Re: [PATCH 1/4] mm,hwpoison: Take free pages off the buddy freelists

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, Sep 02, 2020 at 11:45:07AM +0200, Oscar Salvador wrote:
> The crux of the matter is that historically we left poisoned pages
> in the buddy system because we have some checks in place when
> allocating a page that a gatekeeper for poisoned pages.
> Unfortunately, we do have other users (e.g: compaction [1]) that scan
> buddy freelists and try to get a page from there without checking
> whether the page is HWPoison.
> 
> As I stated already, I think it is fundamentally wrong to keep
> HWPoison pages within the buddy systems, checks in place or not.
> 
> Let us fix this we same way we did for soft_offline [2], and take
> the page off the buddy freelist, so it is completely unreachable.
> 
> Note that this is fairly simple to trigger, as we only need
> to poison free buddy pages (madvise MADV_HWPOISON) and then we need
> to run some sort of memory stress system.
> 
> Just for a matter of reference, I put a dump_page in compaction_alloc
> to trigger for HWPoison patches:
> 
> kernel: page:0000000012b2982b refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1d5db
> kernel: flags: 0xfffffc0800000(hwpoison)
> kernel: raw: 000fffffc0800000 ffffea00007573c8 ffffc90000857de0 0000000000000000
> kernel: raw: 0000000000000001 0000000000000000 00000001ffffffff 0000000000000000
> kernel: page dumped because: compaction_alloc
> 
> kernel: CPU: 4 PID: 123 Comm: kcompactd0 Tainted: G            E     5.9.0-rc2-mm1-1-default+ #5
> kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
> kernel: Call Trace:
> kernel:  dump_stack+0x6d/0x8b
> kernel:  compaction_alloc+0xb2/0xc0
> kernel:  migrate_pages+0x2a6/0x12a0
> kernel:  ? isolate_freepages+0xc80/0xc80
> kernel:  ? __ClearPageMovable+0xb0/0xb0
> kernel:  compact_zone+0x5eb/0x11c0
> kernel:  ? finish_task_switch+0x74/0x300
> kernel:  ? lock_timer_base+0xa8/0x170
> kernel:  proactive_compact_node+0x89/0xf0
> kernel:  ? kcompactd+0x2d0/0x3a0
> kernel:  kcompactd+0x2d0/0x3a0
> kernel:  ? finish_wait+0x80/0x80
> kernel:  ? kcompactd_do_work+0x350/0x350
> kernel:  kthread+0x118/0x130
> kernel:  ? kthread_associate_blkcg+0xa0/0xa0
> kernel:  ret_from_fork+0x22/0x30

Thanks for spotting this.

> After that, if e.g: someone faults in the page, that someone will get killed
> unexpectedly.
> 
> [1] https://lore.kernel.org/linux-mm/20190826104144.GA7849@linux/T/#u
> [2] https://patchwork.kernel.org/patch/11694847/
> 
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
>  mm/memory-failure.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 696505f56910..d349dcb45056 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1325,6 +1325,7 @@ int memory_failure(unsigned long pfn, int flags)
>  	struct dev_pagemap *pgmap;
>  	int res;
>  	unsigned long page_flags;
> +	bool retry = true;
>  
>  	if (!sysctl_memory_failure_recovery)
>  		panic("Memory failure on page %lx", pfn);
> @@ -1364,10 +1365,20 @@ int memory_failure(unsigned long pfn, int flags)
>  	 * In fact it's dangerous to directly bump up page count from 0,
>  	 * that may make page_ref_freeze()/page_ref_unfreeze() mismatch.
>  	 */
> +try_again:
>  	if (!(flags & MF_COUNT_INCREASED) && !get_hwpoison_page(p)) {
>  		if (is_free_buddy_page(p)) {
> -			action_result(pfn, MF_MSG_BUDDY, MF_DELAYED);
> -			return 0;
> +			if (take_page_off_buddy(p)) {
> +				action_result(pfn, MF_MSG_BUDDY, MF_DELAYED);
> +				return 0;
> +			} else {
> +				/* We lost the race, try again */
> +				if (retry) {
> +					retry = false;
> +					goto try_again;
> +				}

You might need add calling action_result() here.

> +				return -EBUSY;
> +			}
>  		} else {
>  			action_result(pfn, MF_MSG_KERNEL_HIGH_ORDER, MF_IGNORED);
>  			return -EBUSY;

And the following block also handles buddy pages, so you could also call
take_page_off_buddy() here?

        /*
         * We ignore non-LRU pages for good reasons.
         * - PG_locked is only well defined for LRU pages and a few others
         * - to avoid races with __SetPageLocked()
         * - to avoid races with __SetPageSlab*() (and more non-atomic ops)
         * The check (unnecessarily) ignores LRU pages being isolated and
         * walked by the page reclaim code, however that's not a big loss.
         */
        shake_page(p, 0);
        /* shake_page could have turned it free. */
        if (!PageLRU(p) && is_free_buddy_page(p)) {
                if (flags & MF_COUNT_INCREASED)
                        action_result(pfn, MF_MSG_BUDDY, MF_DELAYED);
                else
                        action_result(pfn, MF_MSG_BUDDY_2ND, MF_DELAYED);
                return 0;
        }

Or if get_hwpoison_page() handles retry inside it as will be done in patch 3/4,
we don't need this block any more?

Thanks,
Naoya Horiguchi