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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>
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@ -249,6 +249,7 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
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max_sectors = min_not_zero(max_hw_sectors, limits->max_dev_sectors);
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max_sectors = min_t(unsigned int, max_sectors, BLK_DEF_MAX_SECTORS);
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limits->max_sectors = max_sectors;
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q->backing_dev_info.io_pages = max_sectors >> (PAGE_SHIFT - 9);
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}
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EXPORT_SYMBOL(blk_queue_max_hw_sectors);
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@ -212,6 +212,7 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
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spin_lock_irq(q->queue_lock);
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q->limits.max_sectors = max_sectors_kb << 1;
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q->backing_dev_info.io_pages = max_sectors_kb >> (PAGE_SHIFT - 10);
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spin_unlock_irq(q->queue_lock);
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return ret;
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@ -136,6 +136,7 @@ struct bdi_writeback {
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struct backing_dev_info {
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struct list_head bdi_list;
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unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
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unsigned long io_pages; /* max allowed IO size */
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unsigned int capabilities; /* Device capabilities */
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congested_fn *congested_fn; /* Function pointer if device is md/dm */
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void *congested_data; /* Pointer to aux data for congested func */
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@ -207,12 +207,21 @@ int __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
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* memory at once.
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*/
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int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
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pgoff_t offset, unsigned long nr_to_read)
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pgoff_t offset, unsigned long nr_to_read)
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{
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struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
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struct file_ra_state *ra = &filp->f_ra;
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unsigned long max_pages;
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if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
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return -EINVAL;
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nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
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/*
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* If the request exceeds the readahead window, allow the read to
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* be up to the optimal hardware IO size
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*/
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max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
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nr_to_read = min(nr_to_read, max_pages);
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while (nr_to_read) {
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int err;
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@ -369,9 +378,17 @@ ondemand_readahead(struct address_space *mapping,
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bool hit_readahead_marker, pgoff_t offset,
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unsigned long req_size)
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{
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unsigned long max = ra->ra_pages;
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struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
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unsigned long max_pages = ra->ra_pages;
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pgoff_t prev_offset;
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/*
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* If the request exceeds the readahead window, allow the read to
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* be up to the optimal hardware IO size
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*/
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if (req_size > max_pages && bdi->io_pages > max_pages)
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max_pages = min(req_size, bdi->io_pages);
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/*
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* start of file
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*/
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@ -385,7 +402,7 @@ ondemand_readahead(struct address_space *mapping,
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if ((offset == (ra->start + ra->size - ra->async_size) ||
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offset == (ra->start + ra->size))) {
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ra->start += ra->size;
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ra->size = get_next_ra_size(ra, max);
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ra->size = get_next_ra_size(ra, max_pages);
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ra->async_size = ra->size;
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goto readit;
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}
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@ -400,16 +417,16 @@ ondemand_readahead(struct address_space *mapping,
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pgoff_t start;
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rcu_read_lock();
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start = page_cache_next_hole(mapping, offset + 1, max);
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start = page_cache_next_hole(mapping, offset + 1, max_pages);
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rcu_read_unlock();
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if (!start || start - offset > max)
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if (!start || start - offset > max_pages)
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return 0;
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ra->start = start;
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ra->size = start - offset; /* old async_size */
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ra->size += req_size;
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ra->size = get_next_ra_size(ra, max);
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ra->size = get_next_ra_size(ra, max_pages);
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ra->async_size = ra->size;
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goto readit;
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}
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@ -417,7 +434,7 @@ ondemand_readahead(struct address_space *mapping,
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/*
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* oversize read
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*/
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if (req_size > max)
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if (req_size > max_pages)
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goto initial_readahead;
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/*
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@ -433,7 +450,7 @@ ondemand_readahead(struct address_space *mapping,
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* Query the page cache and look for the traces(cached history pages)
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* that a sequential stream would leave behind.
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*/
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if (try_context_readahead(mapping, ra, offset, req_size, max))
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if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
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goto readit;
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/*
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@ -444,7 +461,7 @@ ondemand_readahead(struct address_space *mapping,
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initial_readahead:
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ra->start = offset;
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ra->size = get_init_ra_size(req_size, max);
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ra->size = get_init_ra_size(req_size, max_pages);
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ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
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readit:
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@ -454,7 +471,7 @@ ondemand_readahead(struct address_space *mapping,
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* the resulted next readahead window into the current one.
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*/
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if (offset == ra->start && ra->size == ra->async_size) {
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ra->async_size = get_next_ra_size(ra, max);
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ra->async_size = get_next_ra_size(ra, max_pages);
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ra->size += ra->async_size;
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}
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