linux-ext4 - [PATCHv4 17/43] HACK: readahead: alloc huge pages, if allowed

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Date:   Tue, 25 Oct 2016 03:13:16 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCHv4 17/43] HACK: readahead: alloc huge pages, if allowed

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@...ux.intel.com>
---
 mm/readahead.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index c8a955b1297e..f46a9080f6a9 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
-- 
2.9.3

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