[PATCHv5 11/36] HACK: readahead: alloc huge pages, if allowed

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Most page cache allocation happens via readahead (sync or async), so if
we want to have significant number of huge pages in page cache we need
to find a ways to allocate them from readahead.

Unfortunately, huge pages doesn't fit into current readahead design:
128 max readahead window, assumption on page size, PageReadahead() to
track hit/miss.

I haven't found a ways to get it right yet.

This patch just allocates huge page if allowed, but doesn't really
provide any readahead if huge page is allocated. We read out 2M a time
and I would expect spikes in latancy without readahead.

Therefore HACK.

Having that said, I don't think it should prevent huge page support to
be applied. Future will show if lacking readahead is a big deal with
huge pages in page cache.

Any suggestions are welcome.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/readahead.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index fb4c99f85618..87e38b522645 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -174,6 +174,21 @@ int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 		if (page_offset > end_index)
 			break;
 
+		if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
+				(!page_idx || !(page_offset % HPAGE_PMD_NR)) &&
+				page_cache_allow_huge(mapping, page_offset)) {
+			page = __page_cache_alloc_order(gfp_mask | __GFP_COMP,
+					HPAGE_PMD_ORDER);
+			if (page) {
+				prep_transhuge_page(page);
+				page->index = round_down(page_offset,
+						HPAGE_PMD_NR);
+				list_add(&page->lru, &page_pool);
+				ret++;
+				goto start_io;
+			}
+		}
+
 		rcu_read_lock();
 		page = radix_tree_lookup(&mapping->page_tree, page_offset);
 		rcu_read_unlock();
@@ -189,7 +204,7 @@ int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			SetPageReadahead(page);
 		ret++;
 	}
-
+start_io:
 	/*
 	 * Now start the IO.  We ignore I/O errors - if the page is not
 	 * uptodate then the caller will launch readpage again, and
-- 
2.10.2