mm/userfaultfd: prevent busy looping for tasks with signals pending

mm/userfaultfd: prevent busy looping for tasks with signals pending

userfaultfd may use interruptible sleeps to wait on userspace filling
a page fault, which works fine if the task can be reliably put to
sleeping waiting for that. However, if the task has a normal (ie
non-fatal) signal pending, then TASK_INTERRUPTIBLE sleep will simply
cause schedule() to be a no-op.

For a task that registers a page with userfaultfd and then proceeds
to do a write from it, if that task also has a signal pending then
it'll essentially busy loop from do_page_fault() -> handle_userfault()
until that fault has been filled. Normally it'd be expected that the
task would sleep until that happens. Here's a trace from an application
doing just that:

handle_userfault+0x4b8/0xa00 (P)
hugetlb_fault+0xe24/0x1060
handle_mm_fault+0x2bc/0x318
do_page_fault+0x1e8/0x6f0
do_translation_fault+0x9c/0xd0
do_mem_abort+0x44/0xa0
el1_abort+0x3c/0x68
el1h_64_sync_handler+0xd4/0x100
el1h_64_sync+0x6c/0x70
fault_in_readable+0x74/0x108 (P)
iomap_file_buffered_write+0x14c/0x438
blkdev_write_iter+0x1a8/0x340
vfs_write+0x20c/0x348
ksys_write+0x64/0x108
__arm64_sys_write+0x1c/0x38

where the task is looping with 100% CPU time in the above mentioned
fault path.

Since it's impossible to handle signals, or other conditions like
TIF_NOTIFY_SIGNAL that also prevents interruptible sleeping, from the
fault path, use TASK_UNINTERRUPTIBLE with a short timeout even for vmf
modes that would normally ask for INTERRUPTIBLE or KILLABLE sleep. Fatal
signals will still be handled by the caller, and the timeout is short
enough to hopefully not cause any issues. If this is the first invocation
of this fault, eg FAULT_FLAG_TRIED isn't set, then the normal sleep mode
is used.

Cc: stable@vger.kernel.org
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Reported-by: Zhiwei Jiang <qq282012236@gmail.com>
Link: https://lore.kernel.org/io-uring/20250422162913.1242057-1-qq282012236@gmail.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>