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/block/blk-settings.c b/block/blk-settings.c
index f679ae1..65f16cf 100644 (file)
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -249,6 +249,7 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
        max_sectors = min_not_zero(max_hw_sectors, limits->max_dev_sectors);
        max_sectors = min_t(unsigned int, max_sectors, BLK_DEF_MAX_SECTORS);
        limits->max_sectors = max_sectors;
+       q->backing_dev_info.io_pages = max_sectors >> (PAGE_SHIFT - 9);
 }
 EXPORT_SYMBOL(blk_queue_max_hw_sectors);
 

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 9cc8d7c..ea374e8 100644 (file)
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -212,6 +212,7 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
 
        spin_lock_irq(q->queue_lock);
        q->limits.max_sectors = max_sectors_kb << 1;
+       q->backing_dev_info.io_pages = max_sectors_kb >> (PAGE_SHIFT - 10);
        spin_unlock_irq(q->queue_lock);
 
        return ret;

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index c357f27..b8144b2 100644 (file)
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -136,6 +136,7 @@ struct bdi_writeback {
 struct backing_dev_info {
        struct list_head bdi_list;
        unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
+       unsigned long io_pages; /* max allowed IO size */
        unsigned int capabilities; /* Device capabilities */
        congested_fn *congested_fn; /* Function pointer if device is md/dm */
        void *congested_data;   /* Pointer to aux data for congested func */

diff --git a/mm/readahead.c b/mm/readahead.c
index c8a955b..c4ca702 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -207,12 +207,21 @@ out:
  * 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 @@ readit:
         * 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;
        }