Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2]

[Ni zhan Chen]

On 10/26/2012 05:27 AM, Hugh Dickins wrote:
> On Thu, 25 Oct 2012, Ni zhan Chen wrote:
> > On 10/25/2012 02:59 PM, Hugh Dickins wrote:
> > > On Thu, 25 Oct 2012, Ni zhan Chen wrote:
> > > > 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?
> I was describing what would happen in a case that should not exist,
> that you had thought the common case.  In actuality, the entry should
> not be NULL, it should be as you say there.

Thanks for your patience. So in the common case, the entry should be the 
value I mentioned, then why has this check?
if (slot != NULL)
    return -EEXIST;

the common case will return -EEXIST.

> Hugh

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/