Re: [PATCH mmotm] mm: alloc_large_system_hash check order
From: Hugh Dickins <hidden>
Date: 2009-05-01 11:31:45
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linux-mm, lkml
On Thu, 30 Apr 2009, Mel Gorman wrote:
On Wed, Apr 29, 2009 at 10:09:48PM +0100, Hugh Dickins wrote:quoted
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 <redacted>Looks good Reviewed-by: Mel Gorman <redacted>
Thanks.
As I was looking there, it seemed that alloc_large_system_hash() should be using alloc_pages_exact() instead of having its own "give back the spare pages at the end of the buffer" logic. If alloc_pages_exact() was used, then the check for an order >= MAX_ORDER can be pushed down to alloc_pages_exact() where it may catch other unwary callers. How about adding the following patch on top of yours?
Well observed, yes indeed. In fact, it even looks as if, shock horror, alloc_pages_exact() was _plagiarized_ from alloc_large_system_hash(). Blessed be the GPL, I'm sure we can skip the lengthy lawsuits!
quoted hunk ↗ jump to hunk
==== CUT HERE ==== Use alloc_pages_exact() in alloc_large_system_hash() to avoid duplicated logic alloc_large_system_hash() has logic for freeing unused pages at the end of an power-of-two-pages-aligned buffer that is a duplicate of what is in alloc_pages_exact(). This patch converts alloc_large_system_hash() to use alloc_pages_exact(). Signed-off-by: Mel Gorman <redacted> --- mm/page_alloc.c | 27 +++++---------------------- 1 file changed, 5 insertions(+), 22 deletions(-)diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 1b3da0f..c94b140 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c@@ -1942,6 +1942,9 @@ void *alloc_pages_exact(size_t size, gfp_t gfp_mask) unsigned int order = get_order(size); unsigned long addr; + if (order >= MAX_ORDER) + return NULL; +
I suppose there could be an argument about whether we do or do not want to skip the WARN_ON when it's in alloc_pages_exact(). I have no opinion on that; but DaveM's reply on large_system_hash does make it clear that we're not interested in the warning there.
quoted hunk ↗ jump to hunk
addr = __get_free_pages(gfp_mask, order); if (addr) { unsigned long alloc_end = addr + (PAGE_SIZE << order);@@ -4755,28 +4758,8 @@ void *__init alloc_large_system_hash(const char *tablename, table = alloc_bootmem_nopanic(size); else if (hashdist) table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL); - else { - unsigned long order = get_order(size); - - 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. - */
That's actually a helpful comment, it's easy to think we're dealing in powers of two here when we may not be. Maybe retain it with your alloc_pages_exact call?
- 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;
- }
- }
- }
+ else
+ table = alloc_pages_exact(PAGE_ALIGN(size), GFP_ATOMIC);Do you actually need that PAGE_ALIGN on the size?
} while (!table && size > PAGE_SIZE && --log2qty); if (!table)
Andrew noticed another oddity: that if it goes the hashdist __vmalloc() way, it won't be limited by MAX_ORDER. Makes one wonder whether it ought to fall back to __vmalloc() if the alloc_pages_exact() fails. I think that's a change we could make _if_ the large_system_hash users ever ask for it, but _not_ one we should make surreptitiously. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>