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Thu, 02 Jan 2025 09:07:36 +0000 (GMT) Received: from nalasex01a.na.qualcomm.com (nalasex01a.na.qualcomm.com [10.47.209.196]) by NALASPPMTA04.qualcomm.com (8.18.1.2/8.18.1.2) with ESMTPS id 50297Z3J007525 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 2 Jan 2025 09:07:35 GMT Received: from [10.239.132.245] (10.80.80.8) by nalasex01a.na.qualcomm.com (10.47.209.196) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.9; Thu, 2 Jan 2025 01:07:30 -0800 Message-ID: Date: Thu, 2 Jan 2025 17:07:29 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned To: Anshuman Khandual , Catalin Marinas CC: , , , , , , , , , , , , Tingwei Zhang References: <20241209094227.1529977-1-quic_zhenhuah@quicinc.com> <20241209094227.1529977-2-quic_zhenhuah@quicinc.com> <971b9a05-4ae0-4e6c-8e48-e9e529896ecf@arm.com> <22fd3452-748c-4d4b-bd51-08e6faeaa867@quicinc.com> <3aadda40-c973-4703-8ed8-9cf2d3eb70a0@quicinc.com> <3af539ac-d241-4349-be37-9f41ee42c86c@arm.com> Content-Language: en-US From: Zhenhua Huang In-Reply-To: <3af539ac-d241-4349-be37-9f41ee42c86c@arm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nalasex01a.na.qualcomm.com (10.47.209.196) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-ORIG-GUID: rIv5tPZ79GRWYc-1_COEMldHG3_-czBj X-Proofpoint-GUID: rIv5tPZ79GRWYc-1_COEMldHG3_-czBj X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.680,FMLib:17.12.60.29 definitions=2024-09-06_09,2024-09-06_01,2024-09-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 malwarescore=0 mlxscore=0 spamscore=0 impostorscore=0 suspectscore=0 priorityscore=1501 lowpriorityscore=0 clxscore=1015 adultscore=0 phishscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2411120000 definitions=main-2501020077 On 2025/1/2 11:16, Anshuman Khandual wrote: > > > On 12/31/24 11:22, Zhenhua Huang wrote: >> >> >> On 2024/12/30 15:48, Zhenhua Huang wrote: >>> Hi Anshuman, >>> >>> On 2024/12/27 15:49, Anshuman Khandual wrote: >>>> On 12/24/24 19:39, Catalin Marinas wrote: >>>>> On Tue, Dec 24, 2024 at 05:32:06PM +0800, Zhenhua Huang wrote: >>>>>> Thanks Catalin for review! >>>>>> Merry Christmas. >>>>> >>>>> Merry Christmas to you too! >>>>> >>>>>> On 2024/12/21 2:30, Catalin Marinas wrote: >>>>>>> On Mon, Dec 09, 2024 at 05:42:26PM +0800, Zhenhua Huang wrote: >>>>>>>> Fixes: c1cc1552616d ("arm64: MMU initialisation") >>>>>>> >>>>>>> I wouldn't add a fix for the first commit adding arm64 support, we did >>>>>>> not even have memory hotplug at the time (added later in 5.7 by commit >>>>>>> bbd6ec605c0f ("arm64/mm: Enable memory hot remove")). IIUC, this hasn't >>>>>>> been a problem until commit ba72b4c8cf60 ("mm/sparsemem: support >>>>>>> sub-section hotplug"). That commit broke some arm64 assumptions. >>>>>> >>>>>> Shall we add ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") >>>>>> because it broke arm64 assumptions ? >>>>> >>>>> Yes, I think that would be better. And a cc stable to 5.4 (the above >>>>> commit appeared in 5.3). >>>> >>>> Agreed. This is a problem which needs fixing but not sure if proposed patch >>>> here fixes that problem. >>>> >>>>> >>>>>>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>>>>>> index e2739b69e11b..fd59ee44960e 100644 >>>>>>>> --- a/arch/arm64/mm/mmu.c >>>>>>>> +++ b/arch/arm64/mm/mmu.c >>>>>>>> @@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>>>>>>>    { >>>>>>>>        WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>>>>>>> -    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >>>>>>>> +    if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || >>>>>>>> +    !IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) || >>>>>>>> +    !IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION)) >>>>>>>>            return vmemmap_populate_basepages(start, end, node, altmap); >>>>>>>>        else >>>>>>>>            return vmemmap_populate_hugepages(start, end, node, altmap); >>>>>>> >>>>>>> An alternative would be to fix unmap_hotplug_pmd_range() etc. to avoid >>>>>>> nuking the whole vmemmap pmd section if it's not empty. Not sure how >>>>>>> easy that is, whether we have the necessary information (I haven't >>>>>>> looked in detail). >>>>>>> >>>>>>> A potential issue - can we hotplug 128MB of RAM and only unplug 2MB? If >>>>>>> that's possible, the problem isn't solved by this patch. >>>> >>>> Believe this is possible after sub-section hotplug and hotremove support. >>>> >>>>>> >>>>>> Indeed, seems there is no guarantee that plug size must be equal to unplug >>>>>> size... >>>>>> >>>>>> I have two ideas: >>>>>> 1. Completely disable this PMD mapping optimization since there is no >>>>>> guarantee we must align 128M memory for hotplug .. >>>>> >>>>> I'd be in favour of this, at least if CONFIG_MEMORY_HOTPLUG is enabled. >>>>> I think the only advantage here is that we don't allocate a full 2MB >>>>> block for vmemmap when only plugging in a sub-section. >>>> >>>> Agreed, that will be the right fix for the problem which can be back ported. >>>> We will have to prevent PMD/PUD/CONT mappings for both linear and as well as >>> >>> Thanks Anshuman, yeah.. we must handle linear mapping as well. >>> >>>> vmemmap for all non-boot memory sections, that can be hot-unplugged. >>>> >>>> Something like the following ? [untested] >>>> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index 216519663961..56b9c6891f46 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -1171,9 +1171,15 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node, >>>>   int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>>>                  struct vmem_altmap *altmap) >>>>   { >>>> +       unsigned long start_pfn; >>>> +       struct mem_section *ms; >>>> + >>>>          WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >>>> -       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >>>> +       start_pfn = page_to_pfn((struct page *)start); >>>> +       ms = __pfn_to_section(start_pfn); >>>> + >>>> +       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || !early_section(ms)) >>> >>> LGTM. I will follow your and Catalin's suggestion to prepare further patches, Thanks! >>> >>>>                  return vmemmap_populate_basepages(start, end, node, altmap); >>>>          else >>>>                  return vmemmap_populate_hugepages(start, end, node, altmap); >>>> @@ -1334,10 +1340,15 @@ struct range arch_get_mappable_range(void) >>>>   int arch_add_memory(int nid, u64 start, u64 size, >>>>                      struct mhp_params *params) >>>>   { >>>> +       unsigned long start_pfn = page_to_pfn((struct page *)start); >>>> +       struct mem_section *ms = __pfn_to_section(start_pfn); >>>>          int ret, flags = NO_EXEC_MAPPINGS; >>>>          VM_BUG_ON(!mhp_range_allowed(start, size, true)); >>>> +       if (!early_section(ms)) >>>> +               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >>> >>> However, here comes another doubt, given that the subsection size is 2M, shouldn't we have ability to support PMD SECTION MAPPING if CONFIG_ARM64_4K_PAGES? This might be the optimization we want to maintain? >>> >>> Should we remove NO_BLOCK_MAPPINGS and add more constraints to avoid pud_set_huge if CONFIG_ARM64_4K_PAGES ? >>> >> >> BTW, shall we remove the check for !early_section since arch_add_memory is only called during hotplugging case? Correct me please if I'm mistaken :) > > While this is true, still might be a good idea to keep the early_section() > check in place just to be extra careful here. Otherwise an WARN_ON() might > be needed. Make sense. I would like to add some comments and WARN_ON() if early_section(). > >> The idea is like(not fully tested): >> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >> index e2739b69e11b..9afeb35673a3 100644 >> --- a/arch/arm64/mm/mmu.c >> +++ b/arch/arm64/mm/mmu.c >> @@ -45,6 +45,7 @@ >>  #define NO_BLOCK_MAPPINGS      BIT(0) >>  #define NO_CONT_MAPPINGS       BIT(1) >>  #define NO_EXEC_MAPPINGS       BIT(2)  /* assumes FEAT_HPDS is not used */ >> +#define NO_PUD_BLOCK_MAPPINGS  BIT(3)  /* Hotplug case: do not want block mapping for PUD */ > > Since block mappings get created either at PMD or PUD, the existing flag > NO_BLOCK_MAPPINGS should be split into two i.e NO_PMD_BLOCK_MAPPINGS and > NO_PUD_BLOCK_MAPPINGS (possibly expanding into P4D later). Although all > block mappings can still be prevented using the existing flag which can > be derived from the new ones. > > #define NO_BLOCK_MAPPINGS (NO_PMD_BLOCK_MAPPINGS | NO_PUD_BLOCK_MAPPINGS) Thanks, it's more clear. > >> >>  u64 kimage_voffset __ro_after_init; >>  EXPORT_SYMBOL(kimage_voffset); >> @@ -356,10 +357,12 @@ static void alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end, >> >>                 /* >>                  * For 4K granule only, attempt to put down a 1GB block >> +                * Hotplug case: do not attempt 1GB block >>                  */ >>                 if (pud_sect_supported() && >>                    ((addr | next | phys) & ~PUD_MASK) == 0 && >> -                   (flags & NO_BLOCK_MAPPINGS) == 0) { >> +                   (flags & NO_BLOCK_MAPPINGS) == 0 && >> +                   (flags & NO_PUD_BLOCK_MAPPINGS) == 0) { > > After flags being split as suggested above, only the PUD block mapping > flag need to be checked here, and similarly the PMU block mapping flag > needs to be checked in alloc_init_pmd(). > >>                         pud_set_huge(pudp, phys, prot); >> >>                         /* >> @@ -1175,9 +1178,16 @@ int __meminit vmemmap_check_pmd(pmd_t *pmdp, int node, >>  int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node, >>                 struct vmem_altmap *altmap) >>  { >> +       unsigned long start_pfn; >> +       struct mem_section *ms; >> + >>         WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END)); >> >> -       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >> +       start_pfn = page_to_pfn((struct page *)start); >> +       ms = __pfn_to_section(start_pfn); >> + >> +       /* hotplugged section not support hugepages */ >> +       if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) || !early_section(ms)) >>                 return vmemmap_populate_basepages(start, end, node, altmap); >>         else >>                 return vmemmap_populate_hugepages(start, end, node, altmap); >> @@ -1342,6 +1352,16 @@ int arch_add_memory(int nid, u64 start, u64 size, >> >>         VM_BUG_ON(!mhp_range_allowed(start, size, true)); >> >> +       if (IS_ENABLED(CONFIG_ARM64_4K_PAGES)) >> +       /* >> +        * As per subsection granule is 2M, allow PMD block mapping in >> +        * case 4K PAGES. >> +        * Other cases forbid section mapping. >> +        */ >> +               flags |= NO_PUD_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >> +       else >> +               flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >> + >>         if (can_set_direct_map()) >>                 flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >> > > Basically vmmemap will not allow PMD or PUD block mapping for non boot > memory as a 2MB sized sub-section hot removal involves tearing down a > sub PMD i.e (512 * sizeof(struct page)) VA range, which is currently > not supported in unmap_hotplug_range(). > > Although linear mapping could still support PMD block mapping as hot > removing a 2MB sized sub-section will tear down an entire PMD entry. > > Fair enough but seems like this should be done after the fix patch > which prevents all block mappings for early section memory as that s/early section/non early section ? Sure, I will wait for Catalin/Will's comments. > will be an easy back port. But will leave this to upto Catalin/Will > to decide. > >> >> >>>> + >>>>          if (can_set_direct_map()) >>>>                  flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS; >>>> >>>>> >>>>>> 2. If we want to take this optimization. >>>>>> I propose adding one argument to vmemmap_free to indicate if the entire >>>>>> section is freed(based on subsection map). Vmemmap_free is a common function >>>>>> and might affect other architectures... The process would be: >>>>>> vmemmap_free >>>>>>     unmap_hotplug_range //In unmap_hotplug_pmd_range() as you mentioned：if >>>>>> whole section is freed, proceed as usual. Otherwise, *just clear out struct >>>>>> page content but do not free*. >>>>>>     free_empty_tables // will be called only if entire section is freed >>>>>> >>>>>> On the populate side, >>>>>> else if (vmemmap_check_pmd(pmd, node, addr, next)) //implement this function >>>>>>     continue;    //Buffer still exists, just abort.. >>>>>> >>>>>> Could you please comment further whether #2 is feasible ? >>>>> >>>>> vmemmap_free() already gets start/end, so it could at least check the >>>>> alignment and avoid freeing if it's not unplugging a full section. It >>>> >>>> unmap_hotplug_pmd_range() >>>> { >>>>     do { >>>>         if (pmd_sect(pmd)) { >>>>             pmd_clear(pmdp); >>>>             flush_tlb_kernel_range(addr, addr + PAGE_SIZE); >>>>                          if (free_mapped) >>>>                                  free_hotplug_page_range(pmd_page(pmd), >>>>                                                          PMD_SIZE, altmap); >>>>         } >>>>     } while () >>>> } >>>> >>>> Do you mean clearing the PMD entry but not freeing the mapped page for vmemmap ? >>>> In that case should the hot-unplug fail or not ? If we free the pfns (successful >>>> hot-unplug), then leaving behind entire PMD entry for covering the remaining sub >>>> sections, is going to be problematic as it still maps the removed pfns as well ! >>> >>> Could you please help me to understand in which scenarios this might cause issue? I assume we won't touch these struct page further? >>> >>>> >>>>> does leave a 2MB vmemmap block in place when freeing the last subsection >>>>> but it's safer than freeing valid struct page entries. In addition, it >>>>> could query the memory hotplug state with something like >>>>> find_memory_block() and figure out whether the section is empty. >>>> >>>> I guess there are two potential solutions, if unmap_hotplug_pmd_range() were to >>>> handle sub-section removal. >>>> >>>> 1) Skip pmd_clear() when entire section is not covered >>>> >>>> a. pmd_clear() only if all but the current subsection have been removed earlier >>>>     via is_subsection_map_empty() or something similar. >>>> >>>> b. Skip pmd_clear() if the entire section covering that PMD is not being removed >>>>     but that might be problematic, as it still maps potentially unavailable pfns, >>>>     which are now hot-unplugged out. >>>> >>>> 2) Break PMD into base pages >>>> >>>> a. pmd_clear() only if all but the current subsection have been removed earlier >>>>     via is_subsection_map_empty() or something similar. >>>> >>>> b. Break entire PMD into base page mappings and remove entries corresponding to >>>>     the subsection being removed. Although the BBM sequence needs to be followed >>>>     while making sure that no other part of the kernel is accessing subsections, >>>>     that are mapped via the erstwhile PMD but currently not being removed. >>>> >>>>> >>>>> Anyway, I'll be off until the new year, maybe I get other ideas by then. >>>>> >>> >>> >>