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

Matthew Wilcox
2017-02-09T21:18:35Z

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@microsoft.com>