From: Kiryl Shutsemau <kas@kernel.org>
To: Lance Yang <lance.yang@linux.dev>
Cc: jthoughton@google.com, akpm@linux-foundation.org,
david@kernel.org, ljs@kernel.org, ziy@nvidia.com,
baolin.wang@linux.alibaba.com, liam@infradead.org,
nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com,
baohua@kernel.org, usama.arif@linux.dev, shy828301@gmail.com,
zokeefe@google.com, hughd@google.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] mm/khugepaged: Don't collapse uffd-minor-registered VMAs
Date: Fri, 28 Aug 2026 14:07:07 +0100 [thread overview]
Message-ID: <apGHG9LJtDpe2kdG@thinkstation> (raw)
In-Reply-To: <20260828094703.11081-1-lance.yang@linux.dev>
On Fri, Aug 28, 2026 at 05:47:03PM +0800, Lance Yang wrote:
>
> On Fri, Aug 28, 2026 at 12:50:04AM +0000, James Houghton wrote:
> >Userfaultfd minor faults provides userspace with the ability to manually
> >install PTEs with UFFDIO_CONTINUE. Right now, khugepaged collapse can
> >map holes in the VMA when a naturally-aligned THP is present without
> >explicit action from userspace.
> >
> >This is a problem, as it bypasses userfaultfd minor faults that
> >userspace is expecting to handle.
>
> One basic question first. Should MADV_COLLAPSE refuse to collapse a
> UFFD-minor-registered VMA, regardless of whether all PTEs are present?
I think the line is not between MADV_COLLAPSE vs khugepaged, it is if
vma->vm_mm is current->mm or not.
In other words, we might want to allow collapse if workload itself asks
for it, but not for khugepaged or *external* process_madvise(MADV_COLLAPSE).
is_khugepaged is not the right proxy.
--
Kiryl Shutsemau / Kirill A. Shutemov
next prev parent reply other threads:[~2026-08-28 13:07 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-28 0:50 James Houghton
2026-08-28 9:47 ` Lance Yang
2026-08-28 13:07 ` Kiryl Shutsemau [this message]
2026-08-28 19:07 ` James Houghton
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