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>