linux-kernel - [PATCH] mm: readahead: do not cap readahead() and MADV

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Message-Id: <1456277927-12044-1-git-send-email-hannes@cmpxchg.org>
Date:	Tue, 23 Feb 2016 17:38:47 -0800
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: [PATCH] mm: readahead: do not cap readahead() and MADV_WILLNEED

All readahead is currently capped to a maximum of the device readahead
limit, which defaults to 128k. For heuristics-based readahead this
makes perfect sense, too, but unfortunately the limit is also applied
to the explicit readahead() or madvise(MADV_WILLNEED) syscalls, and
128k is an awfully low limit, particularly for bigger machines. It's
not unreasonable for a user on a 100G machine to say, read this 1G
file, and read it now, I'm going to access the whole thing shortly.

Since both readahead() and MADV_WILLNEED take an explicit length
parameter, it seems weird to truncate that request quietly. Just do
what the user asked for and leave the limiting to the heuristics.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 mm/readahead.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 20e58e8..6d182db 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -212,7 +212,6 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 		return -EINVAL;

-	nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
 	while (nr_to_read) {
 		int err;

@@ -485,6 +484,7 @@ void page_cache_sync_readahead(struct address_space *mapping,

 	/* be dumb */
 	if (filp && (filp->f_mode & FMODE_RANDOM)) {
+		req_size = min(req_size, inode_to_bdi(mapping->host)->ra_pages);
 		force_page_cache_readahead(mapping, filp, offset, req_size);
 		return;
 	}
-- 
2.7.1