From: Andrea Righi andrea.righi@canonical.com

Commit ebc5951eea499314f6fbbde20e295f1345c67330 upstream.

[ This fixes a performance issue we're seeing in AWS instances when running swapoff and using the global readahead algorithm. For a particular instance configuration, Without this fix I/O throughput is very low during swapoff (about 15 MB/s) with this patch is reaches 500 MB/s. Tested swapoff with different workloads with this patch applied. 5.10 onwards already have this fix ]

In unuse_pte_range() we blindly swap-in pages without checking if the swap entry is already present in the swap cache.

By doing this, the hit/miss ratio used by the swap readahead heuristic is not properly updated and this leads to non-optimal performance during swapoff.

Tracing the distribution of the readahead size returned by the swap readahead heuristic during swapoff shows that a small readahead size is used most of the time as if we had only misses (this happens both with cluster and vma readahead), for example:

r::swapin_nr_pages(unsigned long offset):unsigned long:$retval COUNT EVENT 36948 $retval = 8 44151 $retval = 4 49290 $retval = 1 527771 $retval = 2

Checking if the swap entry is present in the swap cache, instead, allows to properly update the readahead statistics and the heuristic behaves in a better way during swapoff, selecting a bigger readahead size:

r::swapin_nr_pages(unsigned long offset):unsigned long:$retval COUNT EVENT 1618 $retval = 1 4960 $retval = 2 41315 $retval = 4 103521 $retval = 8

In terms of swapoff performance the result is the following:

Testing environment ===================

- Host: CPU: 1.8GHz Intel Core i7-8565U (quad-core, 8MB cache) HDD: PC401 NVMe SK hynix 512GB MEM: 16GB

- Guest (kvm): 8GB of RAM virtio block driver 16GB swap file on ext4 (/swapfile)

Test case ========= - allocate 85% of memory - `systemctl hibernate` to force all the pages to be swapped-out to the swap file - resume the system - measure the time that swapoff takes to complete: # /usr/bin/time swapoff /swapfile

Result (swapoff time) ====== 5.6 vanilla 5.6 w/ this patch ----------- ----------------- cluster-readahead 22.09s 12.19s vma-readahead 18.20s 15.33s

Conclusion ==========

The specific use case this patch is addressing is to improve swapoff performance in cloud environments when a VM has been hibernated, resumed and all the memory needs to be forced back to RAM by disabling swap.

This change allows to better exploits the advantages of the readahead heuristic during swapoff and this improvement allows to to speed up the resume process of such VMs.

[andrea.righi@canonical.com: update changelog] Link: http://lkml.kernel.org/r/20200418084705.GA147642@xps-13 Signed-off-by: Andrea Righi andrea.righi@canonical.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: "Huang, Ying" ying.huang@intel.com Cc: Minchan Kim minchan@kernel.org Cc: Anchal Agarwal anchalag@amazon.com Cc: Hugh Dickins hughd@google.com Cc: Vineeth Remanan Pillai vpillai@digitalocean.com Cc: Kelley Nielsen kelleynnn@gmail.com Link: http://lkml.kernel.org/r/20200416180132.GB3352@xps-13 Signed-off-by: Linus Torvalds torvalds@linux-foundation.org --- mm/swapfile.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c index f6964212c6c8..fe5995c38ea4 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -1950,10 +1950,14 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,

pte_unmap(pte); swap_map = &si->swap_map[offset]; - vmf.vma = vma; - vmf.address = addr; - vmf.pmd = pmd; - page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, &vmf); + page = lookup_swap_cache(entry, vma, addr); + if (!page) { + vmf.vma = vma; + vmf.address = addr; + vmf.pmd = pmd; + page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, + &vmf); + } if (!page) { if (*swap_map == 0 || *swap_map == SWAP_MAP_BAD) goto try_next;