linux-kernel - Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

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Message-ID: <0925c64b-c721-4dc5-913a-c43a94dc64a3@redhat.com>
Date: Mon, 14 Jul 2025 17:03:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

>> or sth like that would surely clean that up further.
> 
> Well, I plan to make this not a thing soon so I'd rather not.
> 
> The intent is to make _all_ VMA flags work on 32-bit kernels. I have done some
> preparatory work and next cycle intend to do more on this.
> 
> So I'd rather avoid any config changes on this until I've given this a shot.

Sure, if that is in sight.

>>
>>> +/* Does the madvise operation result in discarding of mapped data? */
>>> +static bool is_discard(int behavior)
>>> +{
>>> +	switch (behavior) {
>>> +	case MADV_FREE:
>>> +	case MADV_DONTNEED:
>>> +	case MADV_DONTNEED_LOCKED:
>>> +	case MADV_REMOVE:
>>> +	case MADV_DONTFORK:
>>> +	case MADV_WIPEONFORK:
>>> +	case MADV_GUARD_INSTALL:
>>> +		return true;
>>> +	}
>>> +
>>> +	return false;
>>> +}
>>> +
>>> +/*
>>> + * We are restricted from madvise()'ing mseal()'d VMAs only in very particular
>>> + * circumstances - discarding of data from read-only anonymous SEALED mappings.
>>> + *
>>> + * This is because users cannot trivally discard data from these VMAs, and may
>>
>> s/trivally/trivially/
> 
> Ack thanks - Andrew can you fixup? Can send a fix-patch otherwise.
> 
>>
>>> + * only do so via an appropriate madvise() call.
>>> + */
>>> +static bool can_madvise_modify(struct madvise_behavior *madv_behavior)
>>> +{
>>> +	struct vm_area_struct *vma = madv_behavior->vma;
>>> +
>>> +	/* If the operation won't discard, we're good. */
>>> +	if (!is_discard(madv_behavior->behavior))
>>> +		return true;
>>
>>
>> Conceptually, I would do this first and then handle all the discard cases /
>> exceptions.
> 
> Hm I'm confused :P we do do this first? I think the idea with this is we can
> very cheaply ignore any MADV_ that isn't applicable.
> 
> Did you mean to put this comment under line below?
> 
> I mean it's not exactly a perf hotspot so don't mind moving them around.

I was thinking of this (start with sealed, then go into details about 
discards):

/* If the VMA isn't sealed, we're all good. */
if (can_modify_vma(vma))
	return true;

/* In a sealed VMA, we only care about discard operations. */
if (!is_discard(madv_behavior->behavior))
	return true;

/* But discards of file-backed mappings are fine. */
if (!vma_is_anonymous(vma))
	return true;

...


But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE 
file mapping?

-- 
Cheers,

David / dhildenb