Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE)

[Alexandru Elisei <alexandru.elisei@arm.com>]

On Thu, Nov 30, 2023 at 02:43:48PM +0100, David Hildenbrand wrote:
> On 30.11.23 14:32, Alexandru Elisei wrote:
> > Hi,
> > 
> > On Thu, Nov 30, 2023 at 01:49:34PM +0100, David Hildenbrand wrote:
> > > > > > +
> > > > > > +out_retry:
> > > > > > +	put_page(page);
> > > > > > +	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > > > > > +		vma_end_read(vma);
> > > > > > +	if (fault_flag_allow_retry_first(vmf->flags)) {
> > > > > > +		err = VM_FAULT_RETRY;
> > > > > > +	} else {
> > > > > > +		/* Replay the fault. */
> > > > > > +		err = 0;
> > > > > 
> > > > > Hello!
> > > > > 
> > > > > Unfortunately, if the page continues to be pinned, it seems like fault will continue to occur.
> > > > > I guess it makes system stability issue. (but I'm not familiar with that, so please let me know if I'm mistaken!)
> > > > > 
> > > > > How about migrating the page when migration problem repeats.
> > > > 
> > > > Yes, I had the same though in the previous iteration of the series, the
> > > > page was migrated out of the VMA if tag storage couldn't be reserved.
> > > > 
> > > > Only short term pins are allowed on MIGRATE_CMA pages, so I expect that the
> > > > pin will be released before the fault is replayed. Because of this, and
> > > > because it makes the code simpler, I chose not to migrate the page if tag
> > > > storage couldn't be reserved.
> > > 
> > > There are still some cases that are theoretically problematic: vmsplice()
> > > can pin pages forever and doesn't use FOLL_LONGTERM yet.
> > > 
> > > All these things also affect other users that rely on movability (e.g., CMA,
> > > memory hotunplug).
> > 
> > I wasn't aware of that, thank you for the information. Then to ensure that the
> > process doesn't hang by replying the loop indefinitely, I'll migrate the page if
> > tag storage cannot be reserved. Looking over the code again, I think I can reuse
> > the same function that migrates tag storage pages out of the MTE VMA (added in
> > patch #21), so no major changes needed.
> 
> It's going to be interesting if migrating that page fails because it is
> pinned :/

I imagine that having both the page **and** its tag storage pinned longterm
without FOLL_LONGTERM is going to be exceedingly rare.

Am I mistaken in believing that the problematic vmsplice() behaviour is
recognized as something that needs to be fixed?

Thanks,
Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>