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

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Message-ID: <CAMgjq7BhfGC7jVHQ62wAJBfTKCDG2+VdgpjiZ7hjxXeC5fHg-w@mail.gmail.com>
Date: Fri, 8 Aug 2025 01:27:22 +0800
From: Kairui Song <ryncsn@...il.com>
To: Jann Horn <jannh@...gle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Pedro Falcato <pfalcato@...e.de>, Matthew Wilcox <willy@...radead.org>, 
	Hugh Dickins <hughd@...gle.com>, David Hildenbrand <david@...hat.com>, Chris Li <chrisl@...nel.org>, 
	Barry Song <baohua@...nel.org>, Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] mm/mincore: avoid touching the PTL

On Fri, Aug 8, 2025 at 12:06 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Aug 7, 2025 at 5:27 PM Kairui Song <ryncsn@...il.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.

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?

> 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.