Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

[Al Viro]

On Sat, Jun 13, 2020 at 04:41:18PM +0100, Al Viro wrote:
> On Sat, Jun 13, 2020 at 04:31:02PM +0100, Al Viro wrote:
> > On Sat, Jun 13, 2020 at 07:12:36PM +0530, afzal mohammed wrote:
> > > Hi,
> > > 
> > > On Sat, Jun 13, 2020 at 01:56:15PM +0100, Al Viro wrote:
> > > 
> > > > Incidentally, what about get_user()/put_user()?  _That_ is where it's
> > > > going to really hurt...
> > > 
> > > All other uaccess routines are also planned to be added, posting only
> > > copy_{from,to}_user() was to get early feedback (mentioned in the
> > > cover letter)
> > 
> > Sure, but what I mean is that I'd expect the performance loss to be
> > dominated by that, not by copy_from_user/copy_to_user on large amounts
> > of data.  Especially on the loads like kernel builds - a lot of stat()
> > and getdents() calls there.
> 
> To clarify: stat() means small copy_to_user(), getdents() - a mix of
> put_user() and small copy_to_user().  I would be very surprised if it
> does not hurt a lot.

PS: there's another fun issue here:

fill a file with zeroes
mmap that file in two areas, MAP_SHARED
thread 1:
munmap() the first area
fill the second one with 'X'
thread 2:
write() from the first area into pipe

One could expect that nothing by zeroes gets written into
pipe - it might be a short write() (or -EFAULT), but finding
any 'X' there would be a bug.

Your patches allow for a possibility of write() doing
get_user_pages_fast(), getting the first page just as
munmap() is about to remove it from page tables and bugger
off.  Then thread 1 proceeds with the store (via the
second area).  And then thread 2 does memcpy() from that
thing via a kmap_atomic()-created alias, observing the
effect of the store.

That might or might not be a POSIX violation, but it does
look like a QoI issue...