linux-kernel - Re: [REGRESSION] Re: [PATCH v8 15/19] mm: don't allow huge faults for files with pre content watches

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Message-ID: <CAHk-=wjMPZ7htPTzxtF52-ZPShfFOQ4R-pHVxLO+pfOW5avC4Q@mail.gmail.com>
Date: Fri, 31 Jan 2025 11:59:56 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Josef Bacik <josef@...icpanda.com>, kernel-team@...com, linux-fsdevel@...r.kernel.org, 
	jack@...e.cz, amir73il@...il.com, brauner@...nel.org, viro@...iv.linux.org.uk, 
	linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-mm@...ck.org, 
	linux-ext4@...r.kernel.org, Peter Xu <peterx@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [REGRESSION] Re: [PATCH v8 15/19] mm: don't allow huge faults for
 files with pre content watches

On Fri, 31 Jan 2025 at 11:17, Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> 20bf82a898b6 ("mm: don't allow huge faults for files with pre content watches")
>
> This breaks huge_fault support for PFNMAPs that was recently added in
> v6.12 and is used by vfio-pci to fault device memory using PMD and PUD
> order mappings.

Surely only for content watches?

Which shouldn't be a valid situation *anyway*.

IOW, there must be some unrelated bug somewhere: either somebody is
allowed to set a pre-content match on a special device.

That should be disabled by the whole

        /*
         * If there are permission event watchers but no pre-content event
         * watchers, set FMODE_NONOTIFY | FMODE_NONOTIFY_PERM to indicate that.
         */

thing in file_set_fsnotify_mode() which only allows regular files and
directories to be notified on.

Or, alternatively, that check for huge-fault disabling is just
checking the wrong bits.

Or - quite possibly - I am missing something obvious?

             Linus