Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

On Mon, Oct 25, 2021 at 09:00:43PM +0200, Andreas Gruenbacher wrote:
> On Fri, Oct 22, 2021 at 9:23 PM Linus Torvalds
> <[email protected]> wrote:
> > On Fri, Oct 22, 2021 at 8:06 AM Catalin Marinas <[email protected]> wrote:
> > > Probing only the first byte(s) in fault_in() would be ideal, no need to
> > > go through all filesystems and try to change the uaccess/probing order.
> >
> > Let's try that. Or rather: probing just the first page - since there
> > are users like that btrfs ioctl, and the direct-io path.
> 
> For direct I/O, we actually only want to trigger page fault-in so that
> we can grab page references with bio_iov_iter_get_pages. Probing for
> sub-page error domains will only slow things down. If we hit -EFAULT
> during the actual copy-in or copy-out, we know that the error can't be
> page fault related. Similarly, in the buffered I/O case, we only
> really care about the next byte, so any probing beyond that is
> unnecessary.
> 
> So maybe we should split the sub-page error domain probing off from
> the fault-in functions. Or at least add an argument to the fault-in
> functions that specifies the amount of memory to probe.

My preferred option is not to touch fault-in for sub-page faults (though
I have some draft patches, they need testing).

All this fault-in and uaccess with pagefaults_disabled() is needed to
avoid a deadlock when the uaccess fault handling would take the same
lock. With sub-page faults, the kernel cannot fix it up anyway, so the
arch code won't even attempt call handle_mm_fault() (it is not an mm
fault). But the problem is the copy_*_user() etc. API that can only
return the number of bytes not copied. That's what I think should be
fixed. fault_in() feels like the wrong place to address this when it's
not an mm fault.

As for fault_in() getting another argument with the amount of sub-page
probing to do, I think the API gets even more confusing. I was also
thinking, with your patches for fault_in() now returning size_t, is the
expectation to be precise in what cannot be copied? We don't have such
requirement for copy_*_user().

While more intrusive, I'd rather change copy_page_from_iter_atomic()
etc. to take a pointer where to write back an error code. If it's
-EFAULT, retry the loop. If it's -EACCES/EPERM just bail out. Or maybe
simply a bool set if there was an mm fault to be retried. Yet another
option to return an -EAGAIN if it could not process the mm fault due to
page faults being disabled.

Happy to give this a try, unless there's a strong preference for the
fault_in() fix-up (well, I can do both options and post them).

-- 
Catalin