mm, numa: rework do_pages_move
Patch series "unclutter thp migration" Motivation: THP migration is hacked into the generic migration with rather surprising semantic. The migration allocation callback is supposed to check whether the THP can be migrated at once and if that is not the case then it allocates a simple page to migrate. unmap_and_move then fixes that up by splitting the THP into small pages while moving the head page to the newly allocated order-0 page. Remaining pages are moved to the LRU list by split_huge_page. The same happens if the THP allocation fails. This is really ugly and error prone [2]. I also believe that split_huge_page to the LRU lists is inherently wrong because all tail pages are not migrated. Some callers will just work around that by retrying (e.g. memory hotplug). There are other pfn walkers which are simply broken though. e.g. madvise_inject_error will migrate head and then advances next pfn by the huge page size. do_move_page_to_node_array, queue_pages_range (migrate_pages, mbind), will simply split the THP before migration if the THP migration is not supported then falls back to single page migration but it doesn't handle tail pages if the THP migration path is not able to allocate a fresh THP so we end up with ENOMEM and fail the whole migration which is a questionable behavior. Page compaction doesn't try to migrate large pages so it should be immune. The first patch reworks do_pages_move which relies on a very ugly calling semantic when the return status is pushed to the migration path via private pointer. It uses pre allocated fixed size batching to achieve that. We simply cannot do the same if a THP is to be split during the migration path which is done in the patch 3. Patch 2 is follow up cleanup which removes the mentioned return status calling convention ugliness. On a side note: There are some semantic issues I have encountered on the way when working on patch 1 but I am not addressing them here. E.g. trying to move THP tail pages will result in either success or EBUSY (the later one more likely once we isolate head from the LRU list). Hugetlb reports EACCESS on tail pages. Some errors are reported via status parameter but migration failures are not even though the original `reason' argument suggests there was an intention to do so. From a quick look into git history this never worked. I have tried to keep the semantic unchanged. Then there is a relatively minor thing that the page isolation might fail because of pages not being on the LRU - e.g. because they are sitting on the per-cpu LRU caches. Easily fixable. This patch (of 3): do_pages_move is supposed to move user defined memory (an array of addresses) to the user defined numa nodes (an array of nodes one for each address). The user provided status array then contains resulting numa node for each address or an error. The semantic of this function is little bit confusing because only some errors are reported back. Notably migrate_pages error is only reported via the return value. This patch doesn't try to address these semantic nuances but rather change the underlying implementation. Currently we are processing user input (which can be really large) in batches which are stored to a temporarily allocated page. Each address is resolved to its struct page and stored to page_to_node structure along with the requested target numa node. The array of these structures is then conveyed down the page migration path via private argument. new_page_node then finds the corresponding structure and allocates the proper target page. What is the problem with the current implementation and why to change it? Apart from being quite ugly it also doesn't cope with unexpected pages showing up on the migration list inside migrate_pages path. That doesn't happen currently but the follow up patch would like to make the thp migration code more clear and that would need to split a THP into the list for some cases. How does the new implementation work? Well, instead of batching into a fixed size array we simply batch all pages that should be migrated to the same node and isolate all of them into a linked list which doesn't require any additional storage. This should work reasonably well because page migration usually migrates larger ranges of memory to a specific node. So the common case should work equally well as the current implementation. Even if somebody constructs an input where the target numa nodes would be interleaved we shouldn't see a large performance impact because page migration alone doesn't really benefit from batching. mmap_sem batching for the lookup is quite questionable and isolate_lru_page which would benefit from batching is not using it even in the current implementation. Link: http://lkml.kernel.org/r/20180103082555.14592-2-mhocko@kernel.orgSigned-off-by: NMichal Hocko <mhocko@suse.com> Acked-by: NKirill A. Shutemov <kirill@shutemov.name> Reviewed-by: NAndrew Morton <akpm@linux-foundation.org> Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com> Cc: Zi Yan <zi.yan@cs.rutgers.edu> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Andrea Reale <ar@linux.vnet.ibm.com> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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