Re: [PATCH 09/15] mm: Allow large pages to be added to the page cache

[Kirill A. Shutemov]

On Tue, Sep 24, 2019 at 05:52:08PM -0700, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> We return -EEXIST if there are any non-shadow entries in the page
> cache in the range covered by the large page.  If there are multiple
> shadow entries in the range, we set *shadowp to one of them (currently
> the one at the highest index).  If that turns out to be the wrong
> answer, we can implement something more complex.  This is mostly
> modelled after the equivalent function in the shmem code.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>  mm/filemap.c | 37 ++++++++++++++++++++++++++-----------
>  1 file changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bab97addbb1d..afe8f5d95810 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -855,6 +855,7 @@ static int __add_to_page_cache_locked(struct page *page,
>  	int huge = PageHuge(page);
>  	struct mem_cgroup *memcg;
>  	int error;
> +	unsigned int nr = 1;
>  	void *old;
>  
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> @@ -866,31 +867,45 @@ static int __add_to_page_cache_locked(struct page *page,
>  					      gfp_mask, &memcg, false);
>  		if (error)
>  			return error;
> +		xas_set_order(&xas, offset, compound_order(page));
> +		nr = compound_nr(page);
>  	}
>  
> -	get_page(page);
> +	page_ref_add(page, nr);
>  	page->mapping = mapping;
>  	page->index = offset;
>  
>  	do {
> +		unsigned long exceptional = 0;
> +		unsigned int i = 0;
> +
>  		xas_lock_irq(&xas);
> -		old = xas_load(&xas);
> -		if (old && !xa_is_value(old))
> +		xas_for_each_conflict(&xas, old) {
> +			if (!xa_is_value(old))
> +				break;
> +			exceptional++;
> +			if (shadowp)
> +				*shadowp = old;
> +		}
> +		if (old)
>  			xas_set_err(&xas, -EEXIST);

This made me confused.

Do we rely on 'old' to be NULL if the loop has completed without 'break'?
It's not very obvious.

Can we have a comment or call xas_set_err() within the loop next to the
'break'?

> -		xas_store(&xas, page);
> +		xas_create_range(&xas);
>  		if (xas_error(&xas))
>  			goto unlock;
>  
> -		if (xa_is_value(old)) {
> -			mapping->nrexceptional--;
> -			if (shadowp)
> -				*shadowp = old;
> +next:
> +		xas_store(&xas, page);
> +		if (++i < nr) {
> +			xas_next(&xas);
> +			goto next;
>  		}
> -		mapping->nrpages++;
> +		mapping->nrexceptional -= exceptional;
> +		mapping->nrpages += nr;
>  
>  		/* hugetlb pages do not participate in page cache accounting */
>  		if (!huge)
> -			__inc_node_page_state(page, NR_FILE_PAGES);
> +			__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES,
> +						nr);

We also need to bump NR_FILE_THPS here.

>  unlock:
>  		xas_unlock_irq(&xas);
>  	} while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK));
> @@ -907,7 +922,7 @@ static int __add_to_page_cache_locked(struct page *page,
>  	/* Leave page->index set: truncation relies upon it */
>  	if (!huge)
>  		mem_cgroup_cancel_charge(page, memcg, false);
> -	put_page(page);
> +	page_ref_sub(page, nr);
>  	return xas_error(&xas);
>  }
>  ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
> -- 
> 2.23.0
> 
> 

-- 
 Kirill A. Shutemov