Linux-mm Archive on
 help / color / Atom feed
From: Hugh Dickins <>
To: Andrew Morton <>
Cc: Konstantin Khlebnikov <>,
	 "Alex Xu (Hello71)" <>,
	 Vineeth Pillai <>,
	 Kelley Nielsen <>,
	Rik van Riel <>,
	 Huang Ying <>,
	Hugh Dickins <>,,
Subject: [PATCH 2/4] mm: swapoff: remove too limiting SWAP_UNUSE_MAX_TRIES
Date: Mon, 8 Apr 2019 12:58:14 -0700 (PDT)
Message-ID: <alpine.LSU.2.11.1904081256170.1523@eggly.anvils> (raw)
In-Reply-To: <alpine.LSU.2.11.1904081249370.1523@eggly.anvils>

SWAP_UNUSE_MAX_TRIES 3 appeared to work well in earlier testing, but
further testing has proved it to be a source of unnecessary swapoff
EBUSY failures (which can then be followed by unmount EBUSY failures).

When mmget_not_zero() or shmem's igrab() fails, there is an mm exiting
or inode being evicted, freeing up swap independent of try_to_unuse().
Those typically completed much sooner than the old quadratic swapoff,
but now it's more common that swapoff may need to wait for them.

It's possible to move those cases from init_mm.mmlist and shmem_swaplist
to separate "exiting" swaplists, and try_to_unuse() then wait for those
lists to be emptied; but we've not bothered with that in the past, and
don't want to risk missing some other forgotten case. So just revert
to cycling around until the swap is gone, without any retries limit.

Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
Signed-off-by: Hugh Dickins <>

 mm/swapfile.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

--- 5.1-rc4/mm/swapfile.c	2019-03-17 16:18:15.713823942 -0700
+++ linux/mm/swapfile.c	2019-04-07 19:15:01.269054187 -0700
@@ -2023,7 +2023,6 @@ static unsigned int find_next_to_unuse(s
  * If the boolean frontswap is true, only unuse pages_to_unuse pages;
  * pages_to_unuse==0 means all pages; ignored if frontswap is false
 int try_to_unuse(unsigned int type, bool frontswap,
 		 unsigned long pages_to_unuse)
@@ -2035,7 +2034,6 @@ int try_to_unuse(unsigned int type, bool
 	struct page *page;
 	swp_entry_t entry;
 	unsigned int i;
-	int retries = 0;
 	if (!si->inuse_pages)
 		return 0;
@@ -2117,14 +2115,16 @@ retry:
 	 * If yes, we would need to do retry the unuse logic again.
 	 * Under global memory pressure, swap entries can be reinserted back
 	 * into process space after the mmlist loop above passes over them.
-	 * Its not worth continuosuly retrying to unuse the swap in this case.
-	 * So we try SWAP_UNUSE_MAX_TRIES times.
+	 *
+	 * Limit the number of retries? No: when shmem_unuse()'s igrab() fails,
+	 * a shmem inode using swap is being evicted; and when mmget_not_zero()
+	 * above fails, that mm is likely to be freeing swap from exit_mmap().
+	 * Both proceed at their own independent pace: we could move them to
+	 * separate lists, and wait for those lists to be emptied; but it's
+	 * easier and more robust (though cpu-intensive) just to keep retrying.
-	if (++retries >= SWAP_UNUSE_MAX_TRIES)
-		retval = -EBUSY;
-	else if (si->inuse_pages)
+	if (si->inuse_pages)
 		goto retry;
 	return (retval == FRONTSWAP_PAGES_UNUSED) ? 0 : retval;

  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-08 19:53 [PATCH 0/4] mm: swapoff: fixes for 5.1-rc Hugh Dickins
2019-04-08 19:56 ` [PATCH 1/4] mm: swapoff: shmem_find_swap_entries() filter out other types Hugh Dickins
2019-04-08 19:58 ` Hugh Dickins [this message]
2019-04-08 19:59 ` [PATCH 3/4] mm: swapoff: take notice of completion sooner Hugh Dickins
2019-04-08 20:01 ` [PATCH 4/4] mm: swapoff: shmem_unuse() stop eviction without igrab() Hugh Dickins
2019-04-09  7:50   ` Konstantin Khlebnikov
2019-04-09 18:43     ` Hugh Dickins

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=alpine.LSU.2.11.1904081256170.1523@eggly.anvils \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-mm Archive on

Archives are clonable:
	git clone --mirror linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ \
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone public-inbox