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SubjectRe: [PATCH 09/12] thp: introduce khugepaged_prealloc_page and khugepaged_alloc_page
On Wed, 12 Sep 2012, Xiao Guangrong wrote:
> On 09/12/2012 10:03 AM, Hugh Dickins wrote:
>
> > What brought me to look at it was hitting "BUG at mm/huge_memory.c:1842!"
> > running tmpfs kbuild swapping load (with memcg's memory.limit_in_bytes
> > forcing out to swap), while I happened to have CONFIG_NUMA=y.
> >
> > That's the VM_BUG_ON(*hpage) on entry to khugepaged_alloc_page().
>
> >
> > So maybe 9/12 is just obscuring what was already a BUG, either earlier
> > in your series or elsewhere in mmotm (I've never seen it on 3.6-rc or
> > earlier releases, nor without CONFIG_NUMA). I've not spent any time
> > looking for it, maybe it's obvious - can you spot and fix it?
>
> Hugh,
>
> I think i have already found the reason,

Great, thank you.

> if i am correct, the bug was existing before my patch.

Before your patchset? Are you sure of that?

>
> Could you please try below patch?

I put it on this morning, and ran load all day without a crash:
I think you indeed found the cause.

> And, could please allow me to fix the bug first,
> then post another patch to improve the things you dislike?

Good plan.

I've not yet glanced at your 2/3 and 3/3, I'm afraid.
I think akpm has helpfully included just 1/3 in the new mmotm.

>
> Subject: [PATCH] thp: fix forgetting to reset the page alloc indicator
>
> If NUMA is enabled, the indicator is not reset if the previous page
> request is failed, then it will trigger the BUG_ON in khugepaged_alloc_page
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
> mm/huge_memory.c | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e366ca5..66d2bc6 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1825,6 +1825,7 @@ static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
> return false;
>
> *wait = false;
> + *hpage = NULL;
> khugepaged_alloc_sleep();
> } else if (*hpage) {
> put_page(*hpage);

The unshown line just below this is

*hpage = NULL;

I do wish you would take the "*hpage = NULL;" out of if and else blocks
and place it once below both.

Thanks,
Hugh


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