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SubjectRe: [PATCH v18 06/32] mm/thp: narrow lru locking
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在 2020/9/10 下午9:49, Matthew Wilcox 写道:
> On Mon, Aug 24, 2020 at 08:54:39PM +0800, Alex Shi wrote:
>> lru_lock and page cache xa_lock have no reason with current sequence,
>> put them together isn't necessary. let's narrow the lru locking, but
>> left the local_irq_disable to block interrupt re-entry and statistic update.
>
> What stats are you talking about here?

Hi Matthew,

Thanks for comments!

like __dec_node_page_state(head, NR_SHMEM_THPS); will have preemptive warning...

>
>> +++ b/mm/huge_memory.c
>> @@ -2397,7 +2397,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
>> }
>>
>> static void __split_huge_page(struct page *page, struct list_head *list,
>> - pgoff_t end, unsigned long flags)
>> + pgoff_t end)
>
> Please don't change this whitespace. It's really annoying having to
> adjust the whitespace when renaming a function. Just two tabs indentation
> to give a clear separation of arguments from code is fine.
>
>
> How about this patch instead? It occurred to me we already have
> perfectly good infrastructure to track whether or not interrupts are
> already disabled, and so we should use that instead of ensuring that
> interrupts are disabled, or tracking that ourselves.

So your proposal looks like;
1, xa_lock_irq(&mapping->i_pages); (optional)
2, spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
3, spin_lock_irqsave(&pgdat->lru_lock, flags);

Is there meaningful for the 2nd and 3rd flags?

IIRC, I had a similar proposal as your, the flags used in xa_lock_irqsave(),
but objected by Hugh.

Thanks
Alex

>
> But I may have missed something else that's relying on having
> interrupts disabled. Please check carefully.
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2ccff8472cd4..74cae6c032f9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2376,17 +2376,16 @@ static void __split_huge_page_tail(struct page *head, int tail,
> }
>
> static void __split_huge_page(struct page *page, struct list_head *list,
> - pgoff_t end, unsigned long flags)
> + pgoff_t end)
> {
> struct page *head = compound_head(page);
> pg_data_t *pgdat = page_pgdat(head);
> struct lruvec *lruvec;
> struct address_space *swap_cache = NULL;
> unsigned long offset = 0;
> + unsigned long flags;
> int i;
>
> - lruvec = mem_cgroup_page_lruvec(head, pgdat);
> -
> /* complete memcg works before add pages to LRU */
> mem_cgroup_split_huge_fixup(head);
>
> @@ -2395,9 +2394,13 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>
> offset = swp_offset(entry);
> swap_cache = swap_address_space(entry);
> - xa_lock(&swap_cache->i_pages);
> + xa_lock_irq(&swap_cache->i_pages);
> }
>
> + /* prevent PageLRU to go away from under us, and freeze lru stats */
> + spin_lock_irqsave(&pgdat->lru_lock, flags);
> + lruvec = mem_cgroup_page_lruvec(head, pgdat);
> +
> for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
> __split_huge_page_tail(head, i, lruvec, list);
> /* Some pages can be beyond i_size: drop them from page cache */
> @@ -2417,6 +2420,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> }
>
> ClearPageCompound(head);
> + spin_unlock_irqrestore(&pgdat->lru_lock, flags);
>
> split_page_owner(head, HPAGE_PMD_ORDER);
>
> @@ -2425,18 +2429,16 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> /* Additional pin to swap cache */
> if (PageSwapCache(head)) {
> page_ref_add(head, 2);
> - xa_unlock(&swap_cache->i_pages);
> + xa_unlock_irq(&swap_cache->i_pages);
> } else {
> page_ref_inc(head);
> }
> } else {
> /* Additional pin to page cache */
> page_ref_add(head, 2);
> - xa_unlock(&head->mapping->i_pages);
> + xa_unlock_irq(&head->mapping->i_pages);
> }
>
> - spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> -
> remap_page(head);
>
> for (i = 0; i < HPAGE_PMD_NR; i++) {
> @@ -2574,7 +2576,6 @@ bool can_split_huge_page(struct page *page, int *pextra_pins)
> int split_huge_page_to_list(struct page *page, struct list_head *list)
> {
> struct page *head = compound_head(page);
> - struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
> struct deferred_split *ds_queue = get_deferred_split_queue(head);
> struct anon_vma *anon_vma = NULL;
> struct address_space *mapping = NULL;
> @@ -2640,9 +2641,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> unmap_page(head);
> VM_BUG_ON_PAGE(compound_mapcount(head), head);
>
> - /* prevent PageLRU to go away from under us, and freeze lru stats */
> - spin_lock_irqsave(&pgdata->lru_lock, flags);
> -
> if (mapping) {
> XA_STATE(xas, &mapping->i_pages, page_index(head));
>
> @@ -2650,13 +2648,13 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> * Check if the head page is present in page cache.
> * We assume all tail are present too, if head is there.
> */
> - xa_lock(&mapping->i_pages);
> + xa_lock_irq(&mapping->i_pages);
> if (xas_load(&xas) != head)
> goto fail;
> }
>
> /* Prevent deferred_split_scan() touching ->_refcount */
> - spin_lock(&ds_queue->split_queue_lock);
> + spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> count = page_count(head);
> mapcount = total_mapcount(head);
> if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
> @@ -2664,7 +2662,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> ds_queue->split_queue_len--;
> list_del(page_deferred_list(head));
> }
> - spin_unlock(&ds_queue->split_queue_lock);
> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> if (mapping) {
> if (PageSwapBacked(head))
> __dec_node_page_state(head, NR_SHMEM_THPS);
> @@ -2672,7 +2670,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> __dec_node_page_state(head, NR_FILE_THPS);
> }
>
> - __split_huge_page(page, list, end, flags);
> + __split_huge_page(page, list, end);
> if (PageSwapCache(head)) {
> swp_entry_t entry = { .val = page_private(head) };
>
> @@ -2688,10 +2686,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> dump_page(page, "total_mapcount(head) > 0");
> BUG();
> }
> - spin_unlock(&ds_queue->split_queue_lock);
> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> fail: if (mapping)
> - xa_unlock(&mapping->i_pages);
> - spin_unlock_irqrestore(&pgdata->lru_lock, flags);
> + xa_unlock_irq(&mapping->i_pages);
> remap_page(head);
> ret = -EBUSY;
> }
>

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