On Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev
wrote:
>
> What if the entry is still pud_present, but got remapped after
> READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere?
That can't happen.
The GUP walk doesn't hold any locks, but it *is* done with interrupts
disabled, and any
On Thu, Sep 10, 2020 at 07:11:16PM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 10, 2020 at 02:22:37PM -0700, John Hubbard wrote:
>
> > Or am I way off here, and it really is possible (aside from the current
> > s390 situation) to observe something that "is no longer a page table"?
>
> Yes, that i
On Fri, Sep 11, 2020 at 09:09:39AM +0200, pet...@infradead.org wrote:
> On Thu, Sep 10, 2020 at 06:59:21PM -0300, Jason Gunthorpe wrote:
> > So, I suggest pXX_offset_unlocked()
>
> Urgh, no. Elsewhere in gup _unlocked() means it will take the lock
> itself (get_user_pages_unlocked()) -- although o
On Thu, Sep 10, 2020 at 06:59:21PM -0300, Jason Gunthorpe wrote:
> So, I suggest pXX_offset_unlocked()
Urgh, no. Elsewhere in gup _unlocked() means it will take the lock
itself (get_user_pages_unlocked()) -- although often it seems to mean
the lock is already held (git grep _unlocked and marvel).
On Thu, Sep 10, 2020 at 07:57:49PM +0200, Gerald Schaefer wrote:
> On Thu, 10 Sep 2020 10:02:33 -0300
> Jason Gunthorpe wrote:
>
> > On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
> >
> > > As Gerald mentioned, it is very difficult to explain in a clear way.
> > > Hopefully,
On 9/10/20 3:11 PM, Jason Gunthorpe wrote:
On Thu, Sep 10, 2020 at 02:22:37PM -0700, John Hubbard wrote:
Or am I way off here, and it really is possible (aside from the current
s390 situation) to observe something that "is no longer a page table"?
Yes, that is the issue. Remember there is no
On Thu, Sep 10, 2020 at 02:22:37PM -0700, John Hubbard wrote:
> Or am I way off here, and it really is possible (aside from the current
> s390 situation) to observe something that "is no longer a page table"?
Yes, that is the issue. Remember there is no locking for GUP
fast. While a page table ca
On Thu, Sep 10, 2020 at 12:32:05PM -0700, Linus Torvalds wrote:
> Yeah, I get hung up on naming sometimes. I don't tend to care much
> about private local variables ("i" is a perfectly fine variable name),
> but these kinds of somewhat subtle cross-architecture definitions I
> feel matter.
One of
On 9/10/20 11:13 AM, Jason Gunthorpe wrote:
On Thu, Sep 10, 2020 at 10:35:38AM -0700, Linus Torvalds wrote:
On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev
wrote:
It is only gup_fast case that exposes the issue. It hits because
pointers to stack copies are passed to gup_pXd_range iterators,
On Thu, Sep 10, 2020 at 12:11 PM Gerald Schaefer
wrote:
>
> That sounds a lot like the pXd_offset_orig() from Martins first approach
> in this thread:
> https://lore.kernel.org/linuxppc-dev/20190418100218.0a4afd51@mschwideX1/
I have to admit to finding that name horrible, but aside from that, yes
On Thu, 10 Sep 2020 11:33:17 -0700
Linus Torvalds wrote:
> On Thu, Sep 10, 2020 at 11:13 AM Jason Gunthorpe wrote:
> >
> > So.. To change away from the stack option I think we'd have to pass
> > the READ_ONCE value to pXX_offset() as an extra argument instead of it
> > derefing the pointer inter
On Thu, Sep 10, 2020 at 11:13 AM Jason Gunthorpe wrote:
>
> So.. To change away from the stack option I think we'd have to pass
> the READ_ONCE value to pXX_offset() as an extra argument instead of it
> derefing the pointer internally.
Yeah, but I think that would actually be the better model tha
On Thu, Sep 10, 2020 at 10:35:38AM -0700, Linus Torvalds wrote:
> On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev
> wrote:
> >
> > It is only gup_fast case that exposes the issue. It hits because
> > pointers to stack copies are passed to gup_pXd_range iterators, not
> > pointers to real page ta
On Thu, 10 Sep 2020 10:02:33 -0300
Jason Gunthorpe wrote:
> On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
>
> > As Gerald mentioned, it is very difficult to explain in a clear way.
> > Hopefully, one could make sense ot of it.
>
> I would say the page table API requires thi
On Thu, Sep 10, 2020 at 07:07:57PM +0200, Gerald Schaefer wrote:
> I might have lost track a bit. Are we still talking about possible
> functional impacts of either our current pagetable walking with s390
> (apart from gup_fast), or the proposed generic change (for s390, or
> others?)?
I'm lookin
On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev
wrote:
>
> It is only gup_fast case that exposes the issue. It hits because
> pointers to stack copies are passed to gup_pXd_range iterators, not
> pointers to real page tables itself.
Can we possibly change fast-gup to not do the stack copies?
I
On Thu, 10 Sep 2020 12:10:26 -0300
Jason Gunthorpe wrote:
> On Thu, Sep 10, 2020 at 03:28:03PM +0200, Gerald Schaefer wrote:
> > On Thu, 10 Sep 2020 10:02:33 -0300
> > Jason Gunthorpe wrote:
> >
> > > On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
> > >
> > > > As Geral
On Thu, Sep 10, 2020 at 03:28:03PM +0200, Gerald Schaefer wrote:
> On Thu, 10 Sep 2020 10:02:33 -0300
> Jason Gunthorpe wrote:
>
> > On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
> >
> > > As Gerald mentioned, it is very difficult to explain in a clear way.
> > > Hopefully,
On Thu, 10 Sep 2020 10:02:33 -0300
Jason Gunthorpe wrote:
> On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
>
> > As Gerald mentioned, it is very difficult to explain in a clear way.
> > Hopefully, one could make sense ot of it.
>
> I would say the page table API requires t
On Wed, 9 Sep 2020 15:03:24 -0300
Jason Gunthorpe wrote:
> On Wed, Sep 09, 2020 at 07:25:34PM +0200, Gerald Schaefer wrote:
> > I actually had to draw myself a picture to get some hold of
> > this, or rather a walk-through with a certain pud-crossing
> > range in a folded 3-level scenario. Not su
On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:
> As Gerald mentioned, it is very difficult to explain in a clear way.
> Hopefully, one could make sense ot of it.
I would say the page table API requires this invariant:
pud = pud_offset(p4d, addr);
do {
On Wed, Sep 09, 2020 at 03:03:24PM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 09, 2020 at 07:25:34PM +0200, Gerald Schaefer wrote:
> > I actually had to draw myself a picture to get some hold of
> > this, or rather a walk-through with a certain pud-crossing
> > range in a folded 3-level scenario.
On Wed, Sep 09, 2020 at 07:25:34PM +0200, Gerald Schaefer wrote:
> I actually had to draw myself a picture to get some hold of
> this, or rather a walk-through with a certain pud-crossing
> range in a folded 3-level scenario. Not sure if I would have
> understood my explanation above w/o that, but
On Wed, 9 Sep 2020 09:18:46 -0700
Dave Hansen wrote:
> On 9/9/20 5:29 AM, Gerald Schaefer wrote:
> > This only works well as long there are real pagetable pointers involved,
> > that can also be used for iteration. For gup_fast, or any other future
> > pagetable walkers using the READ_ONCE logic
On 9/9/20 5:29 AM, Gerald Schaefer wrote:
> This only works well as long there are real pagetable pointers involved,
> that can also be used for iteration. For gup_fast, or any other future
> pagetable walkers using the READ_ONCE logic w/o lock, that is not true.
> There are pointers involved to lo
On Tue, 8 Sep 2020 07:30:50 -0700
Dave Hansen wrote:
> On 9/7/20 11:00 AM, Gerald Schaefer wrote:
> > 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.
>
> Would it
On Tue, 8 Sep 2020 07:30:50 -0700
Dave Hansen wrote:
> On 9/7/20 11:00 AM, Gerald Schaefer wrote:
> > 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.
>
> Would it
On 9/7/20 11:00 AM, Gerald Schaefer wrote:
> 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.
Would it be fair to say that the "fake" page table entries s390
allocates o
On Tue, 8 Sep 2020 14:40:10 +0200
Christophe Leroy 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
> >>>
> >>> Commit 1a420
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
Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
code") introduced a subtle but severe bug
On 08.09.20 07:06, Christophe Leroy wrote:
>
>
> Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
>> From: Alexander Gordeev
>>
>> 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 pa
Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
From: Alexander Gordeev
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
From: Alexander Gordeev
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 discus
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