Re: [PATCH 2/2] mremap: use mmu gather logic for tlb flush in mremap

[Linus Torvalds]

On Mon, Nov 28, 2016 at 12:40 AM, Aaron Lu <aaron.lu@xxxxxxxxx> wrote:
> As suggested by Linus, the same mmu gather logic could be used for tlb
> flush in mremap and this patch just did that.

Ok, looking at this patch, I still think it looks like the right thing
to do, but I'm admittedly rather less certain of it.

The main advantage of the mmu_gather thing is that it automatically
takes care of the TLB flush ranges for us, and that's a big deal
during munmap() (where the actual unmapped page range can be _very_
different from the total range), but now that I notice that this
doesn't actually remove any other code (in fact, it adds a line), I'm
wondering if it's worth it. mremap() is already "dense" in the vma
space, unlike munmap (ie you can't move multiple vma's with a single
mremap), so the fancy range optimizations that make a difference on
some architectures aren't much of an issue.

So I guess the MM people should take a look at this and say whether
they think the current state is fine or whether we should do the
mmu_gather thing. People?

However, I also independently think I found an actual bug while
looking at the code as part of looking at the patch.

This part looks racy:

                /*
                 * We are remapping a dirty PTE, make sure to
                 * flush TLB before we drop the PTL for the
                 * old PTE or we may race with page_mkclean().
                 */
                if (pte_present(*old_pte) && pte_dirty(*old_pte))
                        force_flush = true;
                pte = ptep_get_and_clear(mm, old_addr, old_pte);

where the issue is that another thread might make the pte be dirty (in
the hardware walker, so no locking of ours make any difference)
*after* we checked whether it was dirty, but *before* we removed it
from the page tables.

So I think the "check for force-flush" needs to come *after*, and we should do

                pte = ptep_get_and_clear(mm, old_addr, old_pte);
                if (pte_present(pte) && pte_dirty(pte))
                        force_flush = true;

instead.

This happens for the pmd case too.

So now I'm not sure the mmu_gather thing is worth it, but I'm pretty
sure that there remains a (very very) small race that wasn't fixed by
the original fix in commit 5d1904204c99 ("mremap: fix race between
mremap() and page cleanning").

Aaron, sorry for waffling about this, and asking you to look at a
completely different issue instead.

                 Linus