Re: [PATCH v7 3/6] mce: fix set_mce_nospec to always unmap the whole page

From: Jane Chu
Date: Wed Apr 13 2022 - 19:36:57 EST

On 4/11/2022 4:27 PM, Dan Williams wrote:
> On Tue, Apr 5, 2022 at 12:48 PM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
>>
>> The set_memory_uc() approach doesn't work well in all cases.
>> For example, when "The VMM unmapped the bad page from guest
>> physical space and passed the machine check to the guest."
>> "The guest gets virtual #MC on an access to that page.
>> When the guest tries to do set_memory_uc() and instructs
>> cpa_flush() to do clean caches that results in taking another
>> fault / exception perhaps because the VMM unmapped the page
>> from the guest."
>>
>> Since the driver has special knowledge to handle NP or UC,
>
> I think a patch is needed before this one to make this statement true? I.e.:
>
> diff --git a/drivers/acpi/nfit/mce.c b/drivers/acpi/nfit/mce.c
> index ee8d9973f60b..11641f55025a 100644
> --- a/drivers/acpi/nfit/mce.c
> +++ b/drivers/acpi/nfit/mce.c
> @@ -32,6 +32,7 @@ static int nfit_handle_mce(struct notifier_block
> *nb, unsigned long val,
> */
> mutex_lock(&acpi_desc_lock);
> list_for_each_entry(acpi_desc, &acpi_descs, list) {
> + unsigned int align = 1UL << MCI_MISC_ADDR_LSB(mce->misc);
> struct device *dev = acpi_desc->dev;
> int found_match = 0;
>
> @@ -63,8 +64,7 @@ static int nfit_handle_mce(struct notifier_block
> *nb, unsigned long val,
>
> /* If this fails due to an -ENOMEM, there is little we can do */
> nvdimm_bus_add_badrange(acpi_desc->nvdimm_bus,
> - ALIGN(mce->addr, L1_CACHE_BYTES),
> - L1_CACHE_BYTES);
> + ALIGN(mce->addr, align), align);
> nvdimm_region_notify(nfit_spa->nd_region,
> NVDIMM_REVALIDATE_POISON);
>

Dan, I tried the above change, and this is what I got after injecting 8
back-to-back poisons, then read them and received SIGBUS/BUS_MCEERR_AR,
then repair via the v7 patch which works until this change is added.

[ 6240.955331] nfit ACPI0012:00: XXX, align = 100
[ 6240.960300] nfit ACPI0012:00: XXX, ALIGN(mce->addr,
L1_CACHE_BYTES)=1851600400, L1_CACHE_BYTES=40, ALIGN(mce->addr,
align)=1851600400
[..]
[ 6242.052277] nfit ACPI0012:00: XXX, align = 100
[ 6242.057243] nfit ACPI0012:00: XXX, ALIGN(mce->addr,
L1_CACHE_BYTES)=1851601000, L1_CACHE_BYTES=40, ALIGN(mce->addr,
align)=1851601000
[..]
[ 6244.917198] nfit ACPI0012:00: XXX, align = 1000
[ 6244.922258] nfit ACPI0012:00: XXX, ALIGN(mce->addr,
L1_CACHE_BYTES)=1851601200, L1_CACHE_BYTES=40, ALIGN(mce->addr,
align)=1851602000
[..]

All 8 poisons remain uncleared.

Without further investigation, I don't know why the failure.
Could we mark this change to a follow-on task?
The driver knows a lot about how to clear poisons besides hardcoding
poison alignment to 0x40 bytes.

>
>> let's mark the poisoned page with NP and let driver handle it
>> when it comes down to repair.
>>
>> Please refer to discussions here for more details.
>> https://lore.kernel.org/all/CAPcyv4hrXPb1tASBZUg-GgdVs0OOFKXMXLiHmktg_kFi7YBMyQ@xxxxxxxxxxxxxx/
>>
>> Now since poisoned page is marked as not-present, in order to
>> avoid writing to a 'np' page and trigger kernel Oops, also fix
>> pmem_do_write().
>>
>> Fixes: 284ce4011ba6 ("x86/memory_failure: Introduce {set, clear}_mce_nospec()")
>> Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
>> ---
>> arch/x86/kernel/cpu/mce/core.c | 6 +++---
>> arch/x86/mm/pat/set_memory.c | 18 ++++++------------
>> drivers/nvdimm/pmem.c | 31 +++++++------------------------
>> include/linux/set_memory.h | 4 ++--
>> 4 files changed, 18 insertions(+), 41 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
>> index 981496e6bc0e..fa67bb9d1afe 100644
>> --- a/arch/x86/kernel/cpu/mce/core.c
>> +++ b/arch/x86/kernel/cpu/mce/core.c
>> @@ -579,7 +579,7 @@ static int uc_decode_notifier(struct notifier_block *nb, unsigned long val,
>>
>> pfn = mce->addr >> PAGE_SHIFT;
>> if (!memory_failure(pfn, 0)) {
>> - set_mce_nospec(pfn, whole_page(mce));
>> + set_mce_nospec(pfn);
>> mce->kflags |= MCE_HANDLED_UC;
>> }
>>
>> @@ -1316,7 +1316,7 @@ static void kill_me_maybe(struct callback_head *cb)
>>
>> ret = memory_failure(p->mce_addr >> PAGE_SHIFT, flags);
>> if (!ret) {
>> - set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
>> + set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>> sync_core();
>> return;
>> }
>> @@ -1342,7 +1342,7 @@ static void kill_me_never(struct callback_head *cb)
>> p->mce_count = 0;
>> pr_err("Kernel accessed poison in user space at %llx\n", p->mce_addr);
>> if (!memory_failure(p->mce_addr >> PAGE_SHIFT, 0))
>> - set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
>> + set_mce_nospec(p->mce_addr >> PAGE_SHIFT);
>> }
>>
>> static void queue_task_work(struct mce *m, char *msg, void (*func)(struct callback_head *))
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index 93dde949f224..404ffcb3f2cb 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -1926,13 +1926,8 @@ int set_memory_wb(unsigned long addr, int numpages)
>> EXPORT_SYMBOL(set_memory_wb);
>>
>> #ifdef CONFIG_X86_64
>> -/*
>> - * Prevent speculative access to the page by either unmapping
>> - * it (if we do not require access to any part of the page) or
>> - * marking it uncacheable (if we want to try to retrieve data
>> - * from non-poisoned lines in the page).
>> - */
>> -int set_mce_nospec(unsigned long pfn, bool unmap)
>> +/* Prevent speculative access to a page by marking it not-present */
>> +int set_mce_nospec(unsigned long pfn)
>> {
>> unsigned long decoy_addr;
>> int rc;
>> @@ -1954,10 +1949,7 @@ int set_mce_nospec(unsigned long pfn, bool unmap)
>> */
>> decoy_addr = (pfn << PAGE_SHIFT) + (PAGE_OFFSET ^ BIT(63));
>>
>> - if (unmap)
>> - rc = set_memory_np(decoy_addr, 1);
>> - else
>> - rc = set_memory_uc(decoy_addr, 1);
>> + rc = set_memory_np(decoy_addr, 1);
>> if (rc)
>> pr_warn("Could not invalidate pfn=0x%lx from 1:1 map\n", pfn);
>> return rc;
>> @@ -1966,7 +1958,9 @@ int set_mce_nospec(unsigned long pfn, bool unmap)
>> /* Restore full speculative operation to the pfn. */
>> int clear_mce_nospec(unsigned long pfn)
>> {
>> - return set_memory_wb((unsigned long) pfn_to_kaddr(pfn), 1);
>> + unsigned long addr = (unsigned long) pfn_to_kaddr(pfn);
>> +
>> + return change_page_attr_set(&addr, 1, __pgprot(_PAGE_PRESENT), 0);
>
> This probably warrants a set_memory_present() helper.

I had a set_memory_present() helper in an earlier version, but as there
is no other caller, also if I'm not mis-remembering, thought Christoph
had suggested to simplify the code and I agreed. If you feel strong
about adding the helper back, I can do that to improve readibility.

>
>> }
>> EXPORT_SYMBOL_GPL(clear_mce_nospec);
>>
>> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
>> index 58d95242a836..30c71a68175b 100644
>> --- a/drivers/nvdimm/pmem.c
>> +++ b/drivers/nvdimm/pmem.c
>> @@ -158,36 +158,19 @@ static blk_status_t pmem_do_write(struct pmem_device *pmem,
>> struct page *page, unsigned int page_off,
>> sector_t sector, unsigned int len)
>> {
>> - blk_status_t rc = BLK_STS_OK;
>> - bool bad_pmem = false;
>> phys_addr_t pmem_off = sector * 512 + pmem->data_offset;
>> void *pmem_addr = pmem->virt_addr + pmem_off;
>>
>> - if (unlikely(is_bad_pmem(&pmem->bb, sector, len)))
>> - bad_pmem = true;
>> + if (unlikely(is_bad_pmem(&pmem->bb, sector, len))) {
>> + blk_status_t rc = pmem_clear_poison(pmem, pmem_off, len);
>>
>> - /*
>> - * Note that we write the data both before and after
>> - * clearing poison. The write before clear poison
>> - * handles situations where the latest written data is
>> - * preserved and the clear poison operation simply marks
>> - * the address range as valid without changing the data.
>> - * In this case application software can assume that an
>> - * interrupted write will either return the new good
>> - * data or an error.
>> - *
>> - * However, if pmem_clear_poison() leaves the data in an
>> - * indeterminate state we need to perform the write
>> - * after clear poison.
>> - */
>> + if (rc != BLK_STS_OK)
>> + pr_warn_ratelimited("%s: failed to clear poison\n", __func__);
>
> This should be either "dev_warn_ratelimited(to_dev(pmem), ...", or a
> trace point similar to trace_block_rq_complete() that tells userspace
> about adverse I/O completion results.
>
> However, that's probably a discussion for another patch, so I would
> just drop this new addition for now and we can discuss the logging in
> a follow-on patch.

Okay, drop the warning message. If the unexpected pathological scenario
happens, user will see I/O failure.

>
>
>> + return rc;
>> + }
>> flush_dcache_page(page);
>> write_pmem(pmem_addr, page, page_off, len);
>> - if (unlikely(bad_pmem)) {
>> - rc = pmem_clear_poison(pmem, pmem_off, len);
>> - write_pmem(pmem_addr, page, page_off, len);
>> - }
>> -
>> - return rc;
>> + return BLK_STS_OK;
>> }
>>
>> static void pmem_submit_bio(struct bio *bio)
>> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
>> index d6263d7afb55..cde2d8687a7b 100644
>> --- a/include/linux/set_memory.h
>> +++ b/include/linux/set_memory.h
>> @@ -43,10 +43,10 @@ static inline bool can_set_direct_map(void)
>> #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>>
>> #ifdef CONFIG_X86_64
>> -int set_mce_nospec(unsigned long pfn, bool unmap);
>> +int set_mce_nospec(unsigned long pfn);
>> int clear_mce_nospec(unsigned long pfn);
>> #else
>> -static inline int set_mce_nospec(unsigned long pfn, bool unmap)
>> +static inline int set_mce_nospec(unsigned long pfn)
>
> Looks like after this change the "whole_page()" helper can be deleted
> and the ->mce_whole_page flag in the task_struct can also go, right?
> In a follow-on of course.

Okay, will do in a follow-on patch.

>
> Other than those small issues, this looks good!

thanks!
-jane