Re: [PATCH] list corruption by gather_surplus

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Cliff Wickman <cpw@sgi.com> writes:

> From: Cliff Wickman <cpw@sgi.com>
>
> Gentlemen,
> I see that you all have done maintenance on mm/hugetlb.c, so I'm hoping one
> or two of you could comment on a problem and proposed fix.
>
>
> I am seeing list corruption occurring from within gather_surplus_pages()
> (mm/hugetlb.c).  The problem occurs under a heavy load, and seems to be
> because this function drops the hugetlb_lock.
>
> I have CONFIG_DEBUG_LIST=y, and am running an MPI application with 64 threads
> and a library that creates a large heap of hugetlbfs pages for it.
>
> The below patch fixes the problem.
> The gist of this patch is that gather_surplus_pages() does not have to drop
> the lock if alloc_buddy_huge_page() is told whether the lock is
> already held.


But you didn't explain the corruption details right ? What cause the
corruption ? It would be nice to document that in the commit.

>
> But I may be missing some reason why gather_surplus_pages() is unlocking and
> locking the hugetlb_lock several times (besides around the allocator).
>
> Could you take a look and advise?
>
> Signed-off-by: Cliff Wickman <cpw@sgi.com>
> ---
>  mm/hugetlb.c |   28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
>
> Index: linux/mm/hugetlb.c
> ===================================================================
> --- linux.orig/mm/hugetlb.c
> +++ linux/mm/hugetlb.c
> @@ -747,7 +747,9 @@ static int free_pool_huge_page(struct hs
>  	return ret;
>  }
>
> -static struct page *alloc_buddy_huge_page(struct hstate *h, int nid)
> +/* already_locked means the caller has already locked hugetlb_lock */
> +static struct page *alloc_buddy_huge_page(struct hstate *h, int nid,
> +						int already_locked)
>  {

Why ? Why can't we always call this with lock held ?

>  	struct page *page;
>  	unsigned int r_nid;
> @@ -778,7 +780,8 @@ static struct page *alloc_buddy_huge_pag
>  	 * the node values until we've gotten the hugepage and only the

-aneesh

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