Re: [PATCH v2 1/3] LoongArch: Set initial pte entry with PAGE_GLOBAL for kernel space

From: Huacai Chen
Date: Mon Oct 21 2024 - 21:57:04 EST

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On Tue, Oct 22, 2024 at 9:40 AM maobibo <maobibo@xxxxxxxxxxx> wrote:
>
>
>
> On 2024/10/21 下午6:13, Huacai Chen wrote:
> > On Mon, Oct 21, 2024 at 9:23 AM maobibo <maobibo@xxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2024/10/18 下午2:32, Huacai Chen wrote:
> >>> On Fri, Oct 18, 2024 at 2:23 PM maobibo <maobibo@xxxxxxxxxxx> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2024/10/18 下午12:23, Huacai Chen wrote:
> >>>>> On Fri, Oct 18, 2024 at 12:16 PM maobibo <maobibo@xxxxxxxxxxx> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 2024/10/18 下午12:11, Huacai Chen wrote:
> >>>>>>> On Fri, Oct 18, 2024 at 11:44 AM maobibo <maobibo@xxxxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On 2024/10/18 上午11:14, Huacai Chen wrote:
> >>>>>>>>> Hi, Bibo,
> >>>>>>>>>
> >>>>>>>>> I applied this patch but drop the part of arch/loongarch/mm/kasan_init.c:
> >>>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git/commit/?h=loongarch-next&id=15832255e84494853f543b4c70ced50afc403067
> >>>>>>>>>
> >>>>>>>>> Because kernel_pte_init() should operate on page-table pages, not on
> >>>>>>>>> data pages. You have already handle page-table page in
> >>>>>>>>> mm/kasan/init.c, and if we don't drop the modification on data pages
> >>>>>>>>> in arch/loongarch/mm/kasan_init.c, the kernel fail to boot if KASAN is
> >>>>>>>>> enabled.
> >>>>>>>>>
> >>>>>>>> static inline void set_pte(pte_t *ptep, pte_t pteval)
> >>>>>>>> {
> >>>>>>>> WRITE_ONCE(*ptep, pteval);
> >>>>>>>> -
> >>>>>>>> - if (pte_val(pteval) & _PAGE_GLOBAL) {
> >>>>>>>> - pte_t *buddy = ptep_buddy(ptep);
> >>>>>>>> - /*
> >>>>>>>> - * Make sure the buddy is global too (if it's !none,
> >>>>>>>> - * it better already be global)
> >>>>>>>> - */
> >>>>>>>> - if (pte_none(ptep_get(buddy))) {
> >>>>>>>> -#ifdef CONFIG_SMP
> >>>>>>>> - /*
> >>>>>>>> - * For SMP, multiple CPUs can race, so we need
> >>>>>>>> - * to do this atomically.
> >>>>>>>> - */
> >>>>>>>> - __asm__ __volatile__(
> >>>>>>>> - __AMOR "$zero, %[global], %[buddy] \n"
> >>>>>>>> - : [buddy] "+ZB" (buddy->pte)
> >>>>>>>> - : [global] "r" (_PAGE_GLOBAL)
> >>>>>>>> - : "memory");
> >>>>>>>> -
> >>>>>>>> - DBAR(0b11000); /* o_wrw = 0b11000 */
> >>>>>>>> -#else /* !CONFIG_SMP */
> >>>>>>>> - WRITE_ONCE(*buddy, __pte(pte_val(ptep_get(buddy)) | _PAGE_GLOBAL));
> >>>>>>>> -#endif /* CONFIG_SMP */
> >>>>>>>> - }
> >>>>>>>> - }
> >>>>>>>> + DBAR(0b11000); /* o_wrw = 0b11000 */
> >>>>>>>> }
> >>>>>>>>
> >>>>>>>> No, please hold on. This issue exists about twenty years, Do we need be
> >>>>>>>> in such a hurry now?
> >>>>>>>>
> >>>>>>>> why is DBAR(0b11000) added in set_pte()?
> >>>>>>> It exists before, not added by this patch. The reason is explained in
> >>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.12-rc3&id=f93f67d06b1023313ef1662eac490e29c025c030
> >>>>>> why speculative accesses may cause spurious page fault in kernel space
> >>>>>> with PTE enabled? speculative accesses exists anywhere, it does not
> >>>>>> cause spurious page fault.
> >>>>> Confirmed by Ruiyang Wu, and even if DBAR(0b11000) is wrong, that
> >>>>> means another patch's mistake, not this one. This one just keeps the
> >>>>> old behavior.
> >>>>> +CC Ruiyang Wu here.
> >>>> Also from Ruiyang Wu, the information is that speculative accesses may
> >>>> insert stale TLB, however no page fault exception.
> >>>>
> >>>> So adding barrier in set_pte() does not prevent speculative accesses.
> >>>> And you write patch here, however do not know the actual reason?
> >>>>
> >>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v6.12-rc3&id=f93f67d06b1023313ef1662eac490e29c025c030
> >>> I have CCed Ruiyang, whether the description is correct can be judged by him.
> >>
> >> There are some problems to add barrier() in set_pte():
> >>
> >> 1. There is such issue only for HW ptw enabled and kernel address space,
> >> is that? Also it may be two heavy to add barrier in set_pte(), comparing
> >> to do this in flush_cache_vmap().
> > So adding a barrier in set_pte() may not be the best solution for
> > performance, but you cannot say it is a wrong solution. And yes, we
> > can only care the kernel space, which is also the old behavior before
> > this patch, so set_pte() should be:
> >
> > static inline void set_pte(pte_t *ptep, pte_t pteval)
> > {
> > WRITE_ONCE(*ptep, pteval);
> > #ifdef CONFIG_SMP
> > if (pte_val(pteval) & _PAGE_GLOBAL)
> cpu_has_ptw seems also need here, if it is only for hw page walk.
> > DBAR(0b11000); /* o_wrw = 0b11000 */
> > #endif
> > }
> >
> > Putting a dbar unconditionally in set_pte() is my mistake, I'm sorry for that.
> >
> >>
> >> 2. LoongArch is different with other other architectures, two pages are
> >> included in one TLB entry. If there is two consecutive page mapped and
> >> memory access, there will page fault for the second memory access. Such
> >> as:
> >> addr1 =percpu_alloc(pagesize);
> >> val1 = *(int *)addr1;
> >> // With page table walk, addr1 is present and addr2 is pte_none
> >> // TLB entry includes valid pte for addr1, invalid pte for addr2
> >> addr2 =percpu_alloc(pagesize); // will not flush tlb in first time
> >> val2 = *(int *)addr2;
> >> // With page table walk, addr1 is present and addr2 is present also
> >> // TLB entry includes valid pte for addr1, invalid pte for addr2
> >> So there will be page fault when accessing address addr2
> >>
> >> There there is the same problem with user address space. By the way,
> >> there is HW prefetching technology, negative effective of HW prefetching
> >> technology will be tlb added. So there is potential page fault if memory
> >> is allocated and accessed in the first time.
> > As discussed internally, there may be three problems related to
> > speculative access in detail: 1) a load/store after set_pte() is
> > prioritized before, which can be prevented by dbar, 2) a instruction
> > fetch after set_pte() is prioritized before, which can be prevented by
> > ibar, 3) the buddy tlb problem you described here, if I understand
> > Ruiyang's explanation correctly this can only be prevented by the
> > filter in do_page_fault().
> >
> > From experiments, without the patch "LoongArch: Improve hardware page
> > table walker", there are about 80 times of spurious page faults during
> > boot, and increases continually during stress tests. And after that
> > patch which adds a dbar to set_pte(), we cannot observe spurious page
> > faults anymore. Of course this doesn't mean 2) and 3) don't exist, but
> Good experiment result. Could you share me code about page fault
> counting and test cases?
Counting method:
1, Add a simple printk at the beginning of spurious_fault(), and count
the number of printk from dmesg.
2, Test case: boot Fedora to desktop, and then run kernel building
work with "make -j8" in the system.

Huacai

>
> > we can at least say 1) is the main case. On this basis, in "LoongArch:
> > Improve hardware page table walker" we use a relatively cheap dbar
> > (compared to ibar) to prevent the main case, and add a filter to
> > handle 2) and 3). Such a solution is reasonable.
> >
> >
> >>
> >> 3. For speculative execution, if it is user address, there is eret from
> >> syscall. eret will rollback all speculative execution instruction. So it
> >> is only problem for speculative execution. And how to verify whether it
> >> is the problem of speculative execution or it is the problem of clause 2?
> > As described above, if spurious page faults still exist after adding
> > dbar to set_pte(), it may be a problem of clause 2 (case 3 in my
> > description), otherwise it is not a problem of clause 2.
> >
> > At last, this patch itself is attempting to solve the concurrent
> > problem about _PAGE_GLOBAL, so adding pte_alloc_one_kernel() and
> > removing the buddy stuff in set_pte() are what it needs. However it
> > shouldn't touch the logic of dbar in set_pte(), whether "LoongArch:
> > Improve hardware page table walker" is right or wrong.
> yes, I agree. We can discuss set_pte() issue in later. Simple for this
> patch to solve concurrent problem, it is ok
> https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git/diff/mm/kasan/init.c?h=loongarch-next&id=15832255e84494853f543b4c70ced50afc403067
>
> Regards
> Bibo Mao
> >
> >
> > Huacai
> >
> >>
> >> Regards
> >> Bibo Mao
> >>
> >>
> >>>
> >>> Huacai
> >>>
> >>>>
> >>>> Bibo Mao
> >>>>>
> >>>>> Huacai
> >>>>>
> >>>>>>
> >>>>>> Obvious you do not it and you write wrong patch.
> >>>>>>
> >>>>>>>
> >>>>>>> Huacai
> >>>>>>>
> >>>>>>>>
> >>>>>>>> Regards
> >>>>>>>> Bibo Mao
> >>>>>>>>> Huacai
> >>>>>>>>>
> >>>>>>>>> On Mon, Oct 14, 2024 at 11:59 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
> >>>>>>>>>>
> >>>>>>>>>> Unlike general architectures, there are two pages in one TLB entry
> >>>>>>>>>> on LoongArch system. For kernel space, it requires both two pte
> >>>>>>>>>> entries with PAGE_GLOBAL bit set, else HW treats it as non-global
> >>>>>>>>>> tlb, there will be potential problems if tlb entry for kernel space
> >>>>>>>>>> is not global. Such as fail to flush kernel tlb with function
> >>>>>>>>>> local_flush_tlb_kernel_range() which only flush tlb with global bit.
> >>>>>>>>>>
> >>>>>>>>>> With function kernel_pte_init() added, it can be used to init pte
> >>>>>>>>>> table when it is created for kernel address space, and the default
> >>>>>>>>>> initial pte value is PAGE_GLOBAL rather than zero at beginning.
> >>>>>>>>>>
> >>>>>>>>>> Kernel address space areas includes fixmap, percpu, vmalloc, kasan
> >>>>>>>>>> and vmemmap areas set default pte entry with PAGE_GLOBAL set.
> >>>>>>>>>>
> >>>>>>>>>> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> >>>>>>>>>> ---
> >>>>>>>>>> arch/loongarch/include/asm/pgalloc.h | 13 +++++++++++++
> >>>>>>>>>> arch/loongarch/include/asm/pgtable.h | 1 +
> >>>>>>>>>> arch/loongarch/mm/init.c | 4 +++-
> >>>>>>>>>> arch/loongarch/mm/kasan_init.c | 4 +++-
> >>>>>>>>>> arch/loongarch/mm/pgtable.c | 22 ++++++++++++++++++++++
> >>>>>>>>>> include/linux/mm.h | 1 +
> >>>>>>>>>> mm/kasan/init.c | 8 +++++++-
> >>>>>>>>>> mm/sparse-vmemmap.c | 5 +++++
> >>>>>>>>>> 8 files changed, 55 insertions(+), 3 deletions(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/arch/loongarch/include/asm/pgalloc.h b/arch/loongarch/include/asm/pgalloc.h
> >>>>>>>>>> index 4e2d6b7ca2ee..b2698c03dc2c 100644
> >>>>>>>>>> --- a/arch/loongarch/include/asm/pgalloc.h
> >>>>>>>>>> +++ b/arch/loongarch/include/asm/pgalloc.h
> >>>>>>>>>> @@ -10,8 +10,21 @@
> >>>>>>>>>>
> >>>>>>>>>> #define __HAVE_ARCH_PMD_ALLOC_ONE
> >>>>>>>>>> #define __HAVE_ARCH_PUD_ALLOC_ONE
> >>>>>>>>>> +#define __HAVE_ARCH_PTE_ALLOC_ONE_KERNEL
> >>>>>>>>>> #include <asm-generic/pgalloc.h>
> >>>>>>>>>>
> >>>>>>>>>> +static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
> >>>>>>>>>> +{
> >>>>>>>>>> + pte_t *pte;
> >>>>>>>>>> +
> >>>>>>>>>> + pte = (pte_t *) __get_free_page(GFP_KERNEL);
> >>>>>>>>>> + if (!pte)
> >>>>>>>>>> + return NULL;
> >>>>>>>>>> +
> >>>>>>>>>> + kernel_pte_init(pte);
> >>>>>>>>>> + return pte;
> >>>>>>>>>> +}
> >>>>>>>>>> +
> >>>>>>>>>> static inline void pmd_populate_kernel(struct mm_struct *mm,
> >>>>>>>>>> pmd_t *pmd, pte_t *pte)
> >>>>>>>>>> {
> >>>>>>>>>> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
> >>>>>>>>>> index 9965f52ef65b..22e3a8f96213 100644
> >>>>>>>>>> --- a/arch/loongarch/include/asm/pgtable.h
> >>>>>>>>>> +++ b/arch/loongarch/include/asm/pgtable.h
> >>>>>>>>>> @@ -269,6 +269,7 @@ extern void set_pmd_at(struct mm_struct *mm, unsigned long addr, pmd_t *pmdp, pm
> >>>>>>>>>> extern void pgd_init(void *addr);
> >>>>>>>>>> extern void pud_init(void *addr);
> >>>>>>>>>> extern void pmd_init(void *addr);
> >>>>>>>>>> +extern void kernel_pte_init(void *addr);
> >>>>>>>>>>
> >>>>>>>>>> /*
> >>>>>>>>>> * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
> >>>>>>>>>> diff --git a/arch/loongarch/mm/init.c b/arch/loongarch/mm/init.c
> >>>>>>>>>> index 8a87a482c8f4..9f26e933a8a3 100644
> >>>>>>>>>> --- a/arch/loongarch/mm/init.c
> >>>>>>>>>> +++ b/arch/loongarch/mm/init.c
> >>>>>>>>>> @@ -198,9 +198,11 @@ pte_t * __init populate_kernel_pte(unsigned long addr)
> >>>>>>>>>> if (!pmd_present(pmdp_get(pmd))) {
> >>>>>>>>>> pte_t *pte;
> >>>>>>>>>>
> >>>>>>>>>> - pte = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
> >>>>>>>>>> + pte = memblock_alloc_raw(PAGE_SIZE, PAGE_SIZE);
> >>>>>>>>>> if (!pte)
> >>>>>>>>>> panic("%s: Failed to allocate memory\n", __func__);
> >>>>>>>>>> +
> >>>>>>>>>> + kernel_pte_init(pte);
> >>>>>>>>>> pmd_populate_kernel(&init_mm, pmd, pte);
> >>>>>>>>>> }
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/arch/loongarch/mm/kasan_init.c b/arch/loongarch/mm/kasan_init.c
> >>>>>>>>>> index 427d6b1aec09..34988573b0d5 100644
> >>>>>>>>>> --- a/arch/loongarch/mm/kasan_init.c
> >>>>>>>>>> +++ b/arch/loongarch/mm/kasan_init.c
> >>>>>>>>>> @@ -152,6 +152,8 @@ static void __init kasan_pte_populate(pmd_t *pmdp, unsigned long addr,
> >>>>>>>>>> phys_addr_t page_phys = early ?
> >>>>>>>>>> __pa_symbol(kasan_early_shadow_page)
> >>>>>>>>>> : kasan_alloc_zeroed_page(node);
> >>>>>>>>>> + if (!early)
> >>>>>>>>>> + kernel_pte_init(__va(page_phys));
> >>>>>>>>>> next = addr + PAGE_SIZE;
> >>>>>>>>>> set_pte(ptep, pfn_pte(__phys_to_pfn(page_phys), PAGE_KERNEL));
> >>>>>>>>>> } while (ptep++, addr = next, addr != end && __pte_none(early, ptep_get(ptep)));
> >>>>>>>>>> @@ -287,7 +289,7 @@ void __init kasan_init(void)
> >>>>>>>>>> set_pte(&kasan_early_shadow_pte[i],
> >>>>>>>>>> pfn_pte(__phys_to_pfn(__pa_symbol(kasan_early_shadow_page)), PAGE_KERNEL_RO));
> >>>>>>>>>>
> >>>>>>>>>> - memset(kasan_early_shadow_page, 0, PAGE_SIZE);
> >>>>>>>>>> + kernel_pte_init(kasan_early_shadow_page);
> >>>>>>>>>> csr_write64(__pa_symbol(swapper_pg_dir), LOONGARCH_CSR_PGDH);
> >>>>>>>>>> local_flush_tlb_all();
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/arch/loongarch/mm/pgtable.c b/arch/loongarch/mm/pgtable.c
> >>>>>>>>>> index eb6a29b491a7..228ffc1db0a3 100644
> >>>>>>>>>> --- a/arch/loongarch/mm/pgtable.c
> >>>>>>>>>> +++ b/arch/loongarch/mm/pgtable.c
> >>>>>>>>>> @@ -38,6 +38,28 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> >>>>>>>>>> }
> >>>>>>>>>> EXPORT_SYMBOL_GPL(pgd_alloc);
> >>>>>>>>>>
> >>>>>>>>>> +void kernel_pte_init(void *addr)
> >>>>>>>>>> +{
> >>>>>>>>>> + unsigned long *p, *end;
> >>>>>>>>>> + unsigned long entry;
> >>>>>>>>>> +
> >>>>>>>>>> + entry = (unsigned long)_PAGE_GLOBAL;
> >>>>>>>>>> + p = (unsigned long *)addr;
> >>>>>>>>>> + end = p + PTRS_PER_PTE;
> >>>>>>>>>> +
> >>>>>>>>>> + do {
> >>>>>>>>>> + p[0] = entry;
> >>>>>>>>>> + p[1] = entry;
> >>>>>>>>>> + p[2] = entry;
> >>>>>>>>>> + p[3] = entry;
> >>>>>>>>>> + p[4] = entry;
> >>>>>>>>>> + p += 8;
> >>>>>>>>>> + p[-3] = entry;
> >>>>>>>>>> + p[-2] = entry;
> >>>>>>>>>> + p[-1] = entry;
> >>>>>>>>>> + } while (p != end);
> >>>>>>>>>> +}
> >>>>>>>>>> +
> >>>>>>>>>> void pgd_init(void *addr)
> >>>>>>>>>> {
> >>>>>>>>>> unsigned long *p, *end;
> >>>>>>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >>>>>>>>>> index ecf63d2b0582..6909fe059a2c 100644
> >>>>>>>>>> --- a/include/linux/mm.h
> >>>>>>>>>> +++ b/include/linux/mm.h
> >>>>>>>>>> @@ -3818,6 +3818,7 @@ void *sparse_buffer_alloc(unsigned long size);
> >>>>>>>>>> struct page * __populate_section_memmap(unsigned long pfn,
> >>>>>>>>>> unsigned long nr_pages, int nid, struct vmem_altmap *altmap,
> >>>>>>>>>> struct dev_pagemap *pgmap);
> >>>>>>>>>> +void kernel_pte_init(void *addr);
> >>>>>>>>>> void pmd_init(void *addr);
> >>>>>>>>>> void pud_init(void *addr);
> >>>>>>>>>> pgd_t *vmemmap_pgd_populate(unsigned long addr, int node);
> >>>>>>>>>> diff --git a/mm/kasan/init.c b/mm/kasan/init.c
> >>>>>>>>>> index 89895f38f722..ac607c306292 100644
> >>>>>>>>>> --- a/mm/kasan/init.c
> >>>>>>>>>> +++ b/mm/kasan/init.c
> >>>>>>>>>> @@ -106,6 +106,10 @@ static void __ref zero_pte_populate(pmd_t *pmd, unsigned long addr,
> >>>>>>>>>> }
> >>>>>>>>>> }
> >>>>>>>>>>
> >>>>>>>>>> +void __weak __meminit kernel_pte_init(void *addr)
> >>>>>>>>>> +{
> >>>>>>>>>> +}
> >>>>>>>>>> +
> >>>>>>>>>> static int __ref zero_pmd_populate(pud_t *pud, unsigned long addr,
> >>>>>>>>>> unsigned long end)
> >>>>>>>>>> {
> >>>>>>>>>> @@ -126,8 +130,10 @@ static int __ref zero_pmd_populate(pud_t *pud, unsigned long addr,
> >>>>>>>>>>
> >>>>>>>>>> if (slab_is_available())
> >>>>>>>>>> p = pte_alloc_one_kernel(&init_mm);
> >>>>>>>>>> - else
> >>>>>>>>>> + else {
> >>>>>>>>>> p = early_alloc(PAGE_SIZE, NUMA_NO_NODE);
> >>>>>>>>>> + kernel_pte_init(p);
> >>>>>>>>>> + }
> >>>>>>>>>> if (!p)
> >>>>>>>>>> return -ENOMEM;
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> >>>>>>>>>> index edcc7a6b0f6f..c0388b2e959d 100644
> >>>>>>>>>> --- a/mm/sparse-vmemmap.c
> >>>>>>>>>> +++ b/mm/sparse-vmemmap.c
> >>>>>>>>>> @@ -184,6 +184,10 @@ static void * __meminit vmemmap_alloc_block_zero(unsigned long size, int node)
> >>>>>>>>>> return p;
> >>>>>>>>>> }
> >>>>>>>>>>
> >>>>>>>>>> +void __weak __meminit kernel_pte_init(void *addr)
> >>>>>>>>>> +{
> >>>>>>>>>> +}
> >>>>>>>>>> +
> >>>>>>>>>> pmd_t * __meminit vmemmap_pmd_populate(pud_t *pud, unsigned long addr, int node)
> >>>>>>>>>> {
> >>>>>>>>>> pmd_t *pmd = pmd_offset(pud, addr);
> >>>>>>>>>> @@ -191,6 +195,7 @@ pmd_t * __meminit vmemmap_pmd_populate(pud_t *pud, unsigned long addr, int node)
> >>>>>>>>>> void *p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
> >>>>>>>>>> if (!p)
> >>>>>>>>>> return NULL;
> >>>>>>>>>> + kernel_pte_init(p);
> >>>>>>>>>> pmd_populate_kernel(&init_mm, pmd, p);
> >>>>>>>>>> }
> >>>>>>>>>> return pmd;
> >>>>>>>>>> --
> >>>>>>>>>> 2.39.3
> >>>>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>
> >>>>>>
> >>>>
> >>>>
> >>
>

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