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

From: Jan Kara
Date: Wed Feb 13 2019 - 10:06:38 EST

On Tue 12-02-19 16:55:21, Christopher Lameter wrote:
> On Tue, 12 Feb 2019, Jan Kara wrote:
> > > Isn't that already racy? If the mmap user is fast enough can't it
> > > prevent the page from becoming freed in the first place today?
> >
> > No, it cannot. We block page faulting for the file (via a lock), tear down
> > page tables, free pages and blocks. Then we resume faults and return
> > SIGBUS (if the page ends up being after the new end of file in case of
> > truncate) or do new page fault and fresh block allocation (which can end
> > with SIGBUS if the filesystem cannot allocate new block to back the page).
> Well that is already pretty inconsistent behavior. Under what conditions
> is the SIGBUS occurring without the new fault attempt?

I probably didn't express myself clearly enough. I didn't say that SIGBUS
can occur without a page fault. The evaluation of whether a page would be
beyond EOF, page allocation, and block allocation happen only in response
to a page fault...

> If a new fault is attempted then we have resource constraints that could
> have caused a SIGBUS independently of the truncate. So that case is not
> really something special to be considered for truncation.

Agreed. I was just reacting to Jason's question whether an application
cannot prevent page freeing by being aggressive enough.


Jan Kara <jack@xxxxxxxx>