On 26.08.22 18:04, Peter Xu wrote: > On Fri, Aug 26, 2022 at 04:39:08PM +0200, David Hildenbrand wrote: >> On 26.08.22 16:25, Peter Xu wrote: >>> On Fri, Aug 26, 2022 at 12:49:37PM +0200, David Hildenbrand wrote: >>>> On 24.08.22 00:11, Peter Xu wrote: >>>>> Yu Zhao reported a bug after the commit "mm/swap: Add swp_offset_pfn() to >>>>> fetch PFN from swap entry" added a check in swp_offset_pfn() for swap type [1]: >>>>> >>>>> kernel BUG at include/linux/swapops.h:117! >>>>> CPU: 46 PID: 5245 Comm: EventManager_De Tainted: G S O L 6.0.0-dbg-DEV #2 >>>>> RIP: 0010:pfn_swap_entry_to_page+0x72/0xf0 >>>>> Code: c6 48 8b 36 48 83 fe ff 74 53 48 01 d1 48 83 c1 08 48 8b 09 f6 >>>>> c1 01 75 7b 66 90 48 89 c1 48 8b 09 f6 c1 01 74 74 5d c3 eb 9e <0f> 0b >>>>> 48 ba ff ff ff ff 03 00 00 00 eb ae a9 ff 0f 00 00 75 13 48 >>>>> RSP: 0018:ffffa59e73fabb80 EFLAGS: 00010282 >>>>> RAX: 00000000ffffffe8 RBX: 0c00000000000000 RCX: ffffcd5440000000 >>>>> RDX: 1ffffffffff7a80a RSI: 0000000000000000 RDI: 0c0000000000042b >>>>> RBP: ffffa59e73fabb80 R08: ffff9965ca6e8bb8 R09: 0000000000000000 >>>>> R10: ffffffffa5a2f62d R11: 0000030b372e9fff R12: ffff997b79db5738 >>>>> R13: 000000000000042b R14: 0c0000000000042b R15: 1ffffffffff7a80a >>>>> FS: 00007f549d1bb700(0000) GS:ffff99d3cf680000(0000) knlGS:0000000000000000 >>>>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>>> CR2: 0000440d035b3180 CR3: 0000002243176004 CR4: 00000000003706e0 >>>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>>>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>>>> Call Trace: >>>>> <TASK> >>>>> change_pte_range+0x36e/0x880 >>>>> change_p4d_range+0x2e8/0x670 >>>>> change_protection_range+0x14e/0x2c0 >>>>> mprotect_fixup+0x1ee/0x330 >>>>> do_mprotect_pkey+0x34c/0x440 >>>>> __x64_sys_mprotect+0x1d/0x30 >>>>> >>>>> It triggers because pfn_swap_entry_to_page() could be called upon e.g. a >>>>> genuine swap entry. >>>>> >>>>> Fix it by only calling it when it's a write migration entry where the page* >>>>> is used. >>>>> >>>>> [1] https://lore.kernel.org/lkml/CAOUHufaVC2Za-p8m0aiHw6YkheDcrO-C3wRGixwDS32VTS+k1w@xxxxxxxxxxxxxx/ >>>>> >>>>> Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive") >>>>> Cc: David Hildenbrand <david@xxxxxxxxxx> >>>>> Cc: <stable@xxxxxxxxxxxxxxx> >>>>> Reported-by: Yu Zhao <yuzhao@xxxxxxxxxx> >>>>> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> >>>>> --- >>>>> mm/mprotect.c | 3 ++- >>>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/mm/mprotect.c b/mm/mprotect.c >>>>> index f2b9b1da9083..4549f5945ebe 100644 >>>>> --- a/mm/mprotect.c >>>>> +++ b/mm/mprotect.c >>>>> @@ -203,10 +203,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb, >>>>> pages++; >>>>> } else if (is_swap_pte(oldpte)) { >>>>> swp_entry_t entry = pte_to_swp_entry(oldpte); >>>>> - struct page *page = pfn_swap_entry_to_page(entry); >>>>> pte_t newpte; >>>>> >>>>> if (is_writable_migration_entry(entry)) { >>>>> + struct page *page = pfn_swap_entry_to_page(entry); >>>>> + >>>>> /* >>>>> * A protection check is difficult so >>>>> * just be safe and disable write >>>> >>>> >>>> Stumbling over the THP code, I was wondering if we also want to adjust change_huge_pmd() >>>> and hugetlb_change_protection. There are no actual swap entries, so I assume we're fine. >>>> >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 482c1826e723..466364e7fc5f 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -1798,10 +1798,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >>>> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION >>>> if (is_swap_pmd(*pmd)) { >>>> swp_entry_t entry = pmd_to_swp_entry(*pmd); >>>> - struct page *page = pfn_swap_entry_to_page(entry); >>>> >>>> VM_BUG_ON(!is_pmd_migration_entry(*pmd)); >>>> if (is_writable_migration_entry(entry)) { >>>> + struct page *page = pfn_swap_entry_to_page(entry); >>>> pmd_t newpmd; >>>> /* >>>> * A protection check is difficult so >>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>>> index 2480ba627aa5..559465fae5cd 100644 >>>> --- a/mm/hugetlb.c >>>> +++ b/mm/hugetlb.c >>>> @@ -6370,9 +6370,9 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma, >>>> } >>>> if (unlikely(is_hugetlb_entry_migration(pte))) { >>>> swp_entry_t entry = pte_to_swp_entry(pte); >>>> - struct page *page = pfn_swap_entry_to_page(entry); >>>> >>>> if (!is_readable_migration_entry(entry)) { >>>> + struct page *page = pfn_swap_entry_to_page(entry); >>>> pte_t newpte; >>>> >>>> if (PageAnon(page)) >>>> >>>> >>>> @Peter, what's your thought? >>> >>> IMHO they're not needed? >>> >>> The rule is simple in my mind: we should only pass in a pfn-typed swap >>> entry into pfn_swap_entry_to_page() (or the new swp_offset_pfn()), or it's >>> a violation of the API. In these two cases they do not violate the API and >>> they're always safe because they're guaranteed to be pfn swap entries when >>> calling. >> >> I was wondering about extreme corner cases regarding the struct page. >> >> Assume we have a hwpoison_entry that pointed at a valid struct page. We >> can succeed in offlining+removing the section it's located on (I was >> recently challenging if we want to keep that behavior as it's really >> shaky already), freeing the relevant memmap entry and the memory section. >> >> pfn_swap_entry_to_page() -> pfn_to_page() would be problematic if there >> is no memmap anymore. >> >> >> I assume it's ok to always call it for is_pfn_swap_entry(), but in the >> PMD case we only check for is_swap_pmd()? Isn't that problematic? > > I don't know extensively enough on hwpoison on validity of fetching page > from pfn inside on online/offline ops, but.. if the only concern is about > hwpoison entry existance here I think its fine? Because iirc we'l split > thp when any of the subpage got poisoned, so we should never hit a hwpoison > entry in thp path. Ah right, so we're good. Thanks! -- Thanks, David / dhildenb
