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/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Andrea Reale <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Michal Hocko

mm/internal.h

@@ -538,4 +538,5 @@ static inline bool is_migrate_highatomic_page(struct page *page)
 }
 
 void setup_zone_pageset(struct zone *zone);
+extern struct page *alloc_new_node_page(struct page *page, unsigned long node, int **x);
 #endif	/* __MM_INTERNAL_H */

mm/mempolicy.c

@@ -942,7 +942,8 @@ static void migrate_page_add(struct page *page, struct list_head *pagelist,
 	}
 }
 
-static struct page *new_node_page(struct page *page, unsigned long node, int **x)
+/* page allocation callback for NUMA node migration */
+struct page *alloc_new_node_page(struct page *page, unsigned long node, int **x)
 {
 	if (PageHuge(page))
 		return alloc_huge_page_node(page_hstate(compound_head(page)),
@@ -986,7 +987,7 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
 			flags | MPOL_MF_DISCONTIG_OK, &pagelist);
 
 	if (!list_empty(&pagelist)) {
-		err = migrate_pages(&pagelist, new_node_page, NULL, dest,
+		err = migrate_pages(&pagelist, alloc_new_node_page, NULL, dest,
 					MIGRATE_SYNC, MR_SYSCALL);
 		if (err)
 			putback_movable_pages(&pagelist);

mm/migrate.c

@@ -1444,141 +1444,103 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 }
 
 #ifdef CONFIG_NUMA
-/*
- * Move a list of individual pages
- */
-struct page_to_node {
-	unsigned long addr;
-	struct page *page;
-	int node;
-	int status;
-};
 
-static struct page *new_page_node(struct page *p, unsigned long private,
-		int **result)
+static int store_status(int __user *status, int start, int value, int nr)
 {
-	struct page_to_node *pm = (struct page_to_node *)private;
-
-	while (pm->node != MAX_NUMNODES && pm->page != p)
-		pm++;
+	while (nr-- > 0) {
+		if (put_user(value, status + start))
+			return -EFAULT;
+		start++;
+	}
 
-	if (pm->node == MAX_NUMNODES)
-		return NULL;
+	return 0;
+}
 
-	*result = &pm->status;
+static int do_move_pages_to_node(struct mm_struct *mm,
+		struct list_head *pagelist, int node)
+{
+	int err;
 
-	if (PageHuge(p))
-		return alloc_huge_page_node(page_hstate(compound_head(p)),
-					pm->node);
-	else if (thp_migration_supported() && PageTransHuge(p)) {
-		struct page *thp;
+	if (list_empty(pagelist))
+		return 0;
 
-		thp = alloc_pages_node(pm->node,
-			(GFP_TRANSHUGE | __GFP_THISNODE) & ~__GFP_RECLAIM,
-			HPAGE_PMD_ORDER);
-		if (!thp)
-			return NULL;
-		prep_transhuge_page(thp);
-		return thp;
-	} else
-		return __alloc_pages_node(pm->node,
-				GFP_HIGHUSER_MOVABLE | __GFP_THISNODE, 0);
+	err = migrate_pages(pagelist, alloc_new_node_page, NULL, node,
+			MIGRATE_SYNC, MR_SYSCALL);
+	if (err)
+		putback_movable_pages(pagelist);
+	return err;
 }
 
 /*
- * Move a set of pages as indicated in the pm array. The addr
- * field must be set to the virtual address of the page to be moved
- * and the node number must contain a valid target node.
- * The pm array ends with node = MAX_NUMNODES.
+ * Resolves the given address to a struct page, isolates it from the LRU and
+ * puts it to the given pagelist.
+ * Returns -errno if the page cannot be found/isolated or 0 when it has been
+ * queued or the page doesn't need to be migrated because it is already on
+ * the target node
  */
-static int do_move_page_to_node_array(struct mm_struct *mm,
-				      struct page_to_node *pm,
-				      int migrate_all)
+static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
+		int node, struct list_head *pagelist, bool migrate_all)
 {
+	struct vm_area_struct *vma;
+	struct page *page;
+	unsigned int follflags;
 	int err;
-	struct page_to_node *pp;
-	LIST_HEAD(pagelist);
 
 	down_read(&mm->mmap_sem);
+	err = -EFAULT;
+	vma = find_vma(mm, addr);
+	if (!vma || addr < vma->vm_start || !vma_migratable(vma))
+		goto out;
 
-	/*
-	 * Build a list of pages to migrate
-	 */
-	for (pp = pm; pp->node != MAX_NUMNODES; pp++) {
-		struct vm_area_struct *vma;
-		struct page *page;
-		struct page *head;
-		unsigned int follflags;
-
-		err = -EFAULT;
-		vma = find_vma(mm, pp->addr);
-		if (!vma || pp->addr < vma->vm_start || !vma_migratable(vma))
-			goto set_status;
-
-		/* FOLL_DUMP to ignore special (like zero) pages */
-		follflags = FOLL_GET | FOLL_DUMP;
-		if (!thp_migration_supported())
-			follflags |= FOLL_SPLIT;
-		page = follow_page(vma, pp->addr, follflags);
+	/* FOLL_DUMP to ignore special (like zero) pages */
+	follflags = FOLL_GET | FOLL_DUMP;
+	if (!thp_migration_supported())
+		follflags |= FOLL_SPLIT;
+	page = follow_page(vma, addr, follflags);
 
-		err = PTR_ERR(page);
-		if (IS_ERR(page))
-			goto set_status;
+	err = PTR_ERR(page);
+	if (IS_ERR(page))
+		goto out;
 
-		err = -ENOENT;
-		if (!page)
-			goto set_status;
+	err = -ENOENT;
+	if (!page)
+		goto out;
 
-		err = page_to_nid(page);
+	err = 0;
+	if (page_to_nid(page) == node)
+		goto out_putpage;
 
-		if (err == pp->node)
-			/*
-			 * Node already in the right place
-			 */
-			goto put_and_set;
+	err = -EACCES;
+	if (page_mapcount(page) > 1 && !migrate_all)
+		goto out_putpage;
 
-		err = -EACCES;
-		if (page_mapcount(page) > 1 &&
-				!migrate_all)
-			goto put_and_set;
-
-		if (PageHuge(page)) {
-			if (PageHead(page)) {
-				isolate_huge_page(page, &pagelist);
-				err = 0;
-				pp->page = page;
-			}
-			goto put_and_set;
+	if (PageHuge(page)) {
+		if (PageHead(page)) {
+			isolate_huge_page(page, pagelist);
+			err = 0;
 		}
+	} else {
+		struct page *head;
 
-		pp->page = compound_head(page);
 		head = compound_head(page);
 		err = isolate_lru_page(head);
-		if (!err) {
-			list_add_tail(&head->lru, &pagelist);
-			mod_node_page_state(page_pgdat(head),
-				NR_ISOLATED_ANON + page_is_file_cache(head),
-				hpage_nr_pages(head));
-		}
-put_and_set:
-		/*
-		 * Either remove the duplicate refcount from
-		 * isolate_lru_page() or drop the page ref if it was
-		 * not isolated.
-		 */
-		put_page(page);
-set_status:
-		pp->status = err;
-	}
-
-	err = 0;
-	if (!list_empty(&pagelist)) {
-		err = migrate_pages(&pagelist, new_page_node, NULL,
-				(unsigned long)pm, MIGRATE_SYNC, MR_SYSCALL);
 		if (err)
-			putback_movable_pages(&pagelist);
-	}
+			goto out_putpage;
 
+		err = 0;
+		list_add_tail(&head->lru, pagelist);
+		mod_node_page_state(page_pgdat(head),
+			NR_ISOLATED_ANON + page_is_file_cache(head),
+			hpage_nr_pages(head));
+	}
+out_putpage:
+	/*
+	 * Either remove the duplicate refcount from
+	 * isolate_lru_page() or drop the page ref if it was
+	 * not isolated.
+	 */
+	put_page(page);
+out:
 	up_read(&mm->mmap_sem);
 	return err;
 }
@@ -1593,79 +1555,79 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 			 const int __user *nodes,
 			 int __user *status, int flags)
 {
-	struct page_to_node *pm;
-	unsigned long chunk_nr_pages;
-	unsigned long chunk_start;
-	int err;
-
-	err = -ENOMEM;
-	pm = (struct page_to_node *)__get_free_page(GFP_KERNEL);
-	if (!pm)
-		goto out;
+	int current_node = NUMA_NO_NODE;
+	LIST_HEAD(pagelist);
+	int start, i;
+	int err = 0, err1;
 
 	migrate_prep();
 
-	/*
-	 * Store a chunk of page_to_node array in a page,
-	 * but keep the last one as a marker
-	 */
-	chunk_nr_pages = (PAGE_SIZE / sizeof(struct page_to_node)) - 1;
-
-	for (chunk_start = 0;
-	     chunk_start < nr_pages;
-	     chunk_start += chunk_nr_pages) {
-		int j;
+	for (i = start = 0; i < nr_pages; i++) {
+		const void __user *p;
+		unsigned long addr;
+		int node;
 
-		if (chunk_start + chunk_nr_pages > nr_pages)
-			chunk_nr_pages = nr_pages - chunk_start;
-
-		/* fill the chunk pm with addrs and nodes from user-space */
-		for (j = 0; j < chunk_nr_pages; j++) {
-			const void __user *p;
-			int node;
-
-			err = -EFAULT;
-			if (get_user(p, pages + j + chunk_start))
-				goto out_pm;
-			pm[j].addr = (unsigned long) p;
-
-			if (get_user(node, nodes + j + chunk_start))
-				goto out_pm;
-
-			err = -ENODEV;
-			if (node < 0 || node >= MAX_NUMNODES)
-				goto out_pm;
-
-			if (!node_state(node, N_MEMORY))
-				goto out_pm;
-
-			err = -EACCES;
-			if (!node_isset(node, task_nodes))
-				goto out_pm;
+		err = -EFAULT;
+		if (get_user(p, pages + i))
+			goto out_flush;
+		if (get_user(node, nodes + i))
+			goto out_flush;
+		addr = (unsigned long)p;
+
+		err = -ENODEV;
+		if (node < 0 || node >= MAX_NUMNODES)
+			goto out_flush;
+		if (!node_state(node, N_MEMORY))
+			goto out_flush;
 
-			pm[j].node = node;
+		err = -EACCES;
+		if (!node_isset(node, task_nodes))
+			goto out_flush;
+
+		if (current_node == NUMA_NO_NODE) {
+			current_node = node;
+			start = i;
+		} else if (node != current_node) {
+			err = do_move_pages_to_node(mm, &pagelist, current_node);
+			if (err)
+				goto out;
+			err = store_status(status, start, current_node, i - start);
+			if (err)
+				goto out;
+			start = i;
+			current_node = node;
 		}
 
-		/* End marker for this chunk */
-		pm[chunk_nr_pages].node = MAX_NUMNODES;
-
-		/* Migrate this chunk */
-		err = do_move_page_to_node_array(mm, pm,
-						 flags & MPOL_MF_MOVE_ALL);
-		if (err < 0)
-			goto out_pm;
+		/*
+		 * Errors in the page lookup or isolation are not fatal and we simply
+		 * report them via status
+		 */
+		err = add_page_for_migration(mm, addr, current_node,
+				&pagelist, flags & MPOL_MF_MOVE_ALL);
+		if (!err)
+			continue;
 
-		/* Return status information */
-		for (j = 0; j < chunk_nr_pages; j++)
-			if (put_user(pm[j].status, status + j + chunk_start)) {
-				err = -EFAULT;
-				goto out_pm;
-			}
-	}
-	err = 0;
+		err = store_status(status, i, err, 1);
+		if (err)
+			goto out_flush;
 
-out_pm:
-	free_page((unsigned long)pm);
+		err = do_move_pages_to_node(mm, &pagelist, current_node);
+		if (err)
+			goto out;
+		if (i > start) {
+			err = store_status(status, start, current_node, i - start);
+			if (err)
+				goto out;
+		}
+		current_node = NUMA_NO_NODE;
+	}
+out_flush:
+	/* Make sure we do not overwrite the existing error */
+	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
+	if (!err1)
+		err1 = store_status(status, start, current_node, i - start);
+	if (!err)
+		err = err1;
 out:
 	return err;
 }