mm: don't cap request size based on read-ahead setting
We ran into a funky issue, where someone doing 256K buffered reads saw
128K requests at the device level. Turns out it is read-ahead capping
the request size, since we use 128K as the default setting. This
doesn't make a lot of sense - if someone is issuing 256K reads, they
should see 256K reads, regardless of the read-ahead setting, if the
underlying device can support a 256K read in a single command.
This patch introduces a bdi hint, io_pages. This is the soft max IO
size for the lower level, I've hooked it up to the bdev settings here.
Read-ahead is modified to issue the maximum of the user request size,
and the read-ahead max size, but capped to the max request size on the
device side. The latter is done to avoid reading ahead too much, if the
application asks for a huge read. With this patch, the kernel behaves
like the application expects.
Link: http://lkml.kernel.org/r/1479498073-8657-1-git-send-email-axboe@fb.com
Signed-off-by: Jens Axboe <axboe@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/readahead.c b/mm/readahead.c
index c8a955b..c4ca702 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -207,12 +207,21 @@ int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
* memory at once.
*/
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
- pgoff_t offset, unsigned long nr_to_read)
+ pgoff_t offset, unsigned long nr_to_read)
{
+ struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
+ struct file_ra_state *ra = &filp->f_ra;
+ unsigned long max_pages;
+
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);
+ /*
+ * If the request exceeds the readahead window, allow the read to
+ * be up to the optimal hardware IO size
+ */
+ max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
+ nr_to_read = min(nr_to_read, max_pages);
while (nr_to_read) {
int err;
@@ -369,10 +378,18 @@ ondemand_readahead(struct address_space *mapping,
bool hit_readahead_marker, pgoff_t offset,
unsigned long req_size)
{
- unsigned long max = ra->ra_pages;
+ struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
+ unsigned long max_pages = ra->ra_pages;
pgoff_t prev_offset;
/*
+ * If the request exceeds the readahead window, allow the read to
+ * be up to the optimal hardware IO size
+ */
+ if (req_size > max_pages && bdi->io_pages > max_pages)
+ max_pages = min(req_size, bdi->io_pages);
+
+ /*
* start of file
*/
if (!offset)
@@ -385,7 +402,7 @@ ondemand_readahead(struct address_space *mapping,
if ((offset == (ra->start + ra->size - ra->async_size) ||
offset == (ra->start + ra->size))) {
ra->start += ra->size;
- ra->size = get_next_ra_size(ra, max);
+ ra->size = get_next_ra_size(ra, max_pages);
ra->async_size = ra->size;
goto readit;
}
@@ -400,16 +417,16 @@ ondemand_readahead(struct address_space *mapping,
pgoff_t start;
rcu_read_lock();
- start = page_cache_next_hole(mapping, offset + 1, max);
+ start = page_cache_next_hole(mapping, offset + 1, max_pages);
rcu_read_unlock();
- if (!start || start - offset > max)
+ if (!start || start - offset > max_pages)
return 0;
ra->start = start;
ra->size = start - offset; /* old async_size */
ra->size += req_size;
- ra->size = get_next_ra_size(ra, max);
+ ra->size = get_next_ra_size(ra, max_pages);
ra->async_size = ra->size;
goto readit;
}
@@ -417,7 +434,7 @@ ondemand_readahead(struct address_space *mapping,
/*
* oversize read
*/
- if (req_size > max)
+ if (req_size > max_pages)
goto initial_readahead;
/*
@@ -433,7 +450,7 @@ ondemand_readahead(struct address_space *mapping,
* Query the page cache and look for the traces(cached history pages)
* that a sequential stream would leave behind.
*/
- if (try_context_readahead(mapping, ra, offset, req_size, max))
+ if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
goto readit;
/*
@@ -444,7 +461,7 @@ ondemand_readahead(struct address_space *mapping,
initial_readahead:
ra->start = offset;
- ra->size = get_init_ra_size(req_size, max);
+ ra->size = get_init_ra_size(req_size, max_pages);
ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
readit:
@@ -454,7 +471,7 @@ ondemand_readahead(struct address_space *mapping,
* the resulted next readahead window into the current one.
*/
if (offset == ra->start && ra->size == ra->async_size) {
- ra->async_size = get_next_ra_size(ra, max);
+ ra->async_size = get_next_ra_size(ra, max_pages);
ra->size += ra->async_size;
}
