+ mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch added to mm-unstable branch

+ mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: bdi: reset min_ratio after the sysfs attributes are gone
has been added to the -mm mm-unstable branch.  Its filename is
     mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nickolai Zeldovich <nickolai@csail.mit.edu>
Subject: mm: bdi: reset min_ratio after the sysfs attributes are gone
Date: Fri, 16 Oct 2026 23:06:20 -0400

A write to /sys/class/bdi/<dev>/min_ratio (or min_ratio_fine or min_bytes)
that races with the removal of the device leaves the global sum of minimum
dirty shares charged for a bdi that no longer exists.  Until the next
reboot, that share of the dirty threshold is withheld from every other bdi
(__wb_calc_thresh() scales by 100 % minus the sum), and a later min_ratio
store on any bdi that would push the sum to 100 % fails with EINVAL
although every bdi reads 0.  Root privilege is needed for the store; the
removal is any del_gendisk() (a disk or loop device going away) or the
unmount of a filesystem with its own bdi.  On an x86_64 defconfig kernel
(v7.3-rc7), 2000 rounds of LOOP_CTL_ADD/LOOP_CTL_REMOVE on one loop device
with a second thread writing 50 to its min_ratio leak the share on the
first round the store lands in the window; afterwards `echo 99 >
min_ratio` on another loop device fails with EINVAL.  With the fix the
same run keeps the sum at 0 and the store succeeds.  Affected since v5.16,
when the reset in bdi_unregister() was added.

bdi_unregister() gives the bdi's share back with bdi_set_min_ratio(bdi, 0)
and only then calls device_unregister(), which removes the sysfs
attributes.  A min_ratio_store() that gets its kernfs active reference
before the removal runs to completion after the reset:
__bdi_set_min_ratio() takes bdi_lock, sees bdi->min_ratio == 0, adds the
new value to bdi_min_ratio and stores it in the bdi, which bdi_put() then
frees.  Nothing gives that share back.

Move the reset after device_unregister(): once the attributes are gone no
store can run, and bdi_set_min_ratio() needs nothing from the device.

Found while formally verifying bdi_unregister() against the kernel binary.

Link: https://lore.kernel.org/20261017030620.793108-1-nickolai@csail.mit.edu
Fixes: 3c376dfafbf7 ("mm: bdi: initialize bdi_min_ratio when bdi is unregistered")
Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Assisted-by: LLM
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Manjong Lee <mj0123.lee@samsung.com>
Cc: Jan Kara <jack@suse.cz>
---

 mm/backing-dev.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

--- a/mm/backing-dev.c~mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone
+++ a/mm/backing-dev.c
@@ -1162,19 +1162,20 @@ void bdi_unregister(struct backing_dev_i
 	wb_shutdown(&bdi->wb);
 	cgwb_bdi_unregister(bdi);
 
-	/*
-	 * If this BDI's min ratio has been set, use bdi_set_min_ratio() to
-	 * update the global bdi_min_ratio.
-	 */
-	if (bdi->min_ratio)
-		bdi_set_min_ratio(bdi, 0);
-
 	if (bdi->dev) {
 		bdi_debug_unregister(bdi);
 		device_unregister(bdi->dev);
 		bdi->dev = NULL;
 	}
 
+	/*
+	 * If this BDI's min ratio has been set, use bdi_set_min_ratio() to
+	 * update the global bdi_min_ratio. After device_unregister(): the
+	 * sysfs min_ratio stores can no longer run and set it again.
+	 */
+	if (bdi->min_ratio)
+		bdi_set_min_ratio(bdi, 0);
+
 	if (bdi->owner) {
 		put_device(bdi->owner);
 		bdi->owner = NULL;
_

Patches currently in -mm which might be from nickolai@csail.mit.edu are

mm-mlock-use-a-long-for-the-page-count-in-mlock_fixup.patch
mm-truncate-fix-the-zeroing-end-of-pagecache_isize_extended.patch
resource-report-a-devm_request_resource-conflict-under-the-lock.patch
mm-bdi-reset-min_ratio-after-the-sysfs-attributes-are-gone.patch