Re: [PATCH 2/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/8/26 19:19, Alexander Gordeev wrote:
> On Sun, May 31, 2026 at 01:01:06AM +0000, Askar Safin wrote:
>> vmsplice behavior on writable pipe became equivalent to pwritev2.
>> vmsplice behavior on readable pipe already was nearly
>> equivalent to preadv2, but I made this explicit. I. e. I made it
>> obvious from code that vmsplice now is equivalent to preadv2/pwritev2.
>>
>> Also I moved vmsplice to fs/read_write.c, because now it arguably
>> belongs there.
>>
>> Note that SPLICE_F_NONBLOCK behavior slightly changed: previously
>> vmsplice ignored whether the pipe was opened with O_NONBLOCK, and mode
>> of operation depended on whether SPLICE_F_NONBLOCK was passed only.
>> Now the operation will be non-blocking if O_NONBLOCK was passed when
>> opening *or* SPLICE_F_NONBLOCK was passed to vmsplice. Previous
>> behavior was arguably buggy, and new behavior is arguably better.
>>
>> Now SPLICE_F_GIFT is always ignored by all 3 syscalls: splice, tee
>> and vmsplice.
>>
>> Signed-off-by: Askar Safin <safinaskar@gmail.com>
>> ---
>>  fs/read_write.c          |  23 +++++
>>  fs/splice.c              | 192 +--------------------------------------
>>  include/linux/skbuff.h   |   4 +-
>>  include/linux/splice.h   |   2 +-
>>  include/linux/syscalls.h |   4 +-
>>  5 files changed, 29 insertions(+), 196 deletions(-)
> 
> Hi All,
> 
> This patch as commit e2c0b2368081b ("vmsplice: make vmsplice a trivial
> wrapper for preadv2/pwritev2") in linux-next on s390 causes the selftest
> tools/testing/selftests/mm/cow.c to hang:
> 
> # [RUN] vmsplice() + unmap in child ... with PTE-mapped THP (128 kB)
> 
> Recently there has been changes in THP area, so the problem is not
> necessary linked to this patch per se.

If we reach 128 kB, then 64 kB likely worked. Which might hint at a similar problem
as found by the vmsplice01 ltp test case (blocking instead of returning once the
pipe is full).

https://lore.kernel.org/r/20260603-raumfahrt-unmerklich-ertrugen-c4ecae70d5f9@brauner


-- 
Cheers,

David