Re: [GIT PULL 14/14 for v6.17] vfs iomap

[Sasha Levin]

Hey Christian,

On Fri, Jul 25, 2025 at 01:27:20PM +0200, Christian Brauner wrote:
> Hey Linus,
>
> /* Summary */
> This contains the iomap updates for this cycle:
>
> - Refactor the iomap writeback code and split the generic and ioend/bio
>  based writeback code. There are two methods that define the split
>  between the generic writeback code, and the implemementation of it,
>  and all knowledge of ioends and bios now sits below that layer.
>
> - This series adds fuse iomap support for buffered writes and dirty
>  folio writeback. This is needed so that granular uptodate and dirty
>  tracking can be used in fuse when large folios are enabled. This has
>  two big advantages. For writes, instead of the entire folio needing to
>  be read into the page cache, only the relevant portions need to be.
>  For writeback, only the dirty portions need to be written back instead
>  of the entire folio.

While testing with the linus-next tree, it appears that LKFT can trigger
the following warning, but only on arm64 tests (both on real HW as well
as qemu):

[ 333.129662] WARNING: CPU: 1 PID: 2580 at fs/fuse/file.c:2158 fuse_iomap_writeback_range+0x478/0x558 fuse
[  333.132010] Modules linked in: btrfs blake2b_generic xor xor_neon raid6_pq zstd_compress sm3_ce sha3_ce sha512_ce fuse drm backlight ip_tables x_tables
[  333.133982] CPU: 1 UID: 0 PID: 2580 Comm: msync04 Tainted: G        W           6.16.0-rc7 #1 PREEMPT
[  333.134997] Tainted: [W]=WARN
[  333.135497] Hardware name: linux,dummy-virt (DT)
[  333.136114] pstate: 03402009 (nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
WARNING! No debugging info in module fuse, rebuild with DEBUG_KERNEL and DEBUG_INFO
[ 333.137090] pc : fuse_iomap_writeback_range+0x478/0x558 fuse
[ 333.138009] lr : iomap_writeback_folio (fs/iomap/buffered-io.c:1586 fs/iomap/buffered-io.c:1710)
[  333.138510] sp : ffff80008be8f8c0
[  333.138653] x29: ffff80008be8f8c0 x28: fff00000c5198c00 x27: 0000000000000000
[  333.138975] x26: fff00000d32b8c00 x25: 0000000000000000 x24: 0000000000000000
[  333.139309] x23: 0000000000000000 x22: fffffc1fc039ba40 x21: 0000000000001000
[  333.139600] x20: ffff80008be8f9f0 x19: 0000000000000000 x18: 0000000000000000
[  333.139917] x17: 0000000000000000 x16: ffffbb40f61c3a48 x15: 0000000000000000
[  333.142199] x14: ffffbb40f6924788 x13: 0000ffff8e8effff x12: 0000000000000000
[  333.142739] x11: 1ffe0000199a9241 x10: fff00000ccd4920c x9 : ffffbb40f50bba18
[  333.143466] x8 : ffff80008be8f778 x7 : ffffbb40ee180b68 x6 : ffffbb40f76c9000
[  333.143718] x5 : 0000000000000000 x4 : 000000000000000a x3 : 0000000000001000
[  333.143957] x2 : fff00000c0b6e600 x1 : 000000000000ffff x0 : 0bfffe000000400b
[  333.144993] Call trace:
WARNING! No debugging info in module fuse, rebuild with DEBUG_KERNEL and DEBUG_INFO
[ 333.145466] fuse_iomap_writeback_range+0x478/0x558 fuse (P)
[ 333.146136] iomap_writeback_folio (fs/iomap/buffered-io.c:1586 fs/iomap/buffered-io.c:1710)
[ 333.146444] iomap_writepages (fs/iomap/buffered-io.c:1762)
WARNING! No debugging info in module fuse, rebuild with DEBUG_KERNEL and DEBUG_INFO
[ 333.146590] fuse_writepages+0xa0/0xe8 fuse
[ 333.146774] do_writepages (mm/page-writeback.c:2636)
[ 333.146915] filemap_fdatawrite_wbc (mm/filemap.c:386 mm/filemap.c:376)
[ 333.147788] __filemap_fdatawrite_range (mm/filemap.c:420)
[ 333.148440] file_write_and_wait_range (mm/filemap.c:794)
WARNING! No debugging info in module fuse, rebuild with DEBUG_KERNEL and DEBUG_INFO
[ 333.149054] fuse_fsync+0x6c/0x138 fuse
[ 333.149578] vfs_fsync_range (fs/sync.c:188)
[ 333.149892] __arm64_sys_msync (mm/msync.c:96 mm/msync.c:32 mm/msync.c:32)
[ 333.150095] invoke_syscall.constprop.0 (arch/arm64/include/asm/syscall.h:61 arch/arm64/kernel/syscall.c:54)
[ 333.150330] do_el0_svc (include/linux/thread_info.h:135 (discriminator 2) arch/arm64/kernel/syscall.c:140 (discriminator 2) arch/arm64/kernel/syscall.c:151 (discriminator 2))
[ 333.150461] el0_svc (arch/arm64/include/asm/irqflags.h:82 (discriminator 1) arch/arm64/include/asm/irqflags.h:123 (discriminator 1) arch/arm64/include/asm/irqflags.h:136 (discriminator 1) arch/arm64/kernel/entry-common.c:165 (discriminator 1) arch/arm64/kernel/entry-common.c:178 (discriminator 1) arch/arm64/kernel/entry-common.c:768 (discriminator 1))
[ 333.150583] el0t_64_sync_handler (arch/arm64/kernel/entry-common.c:787)
[ 333.150729] el0t_64_sync (arch/arm64/kernel/entry.S:600)
[  333.150862] ---[ end trace 0000000000000000 ]---

I think that this is because the arm64 tests run on
CONFIG_PAGE_SIZE_64KB=y build, but I'm not sure why we don't see it with
4KB pages at all.

An example link to a failing test that has the full log and more
information: https://qa-reports.linaro.org/lkft/sashal-linus-next/build/v6.13-rc7-44385-g8a03a07bad83/testrun/29269158/suite/log-parser-test/test/exception-warning-cpu-pid-at-fsfusefile-fuse_iomap_writeback_range/details/

--
Thanks,
Sasha