Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded

From: Yang Shi
Date: Thu Apr 08 2021 - 16:40:50 EST

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On Thu, Apr 8, 2021 at 11:17 AM Oscar Salvador <osalvador@xxxxxxx> wrote:
>
> On Thu, Apr 08, 2021 at 10:26:54AM -0700, Yang Shi wrote:
>
> > Thanks, Oscar. Yes, kind of. But we have to remember to initialize
> > "nr_succedded" pointer properly for every migrate_pages() callsite,
> > right? And it doesn't prevent from returning wrong value if
> > migrate_pages() is called multiple times by one caller although there
> > might be not such case (calls migrate_pages() multiple times and care
> > about nr_succeded) for now.
>
> Hi Yang,
>
> I might be missing something but AFAICS you only need to initialize
> nr_succeded pointer where it matters.
> The local nr_succeeded in migrate_pages() doesn't go, and so it gets
> initialized every time you call in it to 0.
> And if you pass a valid pointer, *ret_succeeded == nr_succedeed.
>
> I am talking about this (not even compile-tested):
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 3a389633b68f..fd661cb2ce13 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -40,7 +40,8 @@ extern int migrate_page(struct address_space *mapping,
> struct page *newpage, struct page *page,
> enum migrate_mode mode);
> extern int migrate_pages(struct list_head *l, new_page_t new, free_page_t free,
> - unsigned long private, enum migrate_mode mode, int reason);
> + unsigned long private, enum migrate_mode mode, int reason,
> + unsigned int *ret_succeeded);
> extern struct page *alloc_migration_target(struct page *page, unsigned long private);
> extern int isolate_movable_page(struct page *page, isolate_mode_t mode);
> extern void putback_movable_page(struct page *page);
> @@ -58,7 +59,7 @@ extern int migrate_page_move_mapping(struct address_space *mapping,
> static inline void putback_movable_pages(struct list_head *l) {}
> static inline int migrate_pages(struct list_head *l, new_page_t new,
> free_page_t free, unsigned long private, enum migrate_mode mode,
> - int reason)
> + int reason, unsigned int *ret_succeeded)
> { return -ENOSYS; }
> static inline struct page *alloc_migration_target(struct page *page,
> unsigned long private)
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e04f4476e68e..7238e8faff04 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2364,7 +2364,7 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>
> err = migrate_pages(&cc->migratepages, compaction_alloc,
> compaction_free, (unsigned long)cc, cc->mode,
> - MR_COMPACTION);
> + MR_COMPACTION, NULL);
>
> trace_mm_compaction_migratepages(cc->nr_migratepages, err,
> &cc->migratepages);
> diff --git a/mm/gup.c b/mm/gup.c
> index e40579624f10..b70d463aa1fc 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1606,7 +1606,7 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
> put_page(pages[i]);
>
> if (migrate_pages(&cma_page_list, alloc_migration_target, NULL,
> - (unsigned long)&mtc, MIGRATE_SYNC, MR_CONTIG_RANGE)) {
> + (unsigned long)&mtc, MIGRATE_SYNC, MR_CONTIG_RANGE, NULL)) {
> /*
> * some of the pages failed migration. Do get_user_pages
> * without migration.
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 24210c9bd843..a17e0f039076 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1852,7 +1852,8 @@ static int __soft_offline_page(struct page *page)
>
> if (isolate_page(hpage, &pagelist)) {
> ret = migrate_pages(&pagelist, alloc_migration_target, NULL,
> - (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_FAILURE);
> + (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_FAILURE,
> + NULL);
> if (!ret) {
> bool release = !huge;
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0cdbbfbc5757..28496376de94 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1466,7 +1466,8 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
> if (nodes_empty(nmask))
> node_set(mtc.nid, nmask);
> ret = migrate_pages(&source, alloc_migration_target, NULL,
> - (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
> + (unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG,
> + NULL);
> if (ret) {
> list_for_each_entry(page, &source, lru) {
> pr_warn("migrating pfn %lx failed ret:%d ",
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index ab51132547b8..df260ed12102 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1103,7 +1103,8 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
>
> if (!list_empty(&pagelist)) {
> err = migrate_pages(&pagelist, alloc_migration_target, NULL,
> - (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL);
> + (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL,
> + NULL);
> if (err)
> putback_movable_pages(&pagelist);
> }
> @@ -1355,7 +1356,8 @@ static long do_mbind(unsigned long start, unsigned long len,
> if (!list_empty(&pagelist)) {
> WARN_ON_ONCE(flags & MPOL_MF_LAZY);
> nr_failed = migrate_pages(&pagelist, new_page, NULL,
> - start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND);
> + start, MIGRATE_SYNC, MR_MEMPOLICY_MBIND,
> + NULL);
> if (nr_failed)
> putback_movable_pages(&pagelist);
> }
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 695a594e5860..087ed407b3ce 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1493,6 +1493,9 @@ static inline int try_split_thp(struct page *page, struct page **page2,
> * @mode: The migration mode that specifies the constraints for
> * page migration, if any.
> * @reason: The reason for page migration.
> + * @ret_succeeded: A pointer to place the value of the number of pages
> + * migrated successfully. The caller must pass a valid pointer
> + * if they care about it.
> *
> * The function returns after 10 attempts or if no pages are movable any more
> * because the list has become empty or no retryable pages exist any more.
> @@ -1503,7 +1506,7 @@ static inline int try_split_thp(struct page *page, struct page **page2,
> */
> int migrate_pages(struct list_head *from, new_page_t get_new_page,
> free_page_t put_new_page, unsigned long private,
> - enum migrate_mode mode, int reason)
> + enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
> {
> int retry = 1;
> int thp_retry = 1;
> @@ -1654,6 +1657,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> if (!swapwrite)
> current->flags &= ~PF_SWAPWRITE;
>
> + if (ret_succeeded)
> + *ret_succeeded = nr_succeeded;
> +

Thanks a lot for the example code. You didn't miss anything. At first
glance, I thought your suggestion seemed neater. Actually I
misunderstood what Dave said about "That could really have caused some
interesting problems." with multiple calls to migrate_pages(). I was
thinking about:

unsigned long foo()
{
unsigned long *ret_succeeded;

migrate_pages(..., ret_succeeded);

migrate_pages(..., ret_succeeded);

return *ret_succeeded;
}

Of course, this is a hypothetical case now. We don't have to care
about it at all.

> return rc;
> }
>
> @@ -1723,7 +1729,8 @@ static int do_move_pages_to_node(struct mm_struct *mm,
> };
>
> err = migrate_pages(pagelist, alloc_migration_target, NULL,
> - (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL);
> + (unsigned long)&mtc, MIGRATE_SYNC, MR_SYSCALL,
> + NULL);
> if (err)
> putback_movable_pages(pagelist);
> return err;
> @@ -2230,7 +2237,7 @@ int migrate_misplaced_page(struct page *page, struct vm_area_struct *vma,
> list_add(&page->lru, &migratepages);
> nr_remaining = migrate_pages(&migratepages, alloc_misplaced_dst_page,
> NULL, node, MIGRATE_ASYNC,
> - MR_NUMA_MISPLACED);
> + MR_NUMA_MISPLACED, NULL);
> if (nr_remaining) {
> if (!list_empty(&migratepages)) {
> list_del(&page->lru);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 46f3d594369d..0c1bbadd5ca3 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8490,7 +8490,8 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> cc->nr_migratepages -= nr_reclaimed;
>
> ret = migrate_pages(&cc->migratepages, alloc_migration_target,
> - NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
> + NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE,
> + NULL);
> }
> if (ret < 0) {
> putback_movable_pages(&cc->migratepages);
>
>
> As I said I might be missing a point here, but I cannot see the problem
> you describe here.
>
>
> --
> Oscar Salvador
> SUSE L3

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In reply to: Oscar Salvador: "Re: [PATCH 04/10] mm/migrate: make migrate_pages() return nr_succeeded"
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