Subject: Re: [PATCH mmotm] mm: alloc_large_system_hash check order

From: Andrew Morton <[email-masked]>
Date: Wed, 29 Apr 2009 14:28:25 -0700

On Wed, 29 Apr 2009 22:09:48 +0100 (BST)
Hugh Dickins <[email-masked]> wrote:

> On an x86_64 with 4GB ram, tcp_init()'s call to alloc_large_system_hash(),
> to allocate tcp_hashinfo.ehash, is now triggering an mmotm WARN_ON_ONCE on
> order >= MAX_ORDER - it's hoping for order 11.  alloc_large_system_hash()
> had better make its own check on the order.
> 
> Signed-off-by: Hugh Dickins <[email-masked]>
> ---
> Should probably follow
> page-allocator-do-not-sanity-check-order-in-the-fast-path-fix.patch
> 
> Cc'ed DaveM and netdev, just in case they're surprised it was asking for
> so much, or disappointed it's not getting as much as it was asking for.
> 
>  mm/page_alloc.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> --- 2.6.30-rc3-mm1/mm/page_alloc.c	2009-04-29 21:01:08.000000000 +0100
> +++ mmotm/mm/page_alloc.c	2009-04-29 21:12:04.000000000 +0100
> @@ -4765,7 +4765,10 @@ void *__init alloc_large_system_hash(con
>  			table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
>  		else {
>  			unsigned long order = get_order(size);
> -			table = (void*) __get_free_pages(GFP_ATOMIC, order);
> +
> +			if (order < MAX_ORDER)
> +				table = (void *)__get_free_pages(GFP_ATOMIC,
> +								order);
>  			/*
>  			 * If bucketsize is not a power-of-two, we may free
>  			 * some pages at the end of hash table.

yes, the code is a bit odd:

:	do {
: 		size = bucketsize << log2qty;
: 		if (flags & HASH_EARLY)
: 			table = alloc_bootmem_nopanic(size);
: 		else if (hashdist)
: 			table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
: 		else {
: 			unsigned long order = get_order(size);
: 			table = (void*) __get_free_pages(GFP_ATOMIC, order);
: 			/*
: 			 * If bucketsize is not a power-of-two, we may free
: 			 * some pages at the end of hash table.
: 			 */
: 			if (table) {
: 				unsigned long alloc_end = (unsigned long)table +
: 						(PAGE_SIZE << order);
: 				unsigned long used = (unsigned long)table +
: 						PAGE_ALIGN(size);
: 				split_page(virt_to_page(table), order);
: 				while (used < alloc_end) {
: 					free_page(used);
: 					used += PAGE_SIZE;
: 				}
: 			}
: 		}
: 	} while (!table && size > PAGE_SIZE && --log2qty);

In the case where it does the __vmalloc(), the order-11 allocation will
succeed.  But in the other cases, the allocation attempt will need to
be shrunk and we end up with a smaller hash table.  Is that sensible?

If we want to regularise all three cases, doing

	size = min(size, MAX_ORDER);

before starting the loop would be suitable, although the huge
__get_free_pages() might still fail.  (But it will then warn, won't it?
 And nobody is reporting that).

I was a bit iffy about adding the warning in the first place, let it go
through due to its potential to lead us to code which isn't doing what
it thinks it's doing, or is being generally peculiar.

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