb memory used, swap usage fluctuating) > > > triggered this... > > > > > > WARNING: at mm/shmem.c:1151 shmem_getpage_gfp+0xa5c/0xa70() > > > Pid: 29795, comm: trinity-child4 Not tainted 3.7.0-rc2+ #49 > > > > > > 1148 error = shmem_add_to_page_cache(page, > > > mapping, index, > > > 1149 gfp, > > > swp_to_radix_entry(swap)); > > > 1150 /* We already confirmed swap, and make no > > > allocation */ > > > 1151 VM_BUG_ON(error); > > > 1152 } > > That's very surprising. Easy enough to handle an error there, but > > of course I made it a VM_BUG_ON because it violates my assumptions: > > I rather need to understand how this can be, and I've no idea. > > > > Clutching at straws, I expect this is entirely irrelevant, but: > > there isn't a warning on line 1151 of mm/shmem.c in 3.7.0-rc2 nor > > in current linux.git; rather, there's a VM_BUG_ON on line 1149. > > > > So you've inserted a couple of lines for some reason (more useful > > trinity behaviour, perhaps)? And have some config option I'm > > unfamiliar with, that mutates a BUG_ON or VM_BUG_ON into a warning? > > Hi Hugh, > > I think it maybe caused by your commit [d189922862e03ce: shmem: fix negative > rss in memcg memory.stat], one question: Well, yes, I added the VM_BUG_ON in that commit. > > if function shmem_confirm_swap confirm the entry has already brought back > from swap by a racing thread, The reverse: true confirms that the swap entry has not been brought back from swap by a racing thread; false indicates that there has been a race. > then why call shmem_add_to_page_cache to add > page from swapcache to pagecache again? Adding it to pagecache again, after such a race, would set error to -EEXIST (originating from radix_tree_insert); but we don't do that, we add it to pagecache when it has not already been added. Or that's the intention: but Dave seems to have found an unexpected exception, despite us holding the page lock across all this. (But if it weren't for the memcg and replace_page issues, I'd much prefer to let shmem_add_to_page_cache discover the race as before.) Hugh > otherwise, will goto unlock and then go to repeat? where I miss? > > Regards, > Chen -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>