Commit dc30b96a authored Jul 27, 2018 by Markus Stockhausen Committed by Jens Axboe Jul 27, 2018

readahead: stricter check for bdi io_pages



ondemand_readahead() checks bdi->io_pages to cap the maximum pages
that need to be processed. This works until the readit section. If
we would do an async only readahead (async size = sync size) and
target is at beginning of window we expand the pages by another
get_next_ra_size() pages. Btrace for large reads shows that kernel
always issues a doubled size read at the beginning of processing.
Add an additional check for io_pages in the lower part of the func.
The fix helps devices that hard limit bio pages and rely on proper
handling of max_hw_read_sectors (e.g. older FusionIO cards). For
that reason it could qualify for stable.

Fixes: 9491ae4a ("mm: don't cap request size based on read-ahead setting")
Cc: stable@vger.kernel.org
Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent cdcdcaae

mm/readahead.c

+10 −2

Original line number	Diff line number	Diff line
		@@ -386,6 +386,7 @@ ondemand_readahead(struct address_space *mapping,
		{
		struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
		unsigned long max_pages = ra->ra_pages;
		unsigned long add_pages;
		pgoff_t prev_offset;

		/*
		@@ -475,10 +476,17 @@ readit:
		* Will this read hit the readahead marker made by itself?
		* If so, trigger the readahead marker hit now, and merge
		* the resulted next readahead window into the current one.
		* Take care of maximum IO pages as above.
		*/
		if (offset == ra->start && ra->size == ra->async_size) {
		ra->async_size = get_next_ra_size(ra, max_pages);
		ra->size += ra->async_size;
		add_pages = get_next_ra_size(ra, max_pages);
		if (ra->size + add_pages <= max_pages) {
		ra->async_size = add_pages;
		ra->size += add_pages;
		} else {
		ra->size = max_pages;
		ra->async_size = max_pages >> 1;
		}
		}

		return ra_submit(ra, mapping, filp);

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