From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@suse.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>, Oscar Salvador <osalvador@suse.de>,
Matthew Wilcox <willy@infradead.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Minchan Kim <minchan@kernel.org>, Jann Horn <jannh@google.com>,
Jason Gunthorpe <jgg@ziepe.ca>,
Dave Hansen <dave.hansen@intel.com>,
Hugh Dickins <hughd@google.com>, Rik van Riel <riel@surriel.com>,
"Michael S . Tsirkin" <mst@redhat.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Vlastimil Babka <vbabka@suse.cz>,
Richard Henderson <rth@twiddle.net>,
Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
Matt Turner <mattst88@gmail.com>,
Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
Helge Deller <deller@gmx.de>, Chris Zankel <chris@zankel.net>,
Max Filippov <jcmvbkbc@gmail.com>,
linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org,
linux-parisc@vger.kernel.org, linux-xtensa@linux-xtensa.org,
linux-arch@vger.kernel.org
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory
Date: Thu, 18 Feb 2021 12:38:27 +0100 [thread overview]
Message-ID: <740cdd51-137b-2b08-8b7f-9757d8d847cb@redhat.com> (raw)
In-Reply-To: <YC5PEBiGKT5LUt/I@dhcp22.suse.cz>
>>>> If we hit
>>>> hardware errors on pages, ignore them - nothing we really can or
>>>> should do.
>>>> 3. On errors during MADV_POPULATED, some memory might have been
>>>> populated. Callers have to clean up if they care.
>>>
>>> How does caller find out? madvise reports 0 on success so how do you
>>> find out how much has been populated?
>>
>> If there is an error, something might have been populated. In my QEMU
>> implementation, I simply discard the range again, good enough. I don't think
>> we need to really indicate "error and populated" or "error and not
>> populated".
>
> Agreed. The wording just suggests that the syscall actually provides any
> means for an effective way to handle those errors. Maybe you should just
> stick with the first sentence and drop the second.
Makes sense. "On errors during MADV_POPULATE, some memory might have
been populated."
>
>>>> 4. Concurrent changes to the virtual memory layour are tolerated - we
>>>> process each and every PFN only once, though.
>>>
>>> I do not understand this. madvise is about virtual address space not a
>>> physical address space.
>>
>> What I wanted to express: if we detect a change in the mapping we don't
>> restart at the beginning, we always make forward progress. We process each
>> virtual address once (on a per-page basis, thus I accidentally used "PFN").
>
> This is an implicit assumption. Your range can have the same page mapped
> several times in the given address range and all you care about is that
> you fault those which are not present during the virtual address space
> walk. Your syscall can return and large part of the address space might
> be unpopulated because memory reclaim just dropped those pages and that
> would be fine. This shouldn't really imply memory presence - mlock does
> that.
"Concurrent changes to the virtual memory layout are tolerated. The
range is processed exactly once."
>
>>>> 5. If MADV_POPULATE succeeds, all memory in the range can be accessed
>>>> without SIGBUS. (of course, not if user space changed mappings in the
>>>> meantime or KSM kicked in on anonymous memory).
>>>
>>> I do not see how KSM would change anything here and maybe it is not
>>> really important to mention it. KSM should be really transparent from
>>> the users space POV. Parallel and destructive virtual address space
>>> operations are also expected to change the outcome and there is nothing
>>> kernel do about at and provide any meaningful guarantees. I guess we
>>> want to assume a reasonable userspace behavior here.
>>
>> It's just a note that we cannot protect from someone interfering
>> (discard/ksm/whatever). I'm making that clearer in the cover letter.
>
> Again that is implicit expectation. madvise will not work for anybody
> shooting an own foot.
Okay, I'll drop that part, thanks!
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-02-18 11:38 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-17 15:48 [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory David Hildenbrand
2021-02-17 16:46 ` Dave Hansen
2021-02-17 17:06 ` David Hildenbrand
2021-02-17 17:21 ` Vlastimil Babka
2021-02-18 11:07 ` Rolf Eike Beer
2021-02-18 11:27 ` David Hildenbrand
2021-02-18 10:25 ` Michal Hocko
2021-02-18 10:44 ` David Hildenbrand
2021-02-18 10:54 ` David Hildenbrand
2021-02-18 11:28 ` Michal Hocko
2021-02-18 11:27 ` Michal Hocko
2021-02-18 11:38 ` David Hildenbrand [this message]
2021-02-18 12:22 ` [PATCH RFC] madvise.2: Document MADV_POPULATE David Hildenbrand
2021-02-18 22:59 ` [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory Peter Xu
2021-02-19 8:20 ` David Hildenbrand
2021-02-19 16:31 ` Peter Xu
2021-02-19 17:13 ` David Hildenbrand
2021-02-19 19:14 ` David Hildenbrand
2021-02-19 19:25 ` Mike Kravetz
2021-02-20 9:01 ` David Hildenbrand
2021-02-19 19:23 ` Peter Xu
2021-02-19 20:04 ` David Hildenbrand
2021-02-22 12:46 ` Michal Hocko
2021-02-22 12:52 ` David Hildenbrand
2021-02-19 10:35 ` Michal Hocko
2021-02-19 10:43 ` David Hildenbrand
2021-02-19 11:04 ` Michal Hocko
2021-02-19 11:10 ` David Hildenbrand
2021-02-20 9:12 ` David Hildenbrand
2021-02-22 12:56 ` Michal Hocko
2021-02-22 12:59 ` David Hildenbrand
2021-02-22 13:19 ` Michal Hocko
2021-02-22 13:22 ` David Hildenbrand
2021-02-22 14:02 ` Michal Hocko
2021-02-22 15:30 ` David Hildenbrand
2021-02-24 14:25 ` David Hildenbrand
2021-02-24 14:38 ` David Hildenbrand
2021-02-25 8:41 ` David Hildenbrand
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