Commit c4fa6309 authored Sep 06, 2017 by Huang Ying Committed by Linus Torvalds Sep 06, 2017

mm, swap: fix swap readahead marking

In the original implementation, it is possible that the existing pages
in the swap cache (not newly readahead) could be marked as the readahead
pages.  This will cause the statistics of swap readahead be wrong and
influence the swap readahead algorithm too.

This is fixed via marking a page as the readahead page only if it is
newly allocated and read from the disk.

When testing with linpack, after the fixing the swap readahead hit rate
increased from ~66% to ~86%.

Link: http://lkml.kernel.org/r/20170807054038.1843-3-ying.huang@intel.com


Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent cbc65df2

mm/swap_state.c

+11 −7

Original line number	Diff line number	Diff line
		@@ -498,7 +498,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
		unsigned long start_offset, end_offset;
		unsigned long mask;
		struct blk_plug plug;
		bool do_poll = true;
		bool do_poll = true, page_allocated;

		mask = swapin_nr_pages(offset) - 1;
		if (!mask)
		@@ -514,15 +514,19 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
		blk_start_plug(&plug);
		for (offset = start_offset; offset <= end_offset ; offset++) {
		/* Ok, do the async read-ahead now */
		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
		gfp_mask, vma, addr, false);
		page = __read_swap_cache_async(
		swp_entry(swp_type(entry), offset),
		gfp_mask, vma, addr, &page_allocated);
		if (!page)
		continue;
		if (page_allocated) {
		swap_readpage(page, false);
		if (offset != entry_offset &&
		likely(!PageTransCompound(page))) {
		SetPageReadahead(page);
		count_vm_event(SWAP_RA);
		}
		}
		put_page(page);
		}
		blk_finish_plug(&plug);

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