Re: [PATCH 2/5] mm: migrate: simplify the logic for handling permanent failure

[Zi Yan]

On 3 Nov 2020, at 8:03, Yang Shi wrote:

> When unmap_and_move{_huge_page}() returns !-EAGAIN and !MIGRATEPAGE_SUCCESS,
> the page would be put back to LRU or proper list if it is non-LRU movable
> page.  But, the callers always call putback_movable_pages() to put the
> failed pages back later on, so it seems not very efficient to put every
> single page back immediately, and the code looks convoluted.
>
> Put the failed page on a separate list, then splice the list to migrate
> list when all pages are tried.  It is the caller's responsibility to
> call putback_movable_pages() to handle failures.  This also makes the
> code simpler and more readable.
>
> After the change the rules are:
>     * Success: non hugetlb page will be freed, hugetlb page will be put
>                back
>     * -EAGAIN: stay on the from list
>     * -ENOMEM: stay on the from list
>     * Other errno: put on ret_pages list then splice to from list

Can you put this before the switch case in the migrate_pages? That will
be very helpful to understand the code.
>
> The from list would be empty iff all pages are migrated successfully, it

s/iff/if unless you really mean if and only if. :)


Everything else looks good to me. Thanks for making the code cleaner.
With the changes above, you can add Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>.

> was not so before.  This has no impact to current existing callsites.
>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
>  mm/migrate.c | 58 ++++++++++++++++++++++++++--------------------------
>  1 file changed, 29 insertions(+), 29 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 8a2e7e19e27b..c33c92495ead 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1169,7 +1169,8 @@ static int unmap_and_move(new_page_t get_new_page,
>  				   free_page_t put_new_page,
>  				   unsigned long private, struct page *page,
>  				   int force, enum migrate_mode mode,
> -				   enum migrate_reason reason)
> +				   enum migrate_reason reason,
> +				   struct list_head *ret)
>  {
>  	int rc = MIGRATEPAGE_SUCCESS;
>  	struct page *newpage = NULL;
> @@ -1206,7 +1207,14 @@ static int unmap_and_move(new_page_t get_new_page,
>  		 * migrated will have kept its references and be restored.
>  		 */
>  		list_del(&page->lru);
> +	}
>
> +	/*
> +	 * If migration is successful, releases reference grabbed during
> +	 * isolation. Otherwise, restore the page to right list unless
> +	 * we want to retry.
> +	 */
> +	if (rc == MIGRATEPAGE_SUCCESS) {
>  		/*
>  		 * Compaction can migrate also non-LRU pages which are
>  		 * not accounted to NR_ISOLATED_*. They can be recognized
> @@ -1215,35 +1223,16 @@ static int unmap_and_move(new_page_t get_new_page,
>  		if (likely(!__PageMovable(page)))
>  			mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON +
>  					page_is_file_lru(page), -thp_nr_pages(page));
> -	}
>
> -	/*
> -	 * If migration is successful, releases reference grabbed during
> -	 * isolation. Otherwise, restore the page to right list unless
> -	 * we want to retry.
> -	 */
> -	if (rc == MIGRATEPAGE_SUCCESS) {
>  		if (reason != MR_MEMORY_FAILURE)
>  			/*
>  			 * We release the page in page_handle_poison.
>  			 */
>  			put_page(page);
>  	} else {
> -		if (rc != -EAGAIN) {
> -			if (likely(!__PageMovable(page))) {
> -				putback_lru_page(page);
> -				goto put_new;
> -			}
> +		if (rc != -EAGAIN)
> +			list_add_tail(&page->lru, ret);
>
> -			lock_page(page);
> -			if (PageMovable(page))
> -				putback_movable_page(page);
> -			else
> -				__ClearPageIsolated(page);
> -			unlock_page(page);
> -			put_page(page);
> -		}
> -put_new:
>  		if (put_new_page)
>  			put_new_page(newpage, private);
>  		else
> @@ -1274,7 +1263,8 @@ static int unmap_and_move(new_page_t get_new_page,
>  static int unmap_and_move_huge_page(new_page_t get_new_page,
>  				free_page_t put_new_page, unsigned long private,
>  				struct page *hpage, int force,
> -				enum migrate_mode mode, int reason)
> +				enum migrate_mode mode, int reason,
> +				struct list_head *ret)
>  {
>  	int rc = -EAGAIN;
>  	int page_was_mapped = 0;
> @@ -1290,7 +1280,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  	 * kicking migration.
>  	 */
>  	if (!hugepage_migration_supported(page_hstate(hpage))) {
> -		putback_active_hugepage(hpage);
> +		list_move_tail(&hpage->lru, ret);
>  		return -ENOSYS;
>  	}
>
> @@ -1372,8 +1362,10 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>  out_unlock:
>  	unlock_page(hpage);
>  out:
> -	if (rc != -EAGAIN)
> +	if (rc == MIGRATEPAGE_SUCCESS)
>  		putback_active_hugepage(hpage);
> +	else if (rc != -EAGAIN && rc != MIGRATEPAGE_SUCCESS)
> +		list_move_tail(&hpage->lru, ret);
>
>  	/*
>  	 * If migration was not successful and there's a freeing callback, use
> @@ -1404,8 +1396,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>   *
>   * 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.
> - * The caller should call putback_movable_pages() to return pages to the LRU
> - * or free list only if ret != 0.
> + * It is caller's responsibility to call putback_movable_pages() to return pages
> + * to the LRU or free list only if ret != 0.
>   *
>   * Returns the number of pages that were not migrated, or an error code.
>   */
> @@ -1426,6 +1418,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	struct page *page2;
>  	int swapwrite = current->flags & PF_SWAPWRITE;
>  	int rc, nr_subpages;
> +	LIST_HEAD(ret_pages);
>
>  	if (!swapwrite)
>  		current->flags |= PF_SWAPWRITE;
> @@ -1448,11 +1441,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  			if (PageHuge(page))
>  				rc = unmap_and_move_huge_page(get_new_page,
>  						put_new_page, private, page,
> -						pass > 2, mode, reason);
> +						pass > 2, mode, reason,
> +						&ret_pages);
>  			else
>  				rc = unmap_and_move(get_new_page, put_new_page,
>  						private, page, pass > 2, mode,
> -						reason);
> +						reason, &ret_pages);
>
>  			switch(rc) {
>  			case -ENOMEM:
> @@ -1519,6 +1513,12 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	nr_thp_failed += thp_retry;
>  	rc = nr_failed;
>  out:
> +	/*
> +	 * Put the permanent failure page back to migration list, they
> +	 * will be put back to the right list by the caller.
> +	 */
> +	list_splice(&ret_pages, from);
> +
>  	count_vm_events(PGMIGRATE_SUCCESS, nr_succeeded);
>  	count_vm_events(PGMIGRATE_FAIL, nr_failed);
>  	count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
> -- 
> 2.26.2


—
Best Regards,
Yan Zi

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