Re: [PATCHv6 07/37] filemap: allocate huge page in page_cache_read(), if allowed

[Matthew Wilcox]

On Thu, Jan 26, 2017 at 02:57:49PM +0300, Kirill A. Shutemov wrote:
> Later we can add logic to accumulate information from shadow entires to
> return to caller (average eviction time?).

I would say minimum rather than average.  That will become the refault
time of the entire page, so minimum would probably have us making better
decisions?

> +	/* Wipe shadow entires */
> +	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter,
> +			page->index) {
> +		if (iter.index >= page->index + hpage_nr_pages(page))
> +			break;
>  
>  		p = radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
> -		if (!radix_tree_exceptional_entry(p))
> +		if (!p)
> +			continue;

Just FYI, this can't happen.  You're holding the tree lock so nobody
else gets to remove things from the tree.  radix_tree_for_each_slot()
only gives you the full slots; it skips the empty ones for you.  I'm
OK if you want to leave it in out of an abundance of caution.

> +		__radix_tree_replace(&mapping->page_tree, iter.node, slot, NULL,
> +				workingset_update_node, mapping);

I may add an update_node argument to radix_tree_join at some point,
so you can use it here.  Or maybe we don't need to do that, and what
you have here works just fine.

>  		mapping->nrexceptional--;

... because adjusting the exceptional count is going to be a pain.

> +	error = __radix_tree_insert(&mapping->page_tree,
> +			page->index, compound_order(page), page);
> +	/* This shouldn't happen */
> +	if (WARN_ON_ONCE(error))
> +		return error;

A lesser man would have just ignored the return value from
__radix_tree_insert.  I salute you.

> @@ -2078,18 +2155,34 @@ static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask)
>  {
>  	struct address_space *mapping = file->f_mapping;
>  	struct page *page;
> +	pgoff_t hoffset;
>  	int ret;
>  
>  	do {
> -		page = __page_cache_alloc(gfp_mask|__GFP_COLD);
> +		page = page_cache_alloc_huge(mapping, offset, gfp_mask);
> +no_huge:
> +		if (!page)
> +			page = __page_cache_alloc(gfp_mask|__GFP_COLD);
>  		if (!page)
>  			return -ENOMEM;
>  
> -		ret = add_to_page_cache_lru(page, mapping, offset, gfp_mask & GFP_KERNEL);
> -		if (ret == 0)
> +		if (PageTransHuge(page))
> +			hoffset = round_down(offset, HPAGE_PMD_NR);
> +		else
> +			hoffset = offset;
> +
> +		ret = add_to_page_cache_lru(page, mapping, hoffset,
> +				gfp_mask & GFP_KERNEL);
> +
> +		if (ret == -EEXIST && PageTransHuge(page)) {
> +			put_page(page);
> +			page = NULL;
> +			goto no_huge;
> +		} else if (ret == 0) {
>  			ret = mapping->a_ops->readpage(file, page);
> -		else if (ret == -EEXIST)
> +		} else if (ret == -EEXIST) {
>  			ret = 0; /* losing race to add is OK */
> +		}
>  
>  		put_page(page);

If the filesystem returns AOP_TRUNCATED_PAGE, you'll go round this loop
again trying the huge page again, even if the huge page didn't work
the first time.  I would tend to think that if the huge page failed the
first time, we shouldn't try it again, so I propose this:

        struct address_space *mapping = file->f_mapping;
        struct page *page;
        pgoff_t index;
        int ret;
        bool try_huge = true;

        do {
                if (try_huge) {
                        page = page_cache_alloc_huge(gfp_mask|__GFP_COLD);
                        if (page)
                                index = round_down(offset, HPAGE_PMD_NR);
                        else
                                try_huge = false;
                }

                if (!try_huge) {
                        page = __page_cache_alloc(gfp_mask|__GFP_COLD);
                        index = offset;
                }

                if (!page)
                        return -ENOMEM;

                ret = add_to_page_cache_lru(page, mapping, index,
                                                        gfp_mask & GFP_KERNEL);
                if (ret < 0) {
                        if (try_huge) {
                                try_huge = false;
                                ret = AOP_TRUNCATED_PAGE;
                        } else if (ret == -EEXIST)
                                ret = 0; /* losing race to add is OK */
                } else {
                        ret = mapping->a_ops->readpage(file, page);
                }

                put_page(page);
        } while (ret == AOP_TRUNCATED_PAGE);

But ... maybe it's OK to retry the huge page.  I mean, not many
filesystems return AOP_TRUNCATED_PAGE, and they only do so rarely.

Anyway, I'm fine with the patch going in as-is.  I just wanted to type out
my review notes.

Reviewed-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>