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Message-ID: <640fee1d-e76b-4aca-8975-f6bd4f3279d9@lucifer.local>
Date: Mon, 13 Jan 2025 16:47:50 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
liam.howlett@...cle.com, david.laight.linux@...il.com, mhocko@...e.com,
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linux-doc@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH v9 04/17] mm: introduce vma_iter_store_attached() to use
with attached vmas
On Mon, Jan 13, 2025 at 08:31:45AM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 13, 2025 at 3:58 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
> >
> > On Fri, Jan 10, 2025 at 08:25:51PM -0800, Suren Baghdasaryan wrote:
> > > vma_iter_store() functions can be used both when adding a new vma and
> > > when updating an existing one. However for existing ones we do not need
> > > to mark them attached as they are already marked that way. Introduce
> > > vma_iter_store_attached() to be used with already attached vmas.
> >
> > OK I guess the intent of this is to reinstate the previously existing
> > asserts, only explicitly checking those places where we attach.
>
> No, the motivation is to prevern re-attaching an already attached vma
> or re-detaching an already detached vma for state consistency. I guess
> I should amend the description to make that clear.
Sorry for noise, missed this reply.
What I mean by this is, in a past iteration of this series I reviewed code
where you did this but did _not_ differentiate between cases of new VMAs
vs. existing, which caused an assert in your series which I reported.
So I"m saying - now you _are_ differentiating between the two cases.
It's certainly worth belabouring the point of exactly what it is you are
trying to catch here, however! :) So yes please do add a little more to
commit msg that'd be great, thanks!
>
> >
> > I'm a little concerned that by doing this, somebody might simply invoke
> > this function without realising the implications.
>
> Well, in that case somebody should get an assertion. If
> vma_iter_store() is called against already attached vma, we get this
> assertion:
>
> vma_iter_store()
> vma_mark_attached()
> vma_assert_detached()
>
> If vma_iter_store_attached() is called against a detached vma, we get this one:
>
> vma_iter_store_attached()
> vma_assert_attached()
>
> Does that address your concern?
>
> >
> > Can we have something functional like
> >
> > vma_iter_store_new() and vma_iter_store_overwrite()
>
> Ok. A bit more churn but should not be too bad.
>
> >
> > ?
> >
> > I don't like us just leaving vma_iter_store() quietly making an assumption
> > that a caller doesn't necessarily realise.
> >
> > Also it's more greppable this way.
> >
> > I had a look through callers and it does seem you've snagged them all
> > correctly.
> >
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> > > ---
> > > include/linux/mm.h | 12 ++++++++++++
> > > mm/vma.c | 8 ++++----
> > > mm/vma.h | 11 +++++++++--
> > > 3 files changed, 25 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 2b322871da87..2f805f1a0176 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -821,6 +821,16 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
> > > vma_assert_write_locked(vma);
> > > }
> > >
> > > +static inline void vma_assert_attached(struct vm_area_struct *vma)
> > > +{
> > > + VM_BUG_ON_VMA(vma->detached, vma);
> > > +}
> > > +
> > > +static inline void vma_assert_detached(struct vm_area_struct *vma)
> > > +{
> > > + VM_BUG_ON_VMA(!vma->detached, vma);
> > > +}
> > > +
> > > static inline void vma_mark_attached(struct vm_area_struct *vma)
> > > {
> > > vma->detached = false;
> > > @@ -866,6 +876,8 @@ static inline void vma_end_read(struct vm_area_struct *vma) {}
> > > static inline void vma_start_write(struct vm_area_struct *vma) {}
> > > static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> > > { mmap_assert_write_locked(vma->vm_mm); }
> > > +static inline void vma_assert_attached(struct vm_area_struct *vma) {}
> > > +static inline void vma_assert_detached(struct vm_area_struct *vma) {}
> > > static inline void vma_mark_attached(struct vm_area_struct *vma) {}
> > > static inline void vma_mark_detached(struct vm_area_struct *vma) {}
> > >
> > > diff --git a/mm/vma.c b/mm/vma.c
> > > index d603494e69d7..b9cf552e120c 100644
> > > --- a/mm/vma.c
> > > +++ b/mm/vma.c
> > > @@ -660,14 +660,14 @@ static int commit_merge(struct vma_merge_struct *vmg,
> > > vma_set_range(vmg->vma, vmg->start, vmg->end, vmg->pgoff);
> > >
> > > if (expanded)
> > > - vma_iter_store(vmg->vmi, vmg->vma);
> > > + vma_iter_store_attached(vmg->vmi, vmg->vma);
> > >
> > > if (adj_start) {
> > > adjust->vm_start += adj_start;
> > > adjust->vm_pgoff += PHYS_PFN(adj_start);
> > > if (adj_start < 0) {
> > > WARN_ON(expanded);
> > > - vma_iter_store(vmg->vmi, adjust);
> > > + vma_iter_store_attached(vmg->vmi, adjust);
> > > }
> > > }
> >
> > I kind of feel this whole function (that yes, I added :>) though derived
> > from existing logic) needs rework, as it's necessarily rather confusing.
> >
> > But hey, that's on me :)
> >
> > But this does look right... OK see this as a note-to-self...
> >
> > >
> > > @@ -2845,7 +2845,7 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
> > > anon_vma_interval_tree_pre_update_vma(vma);
> > > vma->vm_end = address;
> > > /* Overwrite old entry in mtree. */
> > > - vma_iter_store(&vmi, vma);
> > > + vma_iter_store_attached(&vmi, vma);
> > > anon_vma_interval_tree_post_update_vma(vma);
> > >
> > > perf_event_mmap(vma);
> > > @@ -2925,7 +2925,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> > > vma->vm_start = address;
> > > vma->vm_pgoff -= grow;
> > > /* Overwrite old entry in mtree. */
> > > - vma_iter_store(&vmi, vma);
> > > + vma_iter_store_attached(&vmi, vma);
> > > anon_vma_interval_tree_post_update_vma(vma);
> > >
> > > perf_event_mmap(vma);
> > > diff --git a/mm/vma.h b/mm/vma.h
> > > index 2a2668de8d2c..63dd38d5230c 100644
> > > --- a/mm/vma.h
> > > +++ b/mm/vma.h
> > > @@ -365,9 +365,10 @@ static inline struct vm_area_struct *vma_iter_load(struct vma_iterator *vmi)
> > > }
> > >
> > > /* Store a VMA with preallocated memory */
> > > -static inline void vma_iter_store(struct vma_iterator *vmi,
> > > - struct vm_area_struct *vma)
> > > +static inline void vma_iter_store_attached(struct vma_iterator *vmi,
> > > + struct vm_area_struct *vma)
> > > {
> > > + vma_assert_attached(vma);
> > >
> > > #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
> > > if (MAS_WARN_ON(&vmi->mas, vmi->mas.status != ma_start &&
> > > @@ -390,7 +391,13 @@ static inline void vma_iter_store(struct vma_iterator *vmi,
> > >
> > > __mas_set_range(&vmi->mas, vma->vm_start, vma->vm_end - 1);
> > > mas_store_prealloc(&vmi->mas, vma);
> > > +}
> > > +
> > > +static inline void vma_iter_store(struct vma_iterator *vmi,
> > > + struct vm_area_struct *vma)
> > > +{
> > > vma_mark_attached(vma);
> > > + vma_iter_store_attached(vmi, vma);
> > > }
> > >
> >
> > See comment at top, and we need some comments here to explain why we're
> > going to pains to do this.
>
> Ack. I'll amend the patch description to make that clear.
>
> >
> > What about mm/nommu.c? I guess these cases are always new VMAs.
>
> CONFIG_PER_VMA_LOCK depends on !CONFIG_NOMMU, so for nommu case all
> these attach/detach functions become NOPs.
>
> >
> > We probably definitely need to check this series in a nommu setup, have you
> > done this? As I can see this breaking things. Then again I suppose you'd
> > have expected bots to moan by now...
> >
> > > static inline unsigned long vma_iter_addr(struct vma_iterator *vmi)
> > > --
> > > 2.47.1.613.gc27f4b7a9f-goog
> > >
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