sys-fs/zfs-kmod: disable zfs_dmu_offset_next_sync tunable by default

As a mitigation until more is understood and fixes are tested & reviewed, change
the default of zfs_dmu_offset_next_sync from 1 to 0, as it was before
05b3eb6d232009db247882a39d518e7282630753 upstream.

There are no reported cases of The Bug being hit with zfs_dmu_offset_next_sync=1:
that does not mean this is a cure or a real fix, but it _appears_ to be at least
effective in reducing the chances of it happening. By itself, it's a safe change
anyway, so it feels worth us doing while we wait.

Note that The Bug has been reproduced on 2.1.x as well, hence we do it for both
2.1.13 and 2.2.1.

Bug: openzfs/zfs#11900
Bug: openzfs/zfs#15526
Bug: https://bugs.gentoo.org/917224
Signed-off-by: Sam James <[email protected]>

sys-fs/zfs-kmod/files/zfs-kmod-2.1.13-Disable-zfs_dmu_offset_next_sync-tunable-by-default.patch

@@ -0,0 +1,40 @@
+From 2b266bd36980caefe353411bd56b2487c44aeb6e Mon Sep 17 00:00:00 2001
+From: Sam James <[email protected]>
+Date: Fri, 24 Nov 2023 21:38:06 +0000
+Subject: [PATCH] Disable zfs_dmu_offset_next_sync tunable by default
+
+As a mitigation until more is understood and fixes are tested & reviewed, change
+the default of zfs_dmu_offset_next_sync from 1 to 0, as it was before
+05b3eb6d232009db247882a39d518e7282630753.
+
+There are no reported cases of The Bug being hit with zfs_dmu_offset_next_sync=1:
+that does not mean this is a cure or a real fix, but it _appears_ to be at least
+effective in reducing the chances of it happening. By itself, it's a safe change
+anyway, so it feels worth us doing while we wait.
+
+Bug: https://github.com/openzfs/zfs/issues/11900
+Bug: https://github.com/openzfs/zfs/issues/15526
+Bug: https://bugs.gentoo.org/917224
+Signed-off-by: Sam James <[email protected]>
+--- a/man/man4/zfs.4
++++ b/man/man4/zfs.4
+@@ -1646,7 +1646,7 @@ Allow no-operation writes.
+ The occurrence of nopwrites will further depend on other pool properties
+ .Pq i.a. the checksumming and compression algorithms .
+ .
+-.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 1 Ns | Ns 0 Pq int
++.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 0 Ns | Ns 1 Pq int
+ Enable forcing TXG sync to find holes.
+ When enabled forces ZFS to sync data when
+ .Sy SEEK_HOLE No or Sy SEEK_DATA
+--- a/module/zfs/dmu.c
++++ b/module/zfs/dmu.c
+@@ -80,7 +80,7 @@ unsigned long zfs_per_txg_dirty_frees_percent = 30;
+  * Disabling this option will result in holes never being reported in dirty
+  * files which is always safe.
+  */
+-int zfs_dmu_offset_next_sync = 1;
++int zfs_dmu_offset_next_sync = 0;
+
+ /*
+  * Limit the amount we can prefetch with one call to this amount.  This

sys-fs/zfs-kmod/files/zfs-kmod-2.2.1-Disable-zfs_dmu_offset_next_sync-tunable-by-default.patch

@@ -0,0 +1,43 @@
+From 2b266bd36980caefe353411bd56b2487c44aeb6e Mon Sep 17 00:00:00 2001
+From: Sam James <[email protected]>
+Date: Fri, 24 Nov 2023 21:38:06 +0000
+Subject: [PATCH] Disable zfs_dmu_offset_next_sync tunable by default
+
+As a mitigation until more is understood and fixes are tested & reviewed, change
+the default of zfs_dmu_offset_next_sync from 1 to 0, as it was before
+05b3eb6d232009db247882a39d518e7282630753.
+
+There are no reported cases of The Bug being hit with zfs_dmu_offset_next_sync=1:
+that does not mean this is a cure or a real fix, but it _appears_ to be at least
+effective in reducing the chances of it happening. By itself, it's a safe change
+anyway, so it feels worth us doing while we wait.
+
+Bug: https://github.com/openzfs/zfs/issues/11900
+Bug: https://github.com/openzfs/zfs/issues/15526
+Bug: https://bugs.gentoo.org/917224
+Signed-off-by: Sam James <[email protected]>
+--- a/man/man4/zfs.4
++++ b/man/man4/zfs.4
+@@ -1677,7 +1677,7 @@ Allow no-operation writes.
+ The occurrence of nopwrites will further depend on other pool properties
+ .Pq i.a. the checksumming and compression algorithms .
+ .
+-.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 1 Ns | Ns 0 Pq int
++.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 0 Ns | Ns 1 Pq int
+ Enable forcing TXG sync to find holes.
+ When enabled forces ZFS to sync data when
+ .Sy SEEK_HOLE No or Sy SEEK_DATA
+--- a/module/zfs/dmu.c
++++ b/module/zfs/dmu.c
+@@ -82,7 +82,7 @@ static uint_t zfs_per_txg_dirty_frees_percent = 30;
+  * Disabling this option will result in holes never being reported in dirty
+  * files which is always safe.
+  */
+-static int zfs_dmu_offset_next_sync = 1;
++static int zfs_dmu_offset_next_sync = 0;
+
+ /*
+  * Limit the amount we can prefetch with one call to this amount.  This
+-- 
+2.43.0
+

sys-fs/zfs-kmod/zfs-kmod-2.1.13-r1.ebuild

@@ -0,0 +1,178 @@
+# Copyright 1999-2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+inherit autotools dist-kernel-utils flag-o-matic linux-mod-r1 multiprocessing
+
+DESCRIPTION="Linux ZFS kernel module for sys-fs/zfs"
+HOMEPAGE="https://github.com/openzfs/zfs"
+
+MODULES_KERNEL_MAX=6.5
+MODULES_KERNEL_MIN=3.10
+
+if [[ ${PV} == 9999 ]] ; then
+	EGIT_REPO_URI="https://github.com/openzfs/zfs.git"
+	inherit git-r3
+	unset MODULES_KERNEL_MAX
+else
+	VERIFY_SIG_OPENPGP_KEY_PATH=/usr/share/openpgp-keys/openzfs.asc
+	inherit verify-sig
+
+	MY_PV=${PV/_rc/-rc}
+	SRC_URI="https://github.com/openzfs/zfs/releases/download/zfs-${MY_PV}/zfs-${MY_PV}.tar.gz"
+	SRC_URI+=" verify-sig? ( https://github.com/openzfs/zfs/releases/download/zfs-${MY_PV}/zfs-${MY_PV}.tar.gz.asc )"
+	S="${WORKDIR}/zfs-${PV%_rc?}"
+
+	ZFS_KERNEL_COMPAT="${MODULES_KERNEL_MAX}"
+	# Increments minor eg 5.14 -> 5.15, and still supports override.
+	ZFS_KERNEL_DEP="${ZFS_KERNEL_COMPAT_OVERRIDE:-${ZFS_KERNEL_COMPAT}}"
+	ZFS_KERNEL_DEP="${ZFS_KERNEL_DEP%%.*}.$(( ${ZFS_KERNEL_DEP##*.} + 1))"
+
+	if [[ ${PV} != *_rc* ]] ; then
+		KEYWORDS="~amd64 ~arm64 ~ppc64 ~riscv ~sparc"
+	fi
+fi
+
+LICENSE="CDDL MIT debug? ( GPL-2+ )"
+SLOT="0/${PVR}"
+IUSE="custom-cflags debug +rootfs"
+RESTRICT="test"
+
+BDEPEND="
+	dev-lang/perl
+	app-alternatives/awk
+"
+
+if [[ ${PV} != 9999 ]] ; then
+	BDEPEND+=" verify-sig? ( sec-keys/openpgp-keys-openzfs )"
+
+	IUSE+=" +dist-kernel-cap"
+	RDEPEND="
+		dist-kernel-cap? ( dist-kernel? (
+			<virtual/dist-kernel-${ZFS_KERNEL_DEP}
+		) )
+	"
+fi
+
+# Used to suggest matching USE, but without suggesting to disable
+PDEPEND="dist-kernel? ( ~sys-fs/zfs-${PV}[dist-kernel] )"
+
+PATCHES=(
+	"${FILESDIR}"/${PN}-2.1.11-gentoo.patch
+	"${FILESDIR}"/${PN}-2.1.13-Disable-zfs_dmu_offset_next_sync-tunable-by-default.patch
+)
+
+pkg_pretend() {
+	use rootfs || return 0
+
+	if has_version virtual/dist-kernel && ! use dist-kernel; then
+		ewarn "You have virtual/dist-kernel installed, but"
+		ewarn "USE=\"dist-kernel\" is not enabled for ${CATEGORY}/${PN}"
+		ewarn "It's recommended to globally enable dist-kernel USE flag"
+		ewarn "to auto-trigger initrd rebuilds with kernel updates"
+	fi
+}
+
+pkg_setup() {
+	local CONFIG_CHECK="
+		EFI_PARTITION
+		ZLIB_DEFLATE
+		ZLIB_INFLATE
+		!DEBUG_LOCK_ALLOC
+		!PAX_KERNEXEC_PLUGIN_METHOD_OR
+	"
+	use debug && CONFIG_CHECK+="
+		DEBUG_INFO
+		FRAME_POINTER
+		!DEBUG_INFO_REDUCED
+	"
+	use rootfs && CONFIG_CHECK+="
+		BLK_DEV_INITRD
+		DEVTMPFS
+	"
+
+	kernel_is -lt 5 && CONFIG_CHECK+=" IOSCHED_NOOP"
+
+	if [[ ${PV} != 9999 ]] ; then
+		local kv_major_max kv_minor_max zcompat
+		zcompat="${ZFS_KERNEL_COMPAT_OVERRIDE:-${ZFS_KERNEL_COMPAT}}"
+		kv_major_max="${zcompat%%.*}"
+		zcompat="${zcompat#*.}"
+		kv_minor_max="${zcompat%%.*}"
+		kernel_is -le "${kv_major_max}" "${kv_minor_max}" || die \
+			"Linux ${kv_major_max}.${kv_minor_max} is the latest supported version"
+	fi
+
+	linux-mod-r1_pkg_setup
+}
+
+src_prepare() {
+	default
+
+	# Run unconditionally (bug #792627)
+	eautoreconf
+
+	if [[ ${PV} != 9999 ]] ; then
+		# Set module revision number
+		sed -Ei "s/(Release:.*)1/\1${PR}-gentoo/" META || die
+	fi
+}
+
+src_configure() {
+	use custom-cflags || strip-flags
+	filter-ldflags -Wl,*
+
+	local myconf=(
+		--bindir="${EPREFIX}"/bin
+		--sbindir="${EPREFIX}"/sbin
+		--with-config=kernel
+		--with-linux="${KV_DIR}"
+		--with-linux-obj="${KV_OUT_DIR}"
+		$(use_enable debug)
+
+		# See gentoo.patch
+		GENTOO_MAKEARGS_EVAL="${MODULES_MAKEARGS[*]@Q}"
+		TEST_JOBS="$(makeopts_jobs)"
+	)
+
+	econf "${myconf[@]}"
+}
+
+src_compile() {
+	emake "${MODULES_MAKEARGS[@]}"
+}
+
+src_install() {
+	emake "${MODULES_MAKEARGS[@]}" DESTDIR="${ED}" install
+	modules_post_process
+
+	dodoc AUTHORS COPYRIGHT META README.md
+}
+
+pkg_postinst() {
+	linux-mod-r1_pkg_postinst
+
+	if [[ -z ${ROOT} ]] && use dist-kernel ; then
+		dist-kernel_reinstall_initramfs "${KV_DIR}" "${KV_FULL}"
+	fi
+
+	if use x86 || use arm ; then
+		ewarn "32-bit kernels will likely require increasing vmalloc to"
+		ewarn "at least 256M and decreasing zfs_arc_max to some value less than that."
+	fi
+
+	if has_version sys-boot/grub ; then
+		ewarn "This version of OpenZFS includes support for new feature flags"
+		ewarn "that are incompatible with previous versions. GRUB2 support for"
+		ewarn "/boot with the new feature flags is not yet available."
+		ewarn "Do *NOT* upgrade root pools to use the new feature flags."
+		ewarn "Any new pools will be created with the new feature flags by default"
+		ewarn "and will not be compatible with older versions of OpenZFS. To"
+		ewarn "create a new pool that is backward compatible wih GRUB2, use "
+		ewarn
+		ewarn "zpool create -o compatibility=grub2 ..."
+		ewarn
+		ewarn "Refer to /usr/share/zfs/compatibility.d/grub2 for list of features."
+	fi
+}