Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

From: Jerome Glisse
Date: Thu Feb 14 2019 - 16:39:30 EST

On Thu, Feb 14, 2019 at 12:50:49PM -0800, Matthew Wilcox wrote:
> On Thu, Feb 14, 2019 at 03:26:22PM -0500, Jerome Glisse wrote:
> > On Mon, Feb 11, 2019 at 11:06:54AM -0700, Jason Gunthorpe wrote:
> > > But it also doesnt' trucate/create a hole. Another thread wrote to it
> > > right away and the 'hole' was essentially instantly reallocated. This
> > > is an inherent, pre-existing, race in the ftrucate/etc APIs.
> >
> > So it is kind of a // point to this, but direct I/O do "truncate" pages
> > or more exactly after a write direct I/O invalidate_inode_pages2_range()
> > is call and it will try to unmap and remove from page cache all pages
> > that have been written too.
> Hang on. Pages are tossed out of the page cache _before_ an O_DIRECT
> write starts. The only way what you're describing can happen is if
> there's a race between an O_DIRECT writer and an mmap. Which is either
> an incredibly badly written application or someone trying an exploit.

I believe they are tossed after O_DIRECT starts (dio_complete). But
regardless the issues is that an RDMA can have pin the page long
before the DIO in which case the page can not be toss from the page
cache and what ever is written to the block device will be discarded
once the RDMA unpin the pages. So we would end up in the code path
that spit out big error message in the kernel log.