Re: [PATCH v14 07/20] mm/thp: narrow lru locking

[Matthew Wilcox]

On Mon, Jul 06, 2020 at 05:15:09PM +0800, Alex Shi wrote:
> Hi Kirill & Johannes & Matthew,
> 
> Would you like to give some comments or share your concern of this patchset,
> specialy for THP part? 

I don't have the brain space to understand this patch set fully at
the moment.  I'll note that the realtime folks are doing their best to
stamp out users of local_irq_disable(), so they won't be pleased to see
you adding a new one.  Also, you removed the comment explaining why the
lock needed to be taken.

> Many Thanks
> Alex
> 
> å 2020/7/3 äå1:07, Alex Shi åé:
> > 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.
> > 
> > Hugh Dickins point: split_huge_page_to_list() was already silly,to be
> > using the _irqsave variant: it's just been taking sleeping locks, so
> > would already be broken if entered with interrupts enabled.
> > so we can save passing flags argument down to __split_huge_page().
> > 
> > Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  mm/huge_memory.c | 24 ++++++++++++------------
> >  1 file changed, 12 insertions(+), 12 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index b18f21da4dac..607869330329 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2433,7 +2433,7 @@ 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)
> >  {
> >  	struct page *head = compound_head(page);
> >  	pg_data_t *pgdat = page_pgdat(head);
> > @@ -2442,8 +2442,6 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >  	unsigned long offset = 0;
> >  	int i;
> >  
> > -	lruvec = mem_cgroup_page_lruvec(head, pgdat);
> > -
> >  	/* complete memcg works before add pages to LRU */
> >  	mem_cgroup_split_huge_fixup(head);
> >  
> > @@ -2455,6 +2453,11 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >  		xa_lock(&swap_cache->i_pages);
> >  	}
> >  
> > +	/* lock lru list/PageCompound, ref freezed by page_ref_freeze */
> > +	spin_lock(&pgdat->lru_lock);
> > +
> > +	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 */
> > @@ -2474,6 +2477,8 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >  	}
> >  
> >  	ClearPageCompound(head);
> > +	spin_unlock(&pgdat->lru_lock);
> > +	/* Caller disabled irqs, so they are still disabled here */
> >  
> >  	split_page_owner(head, HPAGE_PMD_ORDER);
> >  
> > @@ -2491,8 +2496,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >  		page_ref_add(head, 2);
> >  		xa_unlock(&head->mapping->i_pages);
> >  	}
> > -
> > -	spin_unlock_irqrestore(&pgdat->lru_lock, flags);
> > +	local_irq_enable();
> >  
> >  	remap_page(head);
> >  
> > @@ -2631,12 +2635,10 @@ 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;
> >  	int count, mapcount, extra_pins, ret;
> > -	unsigned long flags;
> >  	pgoff_t end;
> >  
> >  	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
> > @@ -2697,9 +2699,7 @@ 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);
> > -
> > +	local_irq_disable();
> >  	if (mapping) {
> >  		XA_STATE(xas, &mapping->i_pages, page_index(head));
> >  
> > @@ -2729,7 +2729,7 @@ 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) };
> >  
> > @@ -2748,7 +2748,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
> >  		spin_unlock(&ds_queue->split_queue_lock);
> >  fail:		if (mapping)
> >  			xa_unlock(&mapping->i_pages);
> > -		spin_unlock_irqrestore(&pgdata->lru_lock, flags);
> > +		local_irq_enable();
> >  		remap_page(head);
> >  		ret = -EBUSY;
> >  	}
> >