Re: [PATCH] mm/gup: continue VM_FAULT_RETRY processing event for pre-faults

[Pavel Machek]

On Fri 2019-05-24 15:22:51, Hugh Dickins wrote:
> On Wed, 22 May 2019, Sebastian Andrzej Siewior wrote:
> > On 2019-05-22 12:21:13 [-0700], Andrew Morton wrote:
> > > On Tue, 14 May 2019 17:29:55 +0300 Mike Rapoport <rppt@xxxxxxxxxxxxx> wrote:
> > > 
> > > > When get_user_pages*() is called with pages = NULL, the processing of
> > > > VM_FAULT_RETRY terminates early without actually retrying to fault-in all
> > > > the pages.
> > > > 
> > > > If the pages in the requested range belong to a VMA that has userfaultfd
> > > > registered, handle_userfault() returns VM_FAULT_RETRY *after* user space
> > > > has populated the page, but for the gup pre-fault case there's no actual
> > > > retry and the caller will get no pages although they are present.
> > > > 
> > > > This issue was uncovered when running post-copy memory restore in CRIU
> > > > after commit d9c9ce34ed5c ("x86/fpu: Fault-in user stack if
> > > > copy_fpstate_to_sigframe() fails").
> 
> I've been getting unexplained segmentation violations, and "make" giving
> up early, when running kernel builds under swapping memory pressure: no
> CRIU involved.
> 
> Bisected last night to that same x86/fpu commit, not itself guilty, but
> suffering from the odd behavior of get_user_pages_unlocked() giving up
> too early.
> 
> (I wondered at first if copy_fpstate_to_sigframe() ought to retry if
> non-negative ret < nr_pages, but no, that would be wrong: a present page
> followed by an invalid area would repeatedly return 1 for nr_pages 2.)
> 
> Cc'ing Pavel, who's been having segfault trouble in emacs: maybe same?

The emacs segfault was always during process exit. This sounds different...

I don't see problems with make.

But its true that at least one of affected machines uses swap heavily.

Best regards,
								Pavel