Re: rhashtable: ENOMEM errors when hit with a flood of insertions

Eric Dumazet
2015-12-03T16:08:44Z

On Thu, 2015-12-03 at 20:51 +0800, Herbert Xu wrote:
> On Mon, Nov 30, 2015 at 06:18:59PM +0800, Herbert Xu wrote:
> >
> > OK that's better. I think I see the problem. The test in
> > rhashtable_insert_rehash is racy and if two threads both try
> > to grow the table one of them may be tricked into doing a rehash
> > instead.
> >
> > I'm working on a fix.
>
> While the EBUSY errors are gone for me, I can still see plenty
> of ENOMEM errors. In fact it turns out that the reason is quite
> understandable. When you pound the rhashtable hard so that it
> doesn't actually get a chance to grow the table in process context,
> then the table will only grow with GFP_ATOMIC allocations.
>
> For me this starts failing regularly at around 2^19 entries, which
> requires about 1024 contiguous pages if I'm not mistaken.

Well, it will fail before this point if memory is fragmented.

Anyway, __vmalloc() can be used with GFP_ATOMIC, have you tried this ?

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index a54ff8949f91..9ef5d74963b2 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -120,8 +120,9 @@ static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
if (size <= (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER) ||
gfp != GFP_KERNEL)
tbl = kzalloc(size, gfp | __GFP_NOWARN | __GFP_NORETRY);
- if (tbl == NULL && gfp == GFP_KERNEL)
- tbl = vzalloc(size);
+ if (tbl == NULL)
+ tbl = __vmalloc(size, gfp | __GFP_HIGHMEM | __GFP_ZERO,
+ PAGE_KERNEL);
if (tbl == NULL)
return NULL;