Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()

[Andreas Gruenbacher <agruenba@redhat.com>]

On Tue, Oct 12, 2021 at 1:59 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Mon, Oct 11, 2021 at 2:08 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> >
> > +#ifdef CONFIG_ARM64_MTE
> > +#define FAULT_GRANULE_SIZE     (16)
> > +#define FAULT_GRANULE_MASK     (~(FAULT_GRANULE_SIZE-1))
>
> [...]
>
> > If this looks in the right direction, I'll do some proper patches
> > tomorrow.
>
> Looks fine to me. It's going to be quite expensive and bad for caches, though.
>
> That said, fault_in_writable() is _supposed_ to all be for the slow
> path when things go south and the normal path didn't work out, so I
> think it's fine.

Let me get back to this; I'm actually not convinced that we need to
worry about sub-page-size fault granules in fault_in_pages_readable or
fault_in_pages_writeable.

From a filesystem point of view, we can get into trouble when a
user-space read or write triggers a page fault while we're holding
filesystem locks, and that page fault ends up calling back into the
filesystem. To deal with that, we're performing those user-space
accesses with page faults disabled. When a page fault would occur, we
get back an error instead, and then we try to fault in the offending
pages. If a page is resident and we still get a fault trying to access
it, trying to fault in the same page again isn't going to help and we
have a true error. We're clearly looking at memory at a page
granularity; faults at a sub-page level don't matter at this level of
abstraction (but they do show similar error behavior). To avoid
getting stuck, when it gets a short result or -EFAULT, the filesystem
implements the following backoff strategy: first, it tries to fault in
a number of pages. When the read or write still doesn't make progress,
it scales back and faults in a single page. Finally, when that still
doesn't help, it gives up. This strategy is needed for actual page
faults, but it also handles sub-page faults appropriately as long as
the user-space access functions give sensible results.

What am I missing?

Thanks,
Andreas

> I do wonder how the sub-page granularity works. Is it sufficient to
> just read from it? Because then a _slightly_ better option might be to
> do one write per page (to catch page table writability) and then one
> read per "granule" (to catch pointer coloring or cache poisoning
> issues)?
>
> That said, since this is all preparatory to us wanting to write to it
> eventually anyway, maybe marking it all dirty in the caches is only
> good.
>
>                 Linus
>