Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

[Ni zhan Chen]

On 10/25/2012 02:59 PM, Hugh Dickins wrote:
> 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

Hi Hugh

Thanks for your response. You mean the -EEXIST originating from 
radix_tree_insert, in radix_tree_insert:
if (slot != NULL)
    return -EEXIST;
But why slot should be NULL? if no race, the pagecache related radix 
tree entry should be RADIX_TREE_EXCEPTIONAL_ENTRY+swap_entry_t.val, 
where I miss?

Regards,
Chen

> > otherwise, will goto unlock and then go to repeat? where I miss?
> >
> > Regards,
> > Chen

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