[PATCH 1/2] mm: lock a vma before stack expansion

[PATCH 1/2] mm: lock a vma before stack expansion

[Suren Baghdasaryan]

With recent changes necessitating mmap_lock to be held for write while
expanding a stack, per-VMA locks should follow the same rules and be
write-locked to prevent page faults into the VMA being expanded. Add
the necessary locking.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 204ddcd52625..c66e4622a557 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 		return -ENOMEM;
 	}
 
+	/* Lock the VMA before expanding to prevent concurrent page faults */
+	vma_start_write(vma);
 	/*
 	 * vma->vm_start/vm_end cannot change under us because the caller
 	 * is required to hold the mmap_lock in read mode.  We need the
@@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 		return -ENOMEM;
 	}
 
+	/* Lock the VMA before expanding to prevent concurrent page faults */
+	vma_start_write(vma);
 	/*
 	 * vma->vm_start/vm_end cannot change under us because the caller
 	 * is required to hold the mmap_lock in read mode.  We need the
-- 
2.41.0.255.g8b1d071c50-goog

[PATCH 2/2] mm: lock newly mapped VMA which can be modified after it becomes visible

[Suren Baghdasaryan]

mmap_region adds a newly created VMA into VMA tree and might modify it
afterwards before dropping the mmap_lock. This poses a problem for page
faults handled under per-VMA locks because they don't take the mmap_lock
and can stumble on this VMA while it's still being modified. Currently
this does not pose a problem since post-addition modifications are done
only for file-backed VMAs, which are not handled under per-VMA lock.
However, once support for handling file-backed page faults with per-VMA
locks is added, this will become a race.
Fix this by write-locking the VMA before inserting it into the VMA tree.
Other places where a new VMA is added into VMA tree do not modify it
after the insertion, so do not need the same locking.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mmap.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index c66e4622a557..84c71431a527 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2812,6 +2812,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	if (vma->vm_file)
 		i_mmap_lock_write(vma->vm_file->f_mapping);
 
+	/* Lock the VMA since it is modified after insertion into VMA tree */
+	vma_start_write(vma);
 	vma_iter_store(&vmi, vma);
 	mm->map_count++;
 	if (vma->vm_file) {
-- 
2.41.0.255.g8b1d071c50-goog

Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Andrew Morton]

On Thu,  6 Jul 2023 21:32:10 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> With recent changes necessitating mmap_lock to be held for write while
> expanding a stack, per-VMA locks should follow the same rules and be
> write-locked to prevent page faults into the VMA being expanded. Add
> the necessary locking.

What are the possible runtime effects of this change?

> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> @@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> -- 
> 2.41.0.255.g8b1d071c50-goog

Re: [PATCH 2/2] mm: lock newly mapped VMA which can be modified after it becomes visible

[Liam R. Howlett]

* Suren Baghdasaryan <surenb@google.com> [230707 00:32]:
> mmap_region adds a newly created VMA into VMA tree and might modify it
> afterwards before dropping the mmap_lock. This poses a problem for page
> faults handled under per-VMA locks because they don't take the mmap_lock
> and can stumble on this VMA while it's still being modified. Currently
> this does not pose a problem since post-addition modifications are done
> only for file-backed VMAs, which are not handled under per-VMA lock.
> However, once support for handling file-backed page faults with per-VMA
> locks is added, this will become a race.
> Fix this by write-locking the VMA before inserting it into the VMA tree.
> Other places where a new VMA is added into VMA tree do not modify it
> after the insertion, so do not need the same locking.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  mm/mmap.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c66e4622a557..84c71431a527 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2812,6 +2812,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	if (vma->vm_file)
>  		i_mmap_lock_write(vma->vm_file->f_mapping);
>  
> +	/* Lock the VMA since it is modified after insertion into VMA tree */

So it is modified, but that i_mmap_lock_write() directly above this
comment is potentially moving below the insert and that is why this lock
is needed.

> +	vma_start_write(vma);
>  	vma_iter_store(&vmi, vma);
>  	mm->map_count++;
>  	if (vma->vm_file) {
> -- 
> 2.41.0.255.g8b1d071c50-goog
> 

Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Markus Elfring]

…
> write-locked to prevent page faults into the VMA being expanded. Add
> the necessary locking.

1. Would it a bit nicer to put the second sentence on a separate line
   in such a change description?

2. I noticed that you put the address “stable@vger.kernel.org”
   into the message field “Cc”.
   Would you like to specify such a hint as a tag?

   See also:
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.4#n264

3. How do you think about to add the tag “Fixes”?

4. Will a cover letter become helpful also for the presented small patch series?

Regards,
Markus

Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Suren Baghdasaryan]

On Fri, Jul 7, 2023 at 7:27 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu,  6 Jul 2023 21:32:10 -0700 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > With recent changes necessitating mmap_lock to be held for write while
> > expanding a stack, per-VMA locks should follow the same rules and be
> > write-locked to prevent page faults into the VMA being expanded. Add
> > the necessary locking.
>
> What are the possible runtime effects of this change?

During the stack expansion concurrent page faults would have to wait
and visa versa (stack expansion would have to wait if there are
ongoing page faults). I think it's the same runtime effects as the
recent similar change requiring mmap_lock to be write lock before
stack expansion.

>
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> >               return -ENOMEM;
> >       }
> >
> > +     /* Lock the VMA before expanding to prevent concurrent page faults */
> > +     vma_start_write(vma);
> >       /*
> >        * vma->vm_start/vm_end cannot change under us because the caller
> >        * is required to hold the mmap_lock in read mode.  We need the
> > @@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> >               return -ENOMEM;
> >       }
> >
> > +     /* Lock the VMA before expanding to prevent concurrent page faults */
> > +     vma_start_write(vma);
> >       /*
> >        * vma->vm_start/vm_end cannot change under us because the caller
> >        * is required to hold the mmap_lock in read mode.  We need the
> > --
> > 2.41.0.255.g8b1d071c50-goog
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>

Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Matthew Wilcox]

On Fri, Jul 07, 2023 at 10:00:42PM +0200, Markus Elfring wrote:
> …
> > write-locked to prevent page faults into the VMA being expanded. Add
> > the necessary locking.
> 
> 1. Would it a bit nicer to put the second sentence on a separate line
>    in such a change description?
> 
> 2. I noticed that you put the address “stable@vger.kernel.org”
>    into the message field “Cc”.
>    Would you like to specify such a hint as a tag?
> 
>    See also:
>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.4#n264
> 
> 3. How do you think about to add the tag “Fixes”?
> 
> 4. Will a cover letter become helpful also for the presented small patch series?

Markus, your nitpicking is not useful.  Please stop.

Re: [PATCH 1/2] mm: lock a vma before stack expansion

[Suren Baghdasaryan]

On Fri, Jul 7, 2023 at 8:03 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Jul 07, 2023 at 10:00:42PM +0200, Markus Elfring wrote:
> > …
> > > write-locked to prevent page faults into the VMA being expanded. Add
> > > the necessary locking.
> >
> > 1. Would it a bit nicer to put the second sentence on a separate line
> >    in such a change description?

Maybe. Will do if there is a need to post a v2.

> >
> > 2. I noticed that you put the address “stable@vger.kernel.org”
> >    into the message field “Cc”.
> >    Would you like to specify such a hint as a tag?
> >
> >    See also:
> >    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.4#n264

Yeah, I always forget that :(

> >
> > 3. How do you think about to add the tag “Fixes”?

I thought about it but was not sure which patch I should list under
such tag because the rules for stack expansion changed recently.

> >
> > 4. Will a cover letter become helpful also for the presented small patch series?

Not much to say other than "add some missing locking" :)

>
> Markus, your nitpicking is not useful.  Please stop.

I'll fix the nits, at least the ones I can, if there is a need for v2.

Re: [PATCH 2/2] mm: lock newly mapped VMA which can be modified after it becomes visible

[Suren Baghdasaryan]

On Fri, Jul 7, 2023 at 7:48 PM Liam R. Howlett <Liam.Howlett@oracle.com> wrote:
>
> * Suren Baghdasaryan <surenb@google.com> [230707 00:32]:
> > mmap_region adds a newly created VMA into VMA tree and might modify it
> > afterwards before dropping the mmap_lock. This poses a problem for page
> > faults handled under per-VMA locks because they don't take the mmap_lock
> > and can stumble on this VMA while it's still being modified. Currently
> > this does not pose a problem since post-addition modifications are done
> > only for file-backed VMAs, which are not handled under per-VMA lock.
> > However, once support for handling file-backed page faults with per-VMA
> > locks is added, this will become a race.
> > Fix this by write-locking the VMA before inserting it into the VMA tree.
> > Other places where a new VMA is added into VMA tree do not modify it
> > after the insertion, so do not need the same locking.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/mmap.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index c66e4622a557..84c71431a527 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2812,6 +2812,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >       if (vma->vm_file)
> >               i_mmap_lock_write(vma->vm_file->f_mapping);
> >
> > +     /* Lock the VMA since it is modified after insertion into VMA tree */
>
> So it is modified, but that i_mmap_lock_write() directly above this
> comment is potentially moving below the insert and that is why this lock
> is needed.

Correct, we should not rely on i_mmap_lock_write() which can be moved
(and is suggested to be moved in
https://lore.kernel.org/all/20230606124939.93561-1-yu.ma@intel.com/).


>
> > +     vma_start_write(vma);
> >       vma_iter_store(&vmi, vma);
> >       mm->map_count++;
> >       if (vma->vm_file) {
> > --
> > 2.41.0.255.g8b1d071c50-goog
> >
>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
>

Re: mm: lock a vma before stack expansion

[Markus Elfring]

>> …
>>> write-locked to prevent page faults into the VMA being expanded. Add
>>> the necessary locking.
>>
>> 1. Would it a bit nicer to put the second sentence on a separate line
>>    in such a change description?
>>
>> 2. I noticed that you put the address “stable@vger.kernel.org”
>>    into the message field “Cc”.
>>    Would you like to specify such a hint as a tag?
>>
>>    See also:
>>    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.4#n264
>>
>> 3. How do you think about to add the tag “Fixes”?
>>
>> 4. Will a cover letter become helpful also for the presented small patch series?
>
> Markus, your nitpicking is not useful. …

Do you like patch reviews more by other contributors?

Regards,
Markus

Re: [1/2] mm: lock a vma before stack expansion

[Markus Elfring]

…
>> Markus, your nitpicking is not useful.  Please stop.
>
> I'll fix the nits, at least the ones I can, if there is a need for v2.

Thanks for such a constructive feedback.

Would you like to take any further information better into account
for subsequent patch variants?

Regards,
Markus

Re: [1/2] mm: lock a vma before stack expansion

[Suren Baghdasaryan]

On Fri, Jul 7, 2023 at 10:55 PM Markus Elfring <Markus.Elfring@web.de> wrote:
>
> …
> >> Markus, your nitpicking is not useful.  Please stop.
> >
> > I'll fix the nits, at least the ones I can, if there is a need for v2.
>
> Thanks for such a constructive feedback.
>
> Would you like to take any further information better into account
> for subsequent patch variants?

All feedback is appreciated. The comments you provided so far seem to
be nice-to-haves but not critical enough to post a new version of the
patch IMHO. If something more substantial is found that requires a new
version then I'll address your feedback there as well.
Thanks,
Suren.

>
> Regards,
> Markus

Re: [1/2] mm: lock a vma before stack expansion

[Markus Elfring]

>> Would you like to take any further information better into account
>> for subsequent patch variants?
>
> All feedback is appreciated.

Thanks for such a positive response.


>                              The comments you provided so far seem to
> be nice-to-haves but not critical enough to post a new version of the
> patch IMHO.

I guess that our dialogue can increase the probability for more desirable improvements.

I got used to point items out which can affect also the development process
for Linux software components.
It seems that some contributors stumble still on difficulties to handle
presented patch review approaches in more constructive ways.


>             If something more substantial is found that requires a new
> version then I'll address your feedback there as well.

Will the chances grow to pick further recommendations, guidelines and
related change possibilities up?

Regards,
Markus