Commit 11bc82d6 authored by Andrea Arcangeli's avatar Andrea Arcangeli Committed by Linus Torvalds

mm: compaction: Use async migration for __GFP_NO_KSWAPD and enforce no writeback

__GFP_NO_KSWAPD allocations are usually very expensive and not mandatory
to succeed as they have graceful fallback.  Waiting for I/O in those,
tends to be overkill in terms of latencies, so we can reduce their latency
by disabling sync migrate.

Unfortunately, even with async migration it's still possible for the
process to be blocked waiting for a request slot (e.g.  get_request_wait
in the block layer) when ->writepage is called.  To prevent
__GFP_NO_KSWAPD blocking, this patch prevents ->writepage being called on
dirty page cache for asynchronous migration.


[ Avoid writebacks for NFS, retry locked pages, use bool]
Signed-off-by: default avatarAndrea Arcangeli <>
Signed-off-by: default avatarMel Gorman <>
Cc: Arthur Marsh <>
Cc: Clemens Ladisch <>
Cc: Johannes Weiner <>
Cc: KAMEZAWA Hiroyuki <>
Cc: Minchan Kim <>
Reported-by: default avatarAlex Villacis Lasso <>
Tested-by: default avatarAlex Villacis Lasso <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent b2eef8c0
......@@ -564,7 +564,7 @@ static int fallback_migrate_page(struct address_space *mapping,
* == 0 - success
static int move_to_new_page(struct page *newpage, struct page *page,
int remap_swapcache)
int remap_swapcache, bool sync)
struct address_space *mapping;
int rc;
......@@ -586,6 +586,15 @@ static int move_to_new_page(struct page *newpage, struct page *page,
mapping = page_mapping(page);
if (!mapping)
rc = migrate_page(mapping, newpage, page);
else {
* Do not writeback pages if !sync and migratepage is
* not pointing to migrate_page() which is nonblocking
* (swapcache/tmpfs uses migratepage = migrate_page).
if (PageDirty(page) && !sync &&
mapping->a_ops->migratepage != migrate_page)
rc = -EBUSY;
else if (mapping->a_ops->migratepage)
* Most pages have a mapping and most filesystems
......@@ -598,6 +607,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
newpage, page);
rc = fallback_migrate_page(mapping, newpage, page);
if (rc) {
newpage->mapping = NULL;
......@@ -641,7 +651,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
rc = -EAGAIN;
if (!trylock_page(page)) {
if (!force)
if (!force || !sync)
goto move_newpage;
......@@ -686,7 +696,15 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
if (PageWriteback(page)) {
if (!force || !sync)
* For !sync, there is no point retrying as the retry loop
* is expected to be too short for PageWriteback to be cleared
if (!sync) {
rc = -EBUSY;
goto uncharge;
if (!force)
goto uncharge;
......@@ -757,7 +775,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
if (!page_mapped(page))
rc = move_to_new_page(newpage, page, remap_swapcache);
rc = move_to_new_page(newpage, page, remap_swapcache, sync);
if (rc && remap_swapcache)
remove_migration_ptes(page, page);
......@@ -850,7 +868,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
if (!page_mapped(hpage))
rc = move_to_new_page(new_hpage, hpage, 1);
rc = move_to_new_page(new_hpage, hpage, 1, sync);
if (rc)
remove_migration_ptes(hpage, hpage);
......@@ -2103,7 +2103,7 @@ rebalance:
if (page)
goto got_pg;
sync_migration = true;
sync_migration = !(gfp_mask & __GFP_NO_KSWAPD);
/* Try direct reclaim and then allocating */
page = __alloc_pages_direct_reclaim(gfp_mask, order,
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