On 2025/1/6 12:59, Baolin Wang wrote:
On 2025/1/6 12:07, Matthew Wilcox wrote:
On Mon, Jan 06, 2025 at 11:46:04AM +0800, Baolin Wang wrote:
On 2025/1/2 21:10, Matthew Wilcox wrote:
On Thu, Jan 02, 2025 at 04:40:17PM +0800, Baolin Wang wrote:
With fast swap devices (such as zram), swapin latency is crucial to
applications.
For shmem swapin, similar to anonymous memory swapin, we can skip
the swapcache
operation to improve swapin latency.
OK, but now we have more complexity. Why can't we always skip the
swapcache on swapin?
Skipping swapcache is used to swap-in shmem large folios, avoiding
the large
folios being split. Meanwhile, since the IO latency of syncing swap
devices
is relatively small, it won't cause the IO latency amplification issue.
But for async swap devices, if we swap-in the large folio one-time, I am
afraid the IO latency can be amplified. And I remember we still haven't
reached an agreement here[1], so let's step by step and start with
the sync
swap devices first.
Regardless of whether we choose to swap-in an order-0 or a large folio,
my point is that we should always do it to the pagecache rather than the
swap cache.
IMO, this would miss the swap readahead algorithm in the swap case,
which can benefit the order-0 swap-in. We need more work to ensure that
skipping swapcache is helpful for all cases, which is why I'm starting
with sync swap devices first.
BTW, I used the SSD swap device to test the performance of skipping
swapcache with the following hack changes, and I found that the
performance of order-0 sequential swap-in shows a significant regression.
Without the following changes:
1G order-0 shmem swap-in: 8056 ms
With the following changes (skip swapcache):
1G order-0 shmem swap-in: 38536 ms
diff --git a/mm/page_io.c b/mm/page_io.c
index 9b983de351f9..1e22dedcd584 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -620,7 +620,6 @@ void swap_read_folio(struct folio *folio, struct
swap_iocb **plug)
unsigned long pflags;
bool in_thrashing;
- VM_BUG_ON_FOLIO(!folio_test_swapcache(folio) && !synchronous,
folio);
VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
VM_BUG_ON_FOLIO(folio_test_uptodate(folio), folio);
diff --git a/mm/shmem.c b/mm/shmem.c
index e82ef1ef1c68..2902d3477520 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2295,7 +2295,7 @@ static int shmem_swapin_folio(struct inode *inode,
pgoff_t index,
fallback_order0 = true;
/* Skip swapcache for synchronous device. */
- if (!fallback_order0 && data_race(si->flags &
SWP_SYNCHRONOUS_IO)) {
+ if (!fallback_order0) {
folio = shmem_swap_alloc_folio(inode, vma,
index, swap, order, gfp);
if (!IS_ERR(folio)) {
skip_swapcache = true;
