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Date: Fri, 6 Nov 2020 15:03:22 -0500
From: Zi Yan <ziy@...dia.com>
To: Yang Shi <shy828301@...il.com>
CC: <mhocko@...e.com>, <songliubraving@...com>, <mgorman@...e.de>,
<jack@...e.cz>, <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] mm: migrate: simplify the logic for handling
permanent failure
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@...dia.com>.
> was not so before. This has no impact to current existing callsites.
>
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
> 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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