[PATCHv6 11/37] HACK: readahead: alloc huge pages, if allowed

Kirill A. Shutemov
2017-01-26T11:57:53Z

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 c4ca70239233..289527a06254 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