Messages in this thread | | | Date | Thu, 25 Nov 2021 11:10:43 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults |
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On Wed, Nov 24, 2021 at 03:00:00PM -0800, Linus Torvalds wrote: > On Wed, Nov 24, 2021 at 12:04 PM Matthew Wilcox <willy@infradead.org> wrote: > > (where __copy_to_user_nofault() is a new function that does exactly what > > copy_to_user_nofault() does, but returns the number of bytes copied) > > If we want the "how many bytes" part, then we should just make > copy_to_user_nofault() have the same semantics as a plain > copy_to_user(). > > IOW, change it to return "number of bytes not copied". > > Looking at the current uses, such a change would be trivial. The only > case that wants a 0/-EFAULT error is the bpf_probe_write_user(), > everybody else already just wants "zero for success", so changing > copy_to_user_nofault() would be trivial.
I agree, if we want the number of byte not copied, we should just change copy_{to,from}_user_nofault() and their callers (I can count three each).
For this specific btrfs case, if we want go with tuning the offset based on the fault address, we'd need copy_to_user_nofault() (or a new function) to be exact. IOW, fall back to byte-at-a-time copy until it hits the real faulting address.
-- Catalin
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