Re: [PATCH 1/4] mm/mprotect: Retry on pmd_trans_unstable()

[Peter Xu]

On Fri, Jun 02, 2023 at 07:04:48PM -0700, Yang Shi wrote:
> On Fri, Jun 2, 2023 at 4:06 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > When hit unstable pmd, we should retry the pmd once more because it means
> > we probably raced with a thp insertion.
> >
> > Skipping it might be a problem as no error will be reported to the caller.
> > I assume it means the user will expect prot changed (e.g. mprotect or
> > userfaultfd wr-protections) applied but it's actually not.
> 
> IIRC, mprotect() holds write mmap_lock, so it should not matter. PROT
> NUMA holds read mmap_lock, but returning 0 also doesn't matter (of
> course retry is fine too). just skip that 2M area.

True.

> The userfaultfd-wp is your call :-)

Yeah I think uffd should still be a problem.  I'll reword the commit
message (by dropping mprotect example) in the new version.

If you have time feel free to have a look at patch 4, where I think it's a
bug for pagemap too (I didn't check as close as all the rest; the memcg one
might be suspecious, that's also in patch 4).

Thanks!

-- 
Peter Xu