diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/hmm.rst | 54 |
1 files changed, 2 insertions, 52 deletions
diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst index e63c11f7e0e0..0a5960beccf7 100644 --- a/Documentation/vm/hmm.rst +++ b/Documentation/vm/hmm.rst @@ -339,58 +339,8 @@ 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. +engine to perform copy from and to device memory. For this we need to use +migrate_vma_setup(), migrate_vma_pages(), and migrate_vma_finalize() helpers. Memory cgroup (memcg) and rss accounting |