On Fri, 26 Mar 2021 15:17:00 +0100 Vlastimil Babka <vbabka@xxxxxxx> wrote: > On 3/26/21 12:26 PM, Sergei Trofimovich wrote: > > init_on_free=1 does not guarantee that free pages contain only zero bytes. > > > > Some examples: > > 1. page_poison=on takes presedence over init_on_alloc=1 / ini_on_free=1 > > Yes, and it spits out a message that you enabled both and poisoning takes > precedence. It was that way even before my changes IIRC, but not consistent. Yeah. I probably should not have included this case as page_poison=on actually made my machine boot just fine. My main focus was to understand why I an seeing the crash on kernel with init_on_alloc=1 init_on_free=1 and most debugging options on. My apologies! I'll try to find where this extra poisoning comes from. Making a step back and explaining my setup: Initially it's an ia64 box that manages to consistently corrupt memory on socket free; https://lkml.org/lkml/2021/2/23/653 To get better understanding where corruption comes from I enabled A Lot of VM, pagealloc and slab debugging options. Full config: https://dev.gentoo.org/~slyfox/configs/guppy-config-5.12.0-rc4-00016-g427684abc9fd-dirty I boot machine as: [ 0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-5.12.0-rc4-00016-g427684abc9fd-dirty root=/dev/sda3 ro slab_nomerge memblock=debug debug_pagealloc=1 hardened_usercopy=1 page_owner=on page_poison=0 init_on_alloc=1 init_on_free=1 debug_guardpage_minorder=0 My boot log: https://dev.gentoo.org/~slyfox/bugs/ia64-boot-bug/2021-03-26-init_on_alloc-fail Caveats in reading boot log: - kernel crashes too early: stack unwinder does not have working kmalloc() yet - kernel crashes in MCE handler: normally it should not. It's an unrelated bug that makes backtrace useless. I'll try to fix it later, but it will not be fast. - I added a bunch of printk()s around the crash. The important pernel boot failure part is: [ 0.000000] put_kernel_page: pmd=e000000100000000 [ 0.000000] pmd:(____ptrval____): aaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaa ................................ Note 1: I do not really enable page_poison at runtime and was misleading you in previous emails. (I initially assumed kernel_poison_pages() poisons pages unconditionally but you all explained it does not). Something else manages to poison my pmd(s?). Note 2: I have many other debugging options enabled that might trigger poisoning. > > 2. free_pages_prepare() always poisons pages: > > > > if (want_init_on_free()) > > kernel_init_free_pages(page, 1 << order); > > kernel_poison_pages(page, 1 << order > > kernel_poison_pages() includes a test if poisoning is enabled. And in that case > want_init_on_free() shouldn't be. see init_mem_debugging_and_hardening() I completely missed that! Thank you! Will try to trace real cause of poisoning. > > I observed use of poisoned pages as the crash on ia64 booted with > > init_on_free=1 init_on_alloc=1 (CONFIG_PAGE_POISONING=y config). > > There pmd page contained 0xaaaaaaaa poison pages and led to early crash. > > Hm but that looks lika a sign that ia64 pmd allocation should use __GFP_ZERO and > doesn't. It shouldn't rely on init_on_alloc or init_on_free being enabled. ia64 does use __GFP_ZERO (I even tried to add it manually to pmd_alloc_one() before I realized all _PGTABLEs imply __GFP_ZERO). I'll provide the call chain I arrived at for completeness: - [ia64 boots] - mem_init() (defined at arch/ia64/mm/init.c) -> setup_gate() (defined at arch/ia64/mm/init.c) -> put_kernel_page() (defined at arch/ia64/mm/init.c) -> [NOTE: from now on it's all generic code, not ia64-speficic] -> pmd_alloc() (defined at include/linux/mm.h) -> __pmd_alloc() (defined at mm/memory.c) -> [under #ifndef __PAGETABLE_PMD_FOLDED] pmd_alloc_one() (defined at include/asm-generic/pgalloc.h) -> pmd_alloc_one() [defined at include/asm-generic/pgalloc.h): static inline pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long addr) { struct page *page; gfp_t gfp = GFP_PGTABLE_USER; if (mm == &init_mm) gfp = GFP_PGTABLE_KERNEL; page = alloc_pages(gfp, 0); if (!page) return NULL; if (!pgtable_pmd_page_ctor(page)) { __free_pages(page, 0); return NULL; } return (pmd_t *)page_address(page); } In our case it is a GFP_PGTABLE_KERNEL with __GFP_ZERO and result is poisoned page instead of zeroed page. If I interpret the above correctly it means that something (probably memalloc_free_pages() ?) puts initial free pages as poisoned and later alloc_pages() assumes they are memset()-zero. But I don't see why. > > The change drops the assumption that init_on_free=1 guarantees free > > pages to contain zeros. > > The change assumes that page_poison=on also leaves want_init_on_free() enabled, > but it doesn't. > > > Alternative would be to make interaction between runtime poisoning and > > sanitizing options and build-time debug flags like CONFIG_PAGE_POISONING > > more coherent. I took the simpler path. > > So that was done in 5.11 and the decisions can be seen in > init_mem_debugging_and_hardening(). There might be of course a bug, or later > changes broke something. Which was the version that you observed a bug? > > > Tested the fix on rx3600. > > > > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > CC: linux-mm@xxxxxxxxx > > Signed-off-by: Sergei Trofimovich <slyfox@xxxxxxxxxx> > > --- > > mm/page_alloc.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index cfc72873961d..d57d9b4f7089 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -2301,7 +2301,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order, > > kernel_unpoison_pages(page, 1 << order); > > set_page_owner(page, order, gfp_flags); > > > > - if (!want_init_on_free() && want_init_on_alloc(gfp_flags)) > > + if (want_init_on_alloc(gfp_flags)) > > kernel_init_free_pages(page, 1 << order); > > } > > > > > -- Sergei
