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

From: Matthew Wilcox
Date: Thu Feb 14 2019 - 15:50:55 EST

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.