linux-kernel - Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem mappings

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Message-ID: <b30a6306-d62b-4515-add8-4550d044501c@redhat.com>
Date: Tue, 18 Feb 2025 18:25:35 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
 linux-kselftest@...r.kernel.org, linux-api@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>, Juan Yescas <jyescas@...gle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>
Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem
 mappings

>>
>> QEMU, for example, will issue an mlockall(MCL_CURRENT | MCL_FUTURE); when
>> requested to then exit(); if it fails.
> 
> Hm under what circumstances? I use qemu extensively to test this stuff with
> no issues. Unless you mean it's using it in the 'host' code somehow.

-overcommit mem-lock=on

or (legacy)

-realtime mlock=on

I think.

[...]

>>>
>>> It fails because it tries to 'touch' the memory, but 'touching' guard
>>> region memory causes a segfault. This kind of breaks the idea of
>>> mlock()'ing guard regions.
>>>
>>> I think adding workarounds to make this possible in any way is not really
>>> worth it (and would probably be pretty gross).
>>>
>>> We already document that 'mlock()ing lightweight guard regions will fail'
>>> as per man page so this is all in line with that.
>>
>> Right, and I claim that supporting VM_LOCKONFAULT might likely be as easy as
>> allowing install/remove of guard regions when that flag is set.
> 
> We already allow this flag! VM_LOCKED and VM_HUGETLB are the only flags we
> disallow.

See mlock2();

SYSCALL_DEFINE3(mlock2, unsigned long, start, size_t, len, int, flags)
{
	vm_flags_t vm_flags = VM_LOCKED;

	if (flags & ~MLOCK_ONFAULT)
		return -EINVAL;

	if (flags & MLOCK_ONFAULT)
		vm_flags |= VM_LOCKONFAULT;

	return do_mlock(start, len, vm_flags);
}

VM_LOCKONFAULT always as VM_LOCKED set as well.

-- 
Cheers,

David / dhildenb