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diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst new file mode 100644 index 000000000..cdf391158 --- /dev/null +++ b/Documentation/vm/hmm.rst @@ -0,0 +1,386 @@ +.. hmm: + +===================================== +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 address 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 lever- +aging 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 device can only access memory allocated +through 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 object 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 +complex data sets (list, tree, ...) are 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 program gets 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 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 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 device is using it (blocking CPU access while it happens). + + +Shared address space and migration +================================== + +HMM intends to provide two main features. 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 the 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 is swapped out to disk from the CPU point of view. Using a +struct page gives the easiest and cleanest integration with existing mm mech- +anisms. 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 things 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 keeping both CPU and device page table synchronized, but also lever- +ages 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 an hmm_mirror struct:: + + int hmm_mirror_register(struct hmm_mirror *mirror, + struct mm_struct *mm); + int hmm_mirror_register_locked(struct hmm_mirror *mirror, + struct mm_struct *mm); + + +The locked variant is to be used when the driver is already holding mmap_sem +of the mm in write mode. The mirror struct has a set of callbacks that are used +to propagate CPU page tables:: + + struct hmm_mirror_ops { + /* sync_cpu_device_pagetables() - synchronize page tables + * + * @mirror: pointer to struct hmm_mirror + * @update_type: type of update that occurred to the CPU page table + * @start: virtual start address of the range to update + * @end: virtual end address of the range to update + * + * This callback ultimately originates from mmu_notifiers when the CPU + * page table is updated. The device driver must update its page table + * in response to this callback. The update argument tells what action + * to perform. + * + * The device driver must not return from this callback until the device + * page tables are completely updated (TLBs flushed, etc); this is a + * synchronous call. + */ + void (*update)(struct hmm_mirror *mirror, + enum hmm_update action, + unsigned long start, + unsigned long end); + }; + +The device driver must perform the update action to the range (mark range +read only, or fully unmap, ...). The device must be done with the update before +the driver callback returns. + +When the device driver wants to populate a range of virtual addresses, it can +use either:: + + int hmm_vma_get_pfns(struct vm_area_struct *vma, + struct hmm_range *range, + unsigned long start, + unsigned long end, + hmm_pfn_t *pfns); + int hmm_vma_fault(struct vm_area_struct *vma, + struct hmm_range *range, + unsigned long start, + unsigned long end, + hmm_pfn_t *pfns, + bool write, + bool block); + +The first one (hmm_vma_get_pfns()) will only fetch present CPU page table +entries and will not trigger a page fault on missing or non-present entries. +The second one does trigger a page fault on missing or read-only entry if the +write parameter is true. Page faults use the generic mm page fault code path +just like a CPU page fault. + +Both functions copy CPU page table entries into their pfns array argument. Each +entry in that array corresponds to an address in the virtual range. HMM +provides a set of flags to help the driver identify special CPU page table +entries. + +Locking with the update() callback is the most important aspect the driver must +respect in order to keep things properly synchronized. The usage pattern is:: + + int driver_populate_range(...) + { + struct hmm_range range; + ... + again: + ret = hmm_vma_get_pfns(vma, &range, start, end, pfns); + if (ret) + return ret; + take_lock(driver->update); + if (!hmm_vma_range_done(vma, &range)) { + release_lock(driver->update); + goto again; + } + + // Use pfns array content to update device page table + + release_lock(driver->update); + return 0; + } + +The driver->update lock is the same lock that the driver takes inside its +update() callback. That lock must be held before hmm_vma_range_done() to avoid +any race with a concurrent CPU page table update. + +HMM implements all this on top of the mmu_notifier API because we wanted a +simpler API and also to be able to perform optimizations latter on like doing +concurrent device updates in multi-devices scenario. + +HMM also serves as an impedance mismatch between how CPU page table updates +are done (by CPU write to the page table and TLB flushes) and how devices +update their own page table. Device updates are a multi-step process. First, +appropriate commands are written to a buffer, then this buffer is scheduled for +execution on the device. It is only once the device has executed commands in +the buffer that the update is done. Creating and scheduling the update command +buffer can happen concurrently for multiple devices. Waiting for each device to +report commands as executed is serialized (there is no point in doing this +concurrently). + + +Represent and manage device memory from core kernel point of view +================================================================= + +Several different designs were tried to support device memory. 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. + +HMM provides a set of helpers to register and hotplug device memory as a new +region needing a struct page. This is offered through a very simple API:: + + struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops, + struct device *device, + unsigned long size); + void hmm_devmem_remove(struct hmm_devmem *devmem); + +The hmm_devmem_ops is where most of the important things are:: + + struct hmm_devmem_ops { + void (*free)(struct hmm_devmem *devmem, struct page *page); + int (*fault)(struct hmm_devmem *devmem, + struct vm_area_struct *vma, + unsigned long addr, + struct page *page, + unsigned flags, + pmd_t *pmdp); + }; + +The first callback (free()) happens when the last reference on a device page is +dropped. This means the device page is now free and no longer used by anyone. +The second callback happens whenever the CPU tries to access a device page +which it cannot do. This second callback must trigger a migration back to +system memory. + + +Migration to and from device memory +=================================== + +Because the CPU cannot access device memory, migration must use the device DMA +engine to perform copy from and to device memory. For this we need a new +migration helper:: + + int migrate_vma(const struct migrate_vma_ops *ops, + struct vm_area_struct *vma, + unsigned long mentries, + unsigned long start, + unsigned long end, + unsigned long *src, + unsigned long *dst, + void *private); + +Unlike other migration functions it works on a range of virtual address, there +are two reasons for that. First, device DMA copy has a high setup overhead cost +and thus batching multiple pages is needed as otherwise the migration overhead +makes the whole exercise pointless. The second reason is because the +migration might be for a range of addresses the device is actively accessing. + +The migrate_vma_ops struct defines two callbacks. First one (alloc_and_copy()) +controls destination memory allocation and copy operation. Second one is there +to allow the device driver to perform cleanup operations after migration:: + + struct migrate_vma_ops { + void (*alloc_and_copy)(struct vm_area_struct *vma, + const unsigned long *src, + unsigned long *dst, + unsigned long start, + unsigned long end, + void *private); + void (*finalize_and_map)(struct vm_area_struct *vma, + const unsigned long *src, + const unsigned long *dst, + unsigned long start, + unsigned long end, + void *private); + }; + +It is important to stress that these migration helpers allow for holes in the +virtual address range. Some pages in the range might not be migrated for all +the usual reasons (page is pinned, page is locked, ...). This helper does not +fail but just skips over those pages. + +The alloc_and_copy() might decide to not migrate all pages in the +range (for reasons under the callback control). For those, the callback just +has to leave the corresponding dst entry empty. + +Finally, the migration of the struct page might fail (for file backed page) for +various reasons (failure to freeze reference, or update page cache, ...). If +that happens, then the finalize_and_map() can catch any pages that were not +migrated. Note those pages were still copied to a new page and thus we wasted +bandwidth but this is considered as a rare event and a price that we are +willing to pay to keep all the code simpler. + + +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 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. |