From: Gerald Schaefer <gerald.schaefer@linux•ibm.com>
To: Christophe Leroy <christophe.leroy@csgroup•eu>
Cc: Peter Zijlstra <peterz@infradead•org>,
Catalin Marinas <catalin.marinas@arm•com>,
Dave Hansen <dave.hansen@linux•intel.com>,
linux-mm <linux-mm@kvack•org>, Paul Mackerras <paulus@samba•org>,
linux-sparc <sparclinux@vger•kernel.org>,
Alexander Gordeev <agordeev@linux•ibm.com>,
Claudio Imbrenda <imbrenda@linux•ibm.com>,
Will Deacon <will@kernel•org>,
linux-arch <linux-arch@vger•kernel.org>,
linux-s390 <linux-s390@vger•kernel.org>,
Vasily Gorbik <gor@linux•ibm.com>,
Christian Borntraeger <borntraeger@de•ibm.com>,
Richard Weinberger <richard@nod•at>, linux-x86 <x86@kernel•org>,
Russell King <linux@armlinux•org.uk>,
Jason Gunthorpe <jgg@ziepe•ca>, Ingo Molnar <mingo@redhat•com>,
Andrey Ryabinin <aryabinin@virtuozzo•com>,
Jeff Dike <jdike@addtoit•com>, Arnd Bergmann <arnd@arndb•de>,
John Hubbard <jhubbard@nvidia•com>,
Heiko Carstens <hca@linux•ibm.com>,
linux-um <linux-um@lists•infradead.org>,
Borislav Petkov <bp@alien8•de>, Andy Lutomirski <luto@kernel•org>,
Thomas Gleixner <tglx@linutronix•de>,
linux-arm <linux-arm-kernel@lists•infradead.org>,
Linus Torvalds <torvalds@linux-foundation•org>,
LKML <linux-kernel@vger•kernel.org>,
Andrew Morton <akpm@linux-foundation•org>,
linux-power <linuxppc-dev@lists•ozlabs.org>,
Mike Rapoport <rppt@kernel•org>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding
Date: Tue, 8 Sep 2020 15:38:57 +0200 [thread overview]
Message-ID: <20200908153857.08d09581@thinkpad> (raw)
In-Reply-To: <96b80926-cf5b-1afa-9b7a-949a2188e61f@csgroup.eu>
On Tue, 8 Sep 2020 14:40:10 +0200
Christophe Leroy <christophe.leroy@csgroup•eu> wrote:
>
>
> Le 08/09/2020 à 14:09, Christian Borntraeger a écrit :
> >
> >
> > On 08.09.20 07:06, Christophe Leroy wrote:
> >>
> >>
> >> Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
> >>> From: Alexander Gordeev <agordeev@linux•ibm.com>
> >>>
> >>> Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
> >>> code") introduced a subtle but severe bug on s390 with gup_fast, due to
> >>> dynamic page table folding.
> >>>
> >>> The question "What would it require for the generic code to work for s390"
> >>> has already been discussed here
> >>> https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1
> >>> and ended with a promising approach here
> >>> https://lkml.kernel.org/r/20190419153307.4f2911b5@mschwideX1
> >>> which in the end unfortunately didn't quite work completely.
> >>>
> >>> We tried to mimic static level folding by changing pgd_offset to always
> >>> calculate top level page table offset, and do nothing in folded pXd_offset.
> >>> What has been overlooked is that PxD_SIZE/MASK and thus pXd_addr_end do
> >>> not reflect this dynamic behaviour, and still act like static 5-level
> >>> page tables.
> >>>
> >>
> >> [...]
> >>
> >>>
> >>> Fix this by introducing new pXd_addr_end_folded helpers, which take an
> >>> additional pXd entry value parameter, that can be used on s390
> >>> to determine the correct page table level and return corresponding
> >>> end / boundary. With that, the pointer iteration will always
> >>> happen in gup_pgd_range for s390. No change for other architectures
> >>> introduced.
> >>
> >> Not sure pXd_addr_end_folded() is the best understandable name, allthough I don't have any alternative suggestion at the moment.
> >> Maybe could be something like pXd_addr_end_fixup() as it will disappear in the next patch, or pXd_addr_end_gup() ?
> >>
> >> Also, if it happens to be acceptable to get patch 2 in stable, I think you should switch patch 1 and patch 2 to avoid the step through pXd_addr_end_folded()
> >
> > given that this fixes a data corruption issue, wouldnt it be the best to go forward
> > with this patch ASAP and then handle the other patches on top with all the time that
> > we need?
>
> I have no strong opinion on this, but I feel rather tricky to have to
> change generic part of GUP to use a new fonction then revert that change
> in the following patch, just because you want the first patch in stable
> and not the second one.
>
> Regardless, I was wondering, why do we need a reference to the pXd at
> all when calling pXd_addr_end() ?
>
> Couldn't S390 retrieve the pXd by using the pXd_offset() dance with the
> passed addr ?
Apart from performance impact when re-doing that what has already been
done by the caller, I think we would also break the READ_ONCE semantics.
After all, the pXd_offset() would also require some pXd pointer input,
which we don't have. So we would need to start over again from mm->pgd.
Also, it seems to be more in line with other primitives that take
a pXd value or pointer.
next prev parent reply other threads:[~2020-09-08 13:42 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-07 18:00 [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 1/3] " Gerald Schaefer
2020-09-08 5:06 ` Christophe Leroy
2020-09-08 12:09 ` Christian Borntraeger
2020-09-08 12:40 ` Christophe Leroy
2020-09-08 13:38 ` Gerald Schaefer [this message]
2020-09-08 14:30 ` Dave Hansen
2020-09-08 17:59 ` Gerald Schaefer
2020-09-09 12:29 ` Gerald Schaefer
2020-09-09 16:18 ` Dave Hansen
2020-09-09 17:25 ` Gerald Schaefer
2020-09-09 18:03 ` Jason Gunthorpe
2020-09-10 9:39 ` Alexander Gordeev
2020-09-10 13:02 ` Jason Gunthorpe
2020-09-10 13:28 ` Gerald Schaefer
2020-09-10 15:10 ` Jason Gunthorpe
2020-09-10 17:07 ` Gerald Schaefer
2020-09-10 17:19 ` Jason Gunthorpe
2020-09-10 17:57 ` Gerald Schaefer
2020-09-10 23:21 ` Jason Gunthorpe
2020-09-10 17:35 ` Linus Torvalds
2020-09-10 18:13 ` Jason Gunthorpe
2020-09-10 18:33 ` Linus Torvalds
2020-09-10 19:10 ` Gerald Schaefer
2020-09-10 19:32 ` Linus Torvalds
2020-09-10 21:59 ` Jason Gunthorpe
2020-09-11 7:09 ` peterz
2020-09-11 11:19 ` Jason Gunthorpe
2020-09-11 19:03 ` [PATCH] " Vasily Gorbik
2020-09-11 19:09 ` Linus Torvalds
2020-09-11 19:40 ` Jason Gunthorpe
2020-09-11 20:05 ` Jason Gunthorpe
2020-09-11 20:36 ` [PATCH v2] " Vasily Gorbik
2020-09-15 17:09 ` Vasily Gorbik
2020-09-15 17:14 ` Jason Gunthorpe
2020-09-15 17:18 ` Mike Rapoport
2020-09-15 17:31 ` John Hubbard
2020-09-10 21:22 ` [RFC PATCH v2 1/3] " John Hubbard
2020-09-10 22:11 ` Jason Gunthorpe
2020-09-10 22:17 ` John Hubbard
2020-09-11 12:19 ` Alexander Gordeev
2020-09-11 16:45 ` Linus Torvalds
2020-09-10 13:11 ` Gerald Schaefer
2020-09-07 18:00 ` [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table entry aware Gerald Schaefer
2020-09-08 5:14 ` Christophe Leroy
2020-09-08 7:46 ` Alexander Gordeev
2020-09-08 8:16 ` Christophe Leroy
2020-09-08 14:15 ` Alexander Gordeev
2020-09-09 8:38 ` Christophe Leroy
2020-09-08 14:25 ` Alexander Gordeev
2020-09-08 13:26 ` Jason Gunthorpe
2020-09-08 14:33 ` Dave Hansen
2020-09-07 18:00 ` [RFC PATCH v2 3/3] mm: make generic pXd_addr_end() macros inline functions Gerald Schaefer
2020-09-07 20:15 ` Mike Rapoport
2020-09-08 5:19 ` Christophe Leroy
2020-09-08 15:48 ` Alexander Gordeev
2020-09-08 17:20 ` Christophe Leroy
2020-09-07 20:12 ` [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table folding Mike Rapoport
2020-09-08 5:22 ` Christophe Leroy
2020-09-08 17:36 ` Gerald Schaefer
2020-09-09 16:12 ` Gerald Schaefer
2020-09-08 4:42 ` Christophe Leroy
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