Re: [RFC PATCH 3/3] mm/mincore: avoid touching the PTL

[Jann Horn <jannh@google.com>]

On Thu, Aug 7, 2025 at 7:28 PM Kairui Song <ryncsn@gmail.com> wrote:
> On Fri, Aug 8, 2025 at 12:06 AM Jann Horn <jannh@google.com> wrote:
> >
> > On Thu, Aug 7, 2025 at 5:27 PM Kairui Song <ryncsn@gmail.com> wrote:
> > > mincore only interested in the existence of a page, which is a
> > > changing state by nature, locking and making it stable is not needed.
> > > And now neither mincore_page or mincore_swap requires PTL, this PTL
> > > locking can be dropped.
> >
> > This means you can race such that you end up looking at an unrelated
> > page of another process, right?
>
> I was thinking If the PTE is gone, it will make mincore go check the
> page cache, but even if we hold the PTL here, the next mincore call
> (if called soon enough) could check the page cache using the same
> address. And it never checks any actual page if the PTE is not none.
>
> Perhaps you mean that it's now doing the page / swap cache lookup
> without holding PTL so if the PTE changed, then the lookup could be
> using an invalidated index, and may find an unrelated page.

Yes, that's what I meant.

> A changing PTE also means the mincore return value is changing, and if
> called earlier or later by a little bit, the result of that address
> could be opposite, and mincore only checks if the page existed,
> it's hard to say the returned value is a false positive / negative?
>
> Or could this introduce a new security issue?

I don't have specific security concerns here; but this is a change
that trades accuracy and simplicity for performance.

> > And your patch intentionally allows that to happen in order to make mincore() faster?
>
> When doing a clean up (patch 1) I noticed and didn't understand why we
> need a PTL here. It will no longer block others and go faster as we
> remove one lock, I can drop this one if we are not comfortable with
> it.

If you had a specific performance concern here, I think we could
consider changing this, but in my view it would sort of be breaking
the locking rules (by using a swap index that is not guaranteed to be
kept alive) and would need an explanatory comment explaining the
tradeoff.

Since you only wrote the patch because you thought the lock was
unnecessary, I'd prefer it if you drop this patch.