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diff --git a/Documentation/mm/hmm.rst b/Documentation/mm/hmm.rst new file mode 100644 index 0000000000..0595098a74 --- /dev/null +++ b/Documentation/mm/hmm.rst @@ -0,0 +1,441 @@ +===================================== +Heterogeneous Memory Management (HMM) +===================================== + +Provide infrastructure and helpers to integrate non-conventional memory (device +memory like GPU on board memory) into regular kernel path, with the cornerstone +of this being specialized struct page for such memory (see sections 5 to 7 of +this document). + +HMM also provides optional helpers for SVM (Share Virtual Memory), i.e., +allowing a device to transparently access program addresses coherently with +the CPU meaning that any valid pointer on the CPU is also a valid pointer +for the device. This is becoming mandatory to simplify the use of advanced +heterogeneous computing where GPU, DSP, or FPGA are used to perform various +computations on behalf of a process. + +This document is divided as follows: in the first section I expose the problems +related to using device specific memory allocators. In the second section, I +expose the hardware limitations that are inherent to many platforms. The third +section gives an overview of the HMM design. The fourth section explains how +CPU page-table mirroring works and the purpose of HMM in this context. The +fifth section deals with how device memory is represented inside the kernel. +Finally, the last section presents a new migration helper that allows +leveraging the device DMA engine. + +.. contents:: :local: + +Problems of using a device specific memory allocator +==================================================== + +Devices with a large amount of on board memory (several gigabytes) like GPUs +have historically managed their memory through dedicated driver specific APIs. +This creates a disconnect between memory allocated and managed by a device +driver and regular application memory (private anonymous, shared memory, or +regular file backed memory). From here on I will refer to this aspect as split +address space. I use shared address space to refer to the opposite situation: +i.e., one in which any application memory region can be used by a device +transparently. + +Split address space happens because devices can only access memory allocated +through a device specific API. This implies that all memory objects in a program +are not equal from the device point of view which complicates large programs +that rely on a wide set of libraries. + +Concretely, this means that code that wants to leverage devices like GPUs needs +to copy objects between generically allocated memory (malloc, mmap private, mmap +share) and memory allocated through the device driver API (this still ends up +with an mmap but of the device file). + +For flat data sets (array, grid, image, ...) this isn't too hard to achieve but +for complex data sets (list, tree, ...) it's hard to get right. Duplicating a +complex data set needs to re-map all the pointer relations between each of its +elements. This is error prone and programs get harder to debug because of the +duplicate data set and addresses. + +Split address space also means that libraries cannot transparently use data +they are getting from the core program or another library and thus each library +might have to duplicate its input data set using the device specific memory +allocator. Large projects suffer from this and waste resources because of the +various memory copies. + +Duplicating each library API to accept as input or output memory allocated by +each device specific allocator is not a viable option. It would lead to a +combinatorial explosion in the library entry points. + +Finally, with the advance of high level language constructs (in C++ but in +other languages too) it is now possible for the compiler to leverage GPUs and +other devices without programmer knowledge. Some compiler identified patterns +are only do-able with a shared address space. It is also more reasonable to use +a shared address space for all other patterns. + + +I/O bus, device memory characteristics +====================================== + +I/O buses cripple shared address spaces due to a few limitations. Most I/O +buses only allow basic memory access from device to main memory; even cache +coherency is often optional. Access to device memory from a CPU is even more +limited. More often than not, it is not cache coherent. + +If we only consider the PCIE bus, then a device can access main memory (often +through an IOMMU) and be cache coherent with the CPUs. However, it only allows +a limited set of atomic operations from the device on main memory. This is worse +in the other direction: the CPU can only access a limited range of the device +memory and cannot perform atomic operations on it. Thus device memory cannot +be considered the same as regular memory from the kernel point of view. + +Another crippling factor is the limited bandwidth (~32GBytes/s with PCIE 4.0 +and 16 lanes). This is 33 times less than the fastest GPU memory (1 TBytes/s). +The final limitation is latency. Access to main memory from the device has an +order of magnitude higher latency than when the device accesses its own memory. + +Some platforms are developing new I/O buses or additions/modifications to PCIE +to address some of these limitations (OpenCAPI, CCIX). They mainly allow +two-way cache coherency between CPU and device and allow all atomic operations the +architecture supports. Sadly, not all platforms are following this trend and +some major architectures are left without hardware solutions to these problems. + +So for shared address space to make sense, not only must we allow devices to +access any memory but we must also permit any memory to be migrated to device +memory while the device is using it (blocking CPU access while it happens). + + +Shared address space and migration +================================== + +HMM intends to provide two main features. The first one is to share the address +space by duplicating the CPU page table in the device page table so the same +address points to the same physical memory for any valid main memory address in +the process address space. + +To achieve this, HMM offers a set of helpers to populate the device page table +while keeping track of CPU page table updates. Device page table updates are +not as easy as CPU page table updates. To update the device page table, you must +allocate a buffer (or use a pool of pre-allocated buffers) and write GPU +specific commands in it to perform the update (unmap, cache invalidations, and +flush, ...). This cannot be done through common code for all devices. Hence +why HMM provides helpers to factor out everything that can be while leaving the +hardware specific details to the device driver. + +The second mechanism HMM provides is a new kind of ZONE_DEVICE memory that +allows allocating a struct page for each page of device memory. Those pages +are special because the CPU cannot map them. However, they allow migrating +main memory to device memory using existing migration mechanisms and everything +looks like a page that is swapped out to disk from the CPU point of view. Using a +struct page gives the easiest and cleanest integration with existing mm +mechanisms. Here again, HMM only provides helpers, first to hotplug new ZONE_DEVICE +memory for the device memory and second to perform migration. Policy decisions +of what and when to migrate is left to the device driver. + +Note that any CPU access to a device page triggers a page fault and a migration +back to main memory. For example, when a page backing a given CPU address A is +migrated from a main memory page to a device page, then any CPU access to +address A triggers a page fault and initiates a migration back to main memory. + +With these two features, HMM not only allows a device to mirror process address +space and keeps both CPU and device page tables synchronized, but also +leverages device memory by migrating the part of the data set that is actively being +used by the device. + + +Address space mirroring implementation and API +============================================== + +Address space mirroring's main objective is to allow duplication of a range of +CPU page table into a device page table; HMM helps keep both synchronized. A +device driver that wants to mirror a process address space must start with the +registration of a mmu_interval_notifier:: + + int mmu_interval_notifier_insert(struct mmu_interval_notifier *interval_sub, + struct mm_struct *mm, unsigned long start, + unsigned long length, + const struct mmu_interval_notifier_ops *ops); + +During the ops->invalidate() callback the device driver must perform the +update action to the range (mark range read only, or fully unmap, etc.). The +device must complete the update before the driver callback returns. + +When the device driver wants to populate a range of virtual addresses, it can +use:: + + int hmm_range_fault(struct hmm_range *range); + +It will trigger a page fault on missing or read-only entries if write access is +requested (see below). Page faults use the generic mm page fault code path just +like a CPU page fault. The usage pattern is:: + + int driver_populate_range(...) + { + struct hmm_range range; + ... + + range.notifier = &interval_sub; + range.start = ...; + range.end = ...; + range.hmm_pfns = ...; + + if (!mmget_not_zero(interval_sub->notifier.mm)) + return -EFAULT; + + again: + range.notifier_seq = mmu_interval_read_begin(&interval_sub); + mmap_read_lock(mm); + ret = hmm_range_fault(&range); + if (ret) { + mmap_read_unlock(mm); + if (ret == -EBUSY) + goto again; + return ret; + } + mmap_read_unlock(mm); + + take_lock(driver->update); + if (mmu_interval_read_retry(&ni, range.notifier_seq) { + release_lock(driver->update); + goto again; + } + + /* Use pfns array content to update device page table, + * under the update lock */ + + release_lock(driver->update); + return 0; + } + +The driver->update lock is the same lock that the driver takes inside its +invalidate() callback. That lock must be held before calling +mmu_interval_read_retry() to avoid any race with a concurrent CPU page table +update. + +Leverage default_flags and pfn_flags_mask +========================================= + +The hmm_range struct has 2 fields, default_flags and pfn_flags_mask, that specify +fault or snapshot policy for the whole range instead of having to set them +for each entry in the pfns array. + +For instance if the device driver wants pages for a range with at least read +permission, it sets:: + + range->default_flags = HMM_PFN_REQ_FAULT; + range->pfn_flags_mask = 0; + +and calls hmm_range_fault() as described above. This will fill fault all pages +in the range with at least read permission. + +Now let's say the driver wants to do the same except for one page in the range for +which it wants to have write permission. Now driver set:: + + range->default_flags = HMM_PFN_REQ_FAULT; + range->pfn_flags_mask = HMM_PFN_REQ_WRITE; + range->pfns[index_of_write] = HMM_PFN_REQ_WRITE; + +With this, HMM will fault in all pages with at least read (i.e., valid) and for the +address == range->start + (index_of_write << PAGE_SHIFT) it will fault with +write permission i.e., if the CPU pte does not have write permission set then HMM +will call handle_mm_fault(). + +After hmm_range_fault completes the flag bits are set to the current state of +the page tables, ie HMM_PFN_VALID | HMM_PFN_WRITE will be set if the page is +writable. + + +Represent and manage device memory from core kernel point of view +================================================================= + +Several different designs were tried to support device memory. The first one +used a device specific data structure to keep information about migrated memory +and HMM hooked itself in various places of mm code to handle any access to +addresses that were backed by device memory. It turns out that this ended up +replicating most of the fields of struct page and also needed many kernel code +paths to be updated to understand this new kind of memory. + +Most kernel code paths never try to access the memory behind a page +but only care about struct page contents. Because of this, HMM switched to +directly using struct page for device memory which left most kernel code paths +unaware of the difference. We only need to make sure that no one ever tries to +map those pages from the CPU side. + +Migration to and from device memory +=================================== + +Because the CPU cannot access device memory directly, the device driver must +use hardware DMA or device specific load/store instructions to migrate data. +The migrate_vma_setup(), migrate_vma_pages(), and migrate_vma_finalize() +functions are designed to make drivers easier to write and to centralize common +code across drivers. + +Before migrating pages to device private memory, special device private +``struct page`` need to be created. These will be used as special "swap" +page table entries so that a CPU process will fault if it tries to access +a page that has been migrated to device private memory. + +These can be allocated and freed with:: + + struct resource *res; + struct dev_pagemap pagemap; + + res = request_free_mem_region(&iomem_resource, /* number of bytes */, + "name of driver resource"); + pagemap.type = MEMORY_DEVICE_PRIVATE; + pagemap.range.start = res->start; + pagemap.range.end = res->end; + pagemap.nr_range = 1; + pagemap.ops = &device_devmem_ops; + memremap_pages(&pagemap, numa_node_id()); + + memunmap_pages(&pagemap); + release_mem_region(pagemap.range.start, range_len(&pagemap.range)); + +There are also devm_request_free_mem_region(), devm_memremap_pages(), +devm_memunmap_pages(), and devm_release_mem_region() when the resources can +be tied to a ``struct device``. + +The overall migration steps are similar to migrating NUMA pages within system +memory (see Documentation/mm/page_migration.rst) but the steps are split +between device driver specific code and shared common code: + +1. ``mmap_read_lock()`` + + The device driver has to pass a ``struct vm_area_struct`` to + migrate_vma_setup() so the mmap_read_lock() or mmap_write_lock() needs to + be held for the duration of the migration. + +2. ``migrate_vma_setup(struct migrate_vma *args)`` + + The device driver initializes the ``struct migrate_vma`` fields and passes + the pointer to migrate_vma_setup(). The ``args->flags`` field is used to + filter which source pages should be migrated. For example, setting + ``MIGRATE_VMA_SELECT_SYSTEM`` will only migrate system memory and + ``MIGRATE_VMA_SELECT_DEVICE_PRIVATE`` will only migrate pages residing in + device private memory. If the latter flag is set, the ``args->pgmap_owner`` + field is used to identify device private pages owned by the driver. This + avoids trying to migrate device private pages residing in other devices. + Currently only anonymous private VMA ranges can be migrated to or from + system memory and device private memory. + + One of the first steps migrate_vma_setup() does is to invalidate other + device's MMUs with the ``mmu_notifier_invalidate_range_start(()`` and + ``mmu_notifier_invalidate_range_end()`` calls around the page table + walks to fill in the ``args->src`` array with PFNs to be migrated. + The ``invalidate_range_start()`` callback is passed a + ``struct mmu_notifier_range`` with the ``event`` field set to + ``MMU_NOTIFY_MIGRATE`` and the ``owner`` field set to + the ``args->pgmap_owner`` field passed to migrate_vma_setup(). This is + allows the device driver to skip the invalidation callback and only + invalidate device private MMU mappings that are actually migrating. + This is explained more in the next section. + + While walking the page tables, a ``pte_none()`` or ``is_zero_pfn()`` + entry results in a valid "zero" PFN stored in the ``args->src`` array. + This lets the driver allocate device private memory and clear it instead + of copying a page of zeros. Valid PTE entries to system memory or + device private struct pages will be locked with ``lock_page()``, isolated + from the LRU (if system memory since device private pages are not on + the LRU), unmapped from the process, and a special migration PTE is + inserted in place of the original PTE. + migrate_vma_setup() also clears the ``args->dst`` array. + +3. The device driver allocates destination pages and copies source pages to + destination pages. + + The driver checks each ``src`` entry to see if the ``MIGRATE_PFN_MIGRATE`` + bit is set and skips entries that are not migrating. The device driver + can also choose to skip migrating a page by not filling in the ``dst`` + array for that page. + + The driver then allocates either a device private struct page or a + system memory page, locks the page with ``lock_page()``, and fills in the + ``dst`` array entry with:: + + dst[i] = migrate_pfn(page_to_pfn(dpage)); + + Now that the driver knows that this page is being migrated, it can + invalidate device private MMU mappings and copy device private memory + to system memory or another device private page. The core Linux kernel + handles CPU page table invalidations so the device driver only has to + invalidate its own MMU mappings. + + The driver can use ``migrate_pfn_to_page(src[i])`` to get the + ``struct page`` of the source and either copy the source page to the + destination or clear the destination device private memory if the pointer + is ``NULL`` meaning the source page was not populated in system memory. + +4. ``migrate_vma_pages()`` + + This step is where the migration is actually "committed". + + If the source page was a ``pte_none()`` or ``is_zero_pfn()`` page, this + is where the newly allocated page is inserted into the CPU's page table. + This can fail if a CPU thread faults on the same page. However, the page + table is locked and only one of the new pages will be inserted. + The device driver will see that the ``MIGRATE_PFN_MIGRATE`` bit is cleared + if it loses the race. + + If the source page was locked, isolated, etc. the source ``struct page`` + information is now copied to destination ``struct page`` finalizing the + migration on the CPU side. + +5. Device driver updates device MMU page tables for pages still migrating, + rolling back pages not migrating. + + If the ``src`` entry still has ``MIGRATE_PFN_MIGRATE`` bit set, the device + driver can update the device MMU and set the write enable bit if the + ``MIGRATE_PFN_WRITE`` bit is set. + +6. ``migrate_vma_finalize()`` + + This step replaces the special migration page table entry with the new + page's page table entry and releases the reference to the source and + destination ``struct page``. + +7. ``mmap_read_unlock()`` + + The lock can now be released. + +Exclusive access memory +======================= + +Some devices have features such as atomic PTE bits that can be used to implement +atomic access to system memory. To support atomic operations to a shared virtual +memory page such a device needs access to that page which is exclusive of any +userspace access from the CPU. The ``make_device_exclusive_range()`` function +can be used to make a memory range inaccessible from userspace. + +This replaces all mappings for pages in the given range with special swap +entries. Any attempt to access the swap entry results in a fault which is +resovled by replacing the entry with the original mapping. A driver gets +notified that the mapping has been changed by MMU notifiers, after which point +it will no longer have exclusive access to the page. Exclusive access is +guaranteed to last until the driver drops the page lock and page reference, at +which point any CPU faults on the page may proceed as described. + +Memory cgroup (memcg) and rss accounting +======================================== + +For now, device memory is accounted as any regular page in rss counters (either +anonymous if device page is used for anonymous, file if device page is used for +file backed page, or shmem if device page is used for shared memory). This is a +deliberate choice to keep existing applications, that might start using device +memory without knowing about it, running unimpacted. + +A drawback is that the OOM killer might kill an application using a lot of +device memory and not a lot of regular system memory and thus not freeing much +system memory. We want to gather more real world experience on how applications +and system react under memory pressure in the presence of device memory before +deciding to account device memory differently. + + +Same decision was made for memory cgroup. Device memory pages are accounted +against same memory cgroup a regular page would be accounted to. This does +simplify migration to and from device memory. This also means that migration +back from device memory to regular memory cannot fail because it would +go above memory cgroup limit. We might revisit this choice latter on once we +get more experience in how device memory is used and its impact on memory +resource control. + + +Note that device memory can never be pinned by a device driver nor through GUP +and thus such memory is always free upon process exit. Or when last reference +is dropped in case of shared memory or file backed memory. |