[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ef0e11c5-13cf-4d47-a277-41da317be165@redhat.com>
Date: Mon, 21 Oct 2024 19:00:53 +0200
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, Muchun Song <muchun.song@...ux.dev>,
Richard Henderson <richard.henderson@...aro.org>,
Ivan Kokshaysky <ink@...assic.park.msu.ru>, Matt Turner
<mattst88@...il.com>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
"James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
linux-kselftest@...r.kernel.org, Sidhartha Kumar
<sidhartha.kumar@...cle.com>, Jeff Xu <jeffxu@...omium.org>,
Christoph Hellwig <hch@...radead.org>, linux-api@...r.kernel.org,
John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker
On 21.10.24 18:51, Lorenzo Stoakes wrote:
> On Mon, Oct 21, 2024 at 06:44:04PM +0200, David Hildenbrand wrote:
>> On 21.10.24 18:23, Lorenzo Stoakes wrote:
>>> On Mon, Oct 21, 2024 at 06:00:20PM +0200, David Hildenbrand wrote:
>>> [snip]
>>>>>
>>>>> To summarise for on-list:
>>>>>
>>>>> * MADV_FREE, while ostensibly being a 'lazy free' mechanism, has the
>>>>> ability to be 'cancelled' if you write to the memory. Also, after the
>>>>> freeing is complete, you can write to the memory to reuse it, the mapping
>>>>> is still there.
>>>>>
>>>>> * For hardware poison markers it makes sense to drop them as you're
>>>>> effectively saying 'I am done with this range that is now unbacked and
>>>>> expect to get an empty page should I use it now'. UFFD WP I am not sure
>>>>> about but presumably also fine.
>>>>>
>>>>> * However, guard pages are different - if you 'cancel' and you are left
>>>>> with a block of memory allocated to you by a pthread or userland
>>>>> allocator implementation, you don't want to then no longer be protected
>>>>> from overrunning into other thread memory.
>>>>
>>>> Agreed. What happens on MADV_DONTNEED/MADV_FREE on guard pages? Ignored or
>>>> error? It sounds like a usage "error" to me (in contrast to munmap()).
>>>
>>> It's ignored, no errror. On MADV_DONTNEED we already left the guard pages in
>>> place, from v3 we will do the same for MADV_FREE.
>>>
>>> I'm not sure I'd say it's an error per se, as somebody might have a use case
>>> where they want to zap over a range but keep guard pages, perhaps an allocator
>>> or something?
>>
>> Hm, not sure I see use for that.
>>
>> Staring at madvise_walk_vmas(), we return ENOMEM on VMA holes, but would
>> process PROT_NONE. So current behavior is at least consistent with PROT_NONE
>> handling (where something could be mapped, though).
>
> Err, the handling of holes is terrible, yes we return ENOMEM, but we _carry out
> the whole procedure_ then return an error, an error _indistinguishable from an
> error arising from any of the individual parts_.
>
> Which is just, awful.
Yes, absolutely. I don't know why we decided to continue. And why we
return ENOMEM ...
>
>>
>> No strong opinion.
>
> Well you used up your strong opinion on the naming ;)
He, and I am out of energy for this year ;)
In retrospective, "install or remove a guard PTE" is just much better
than anything else ...
So I should never have been mislead to suggest poison/unpoison as a
replacement for poison/remedy :P
>
>>
>>>
>>> Also the existing logic is that existing markers (HW poison, uffd-simulated HW
>>> poison, uffd wp marker) are retained and no error raised on MADV_DONTNEED, and
>>> no error on MADV_FREE either, so it'd be consistent with existing behaviour.
>>
>>
>> HW poison / uffd-simulated HW poison are expected to be zapped: it's just
>> like a mapped page with HWPOISON. So that is correct.
>
> Well, poison is _not_ zapped on MADV_DONTNEED but _is_ on MADV_FREE :) anyway, I
Huh?
madvise_dontneed_single_vma()->zap_page_range_single(details=NULL)->unmap_single_vma(details=NULL)
... zap_pte_range()
} else if (is_hwpoison_entry(entry) ||
is_poisoned_swp_entry(entry)) {
if (!should_zap_cows(details))
continue;
...
Should just zap them.
What am I missing?
> mean the MADV flags are a confusing mess generally, as per Vlasta's comments
> which to begin with I strongly disagreed with then, discussing further, realsed
> that no this is just a bit insane and had driven _me_ insane.
>
>>
>> UFFD-WP behavior is ... weird. Would not expect MADV_DONTNEED to zap uffd-wp
>> entries.
>>
>>>
>>> Also semantically you are achieving what the calls expect you are freeing the
>>> ranges since the guard page regions are unbacked so are already freed... so yeah
>>> I don't think an error really makes sense here.
>>
>> I you compare it to a VMA hole, it make sense to fail. If we treat it like
>> PROT_NONE, it make sense to skip them.
>>
>>>
>>> We might also be limiting use cases by assuming they might _only_ be used for
>>> allocators and such.
>>
>> I don't buy that as an argument, sorry :)
>>
>> "Let's map the kernel writable into all user space because otherwise we
>> might be limiting use cases"
>
> That's a great idea! Patch series incoming, 1st April 2025... :>)
:) Just flip the bit on x86 and we're done!
>>
>>
>> :P
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
>
> Overall I think just always leaving in place except on remedy err sorry sorry
> unpoison and munmap and not returning an error if encountered elsewhere (other
> than, of course, GUP) is the right way forward and most in line with user
> expectation and practical usage.
Fine with me, make sure to document that is behaves like a PROT_NONE
VMA, not like a memory hole, except when something would trigger a fault
(GUP etc).
--
Cheers,
David / dhildenb
Powered by blists - more mailing lists