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

[Eric Dumazet]

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;
 


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