On Thu 02-11-17 15:35:07, Jaewon Kim wrote:
> online_page_ext and page_ext_init allocate page_ext for each section, but
> they do not allocate if the first PFN is !pfn_present(pfn) or
> !pfn_valid(pfn). Then section->page_ext remains as NULL. lookup_page_ext
> checks NULL only if CONFIG_DEBUG_VM is enabled. For a valid PFN,
> __set_page_owner will try to get page_ext through lookup_page_ext.
> Without CONFIG_DEBUG_VM lookup_page_ext will misuse NULL pointer as value
> 0. This incurrs invalid address access.
>
> This is the panic example when PFN 0x100000 is not valid but PFN 0x13FC00
> is being used for page_ext. section->page_ext is NULL, get_entry returned
> invalid page_ext address as 0x1DFA000 for a PFN 0x13FC00.
>
> <1>[ 11.618085] Unable to handle kernel paging request at virtual address 01dfa014
> <1>[ 11.618140] pgd = ffffffc0c6dc9000
> <1>[ 11.618174] [01dfa014] *pgd=0000000000000000, *pud=0000000000000000
> <4>[ 11.618240] ------------[ cut here ]------------
> <2>[ 11.618278] Kernel BUG at ffffff80082371e0 [verbose debug info unavailable]
> <0>[ 11.618338] Internal error: Oops: 96000045 [#1] PREEMPT SMP
> <4>[ 11.618381] Modules linked in:
> <4>[ 11.618524] task: ffffffc0c6ec9180 task.stack: ffffffc0c6f40000
> <4>[ 11.618569] PC is at __set_page_owner+0x48/0x78
> <4>[ 11.618607] LR is at __set_page_owner+0x44/0x78
> <4>[ 11.626025] [<ffffff80082371e0>] __set_page_owner+0x48/0x78
> <4>[ 11.626071] [<ffffff80081df9f0>] get_page_from_freelist+0x880/0x8e8
> <4>[ 11.626118] [<ffffff80081e00a4>] __alloc_pages_nodemask+0x14c/0xc48
> <4>[ 11.626165] [<ffffff80081e610c>] __do_page_cache_readahead+0xdc/0x264
> <4>[ 11.626214] [<ffffff80081d8824>] filemap_fault+0x2ac/0x550
> <4>[ 11.626259] [<ffffff80082e5cf8>] ext4_filemap_fault+0x3c/0x58
> <4>[ 11.626305] [<ffffff800820a2f8>] __do_fault+0x80/0x120
> <4>[ 11.626347] [<ffffff800820eb4c>] handle_mm_fault+0x704/0xbb0
> <4>[ 11.626393] [<ffffff800809ba70>] do_page_fault+0x2e8/0x394
> <4>[ 11.626437] [<ffffff8008080be4>] do_mem_abort+0x88/0x124
>
> Though the first page is not valid, page_ext could be useful for other
> pages in the section. But checking all PFNs in a section may be time
> consuming job. Let's check each (section count / 16) PFN, then prepare
> page_ext if any PFN is present or valid. And remove the CONFIG_DEBUG_VM in
> lookup_page_ext to avoid panic.
So I would split this patch into two. First one to address the panic
which sounds like a stable material and then the enhancement which will
most likely need a further discussion.
> Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
> ---
> mm/page_ext.c | 29 ++++++++++++++++++++++-------
> 1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 32f18911deda..bf9c99beb312 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -124,7 +124,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> struct page_ext *base;
>
> base = NODE_DATA(page_to_nid(page))->node_page_ext;
> -#if defined(CONFIG_DEBUG_VM)
> /*
> * The sanity checks the page allocator does upon freeing a
> * page can reach here before the page_ext arrays are
> @@ -133,7 +132,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> */
> if (unlikely(!base))
> return NULL;
> -#endif
> index = pfn - round_down(node_start_pfn(page_to_nid(page)),
> MAX_ORDER_NR_PAGES);
> return get_entry(base, index);
> @@ -198,7 +196,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> {
> unsigned long pfn = page_to_pfn(page);
> struct mem_section *section = __pfn_to_section(pfn);
> -#if defined(CONFIG_DEBUG_VM)
> /*
> * The sanity checks the page allocator does upon freeing a
> * page can reach here before the page_ext arrays are
> @@ -207,7 +204,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> */
> if (!section->page_ext)
> return NULL;
> -#endif
> return get_entry(section->page_ext, pfn);
> }
>
> @@ -312,7 +308,17 @@ static int __meminit online_page_ext(unsigned long start_pfn,
> }
>
> for (pfn = start; !fail && pfn < end; pfn += PAGES_PER_SECTION) {
> - if (!pfn_present(pfn))
> + unsigned long t_pfn = pfn;
> + bool present = false;
> +
> + while (t_pfn < ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> + if (pfn_present(t_pfn)) {
> + present = true;
> + break;
> + }
> + t_pfn = ALIGN(pfn + 1, PAGES_PER_SECTION >> 4);
> + }
> + if (!present)
> continue;
> fail = init_section_page_ext(pfn, nid);
> }
> @@ -391,8 +397,17 @@ void __init page_ext_init(void)
> */
> for (pfn = start_pfn; pfn < end_pfn;
> pfn = ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> -
> - if (!pfn_valid(pfn))
> + unsigned long t_pfn = pfn;
> + bool valid = false;
> +
> + while (t_pfn < ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> + if (pfn_valid(t_pfn)) {
> + valid = true;
> + break;
> + }
> + t_pfn = ALIGN(pfn + 1, PAGES_PER_SECTION >> 4);
> + }
> + if (!valid)
> continue;
> /*
> * Nodes's pfns can be overlapping.
> --
> 2.13.0
--
Michal Hocko
SUSE Labs
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