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Message-ID: <z4y455g6vrx5elvtoujht4zexuwoil4voiojs4cbg6ypkrwdpg@zevhspkgnpoe>
Date: Wed, 21 Aug 2024 09:17:56 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
sidhartha.kumar@...cle.com, Bert Karwatzki <spasswolf@....de>,
Jiri Olsa <olsajiri@...il.com>, Kees Cook <kees@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v6 06/20] mm/vma: Change munmap to use
vma_munmap_struct() for accounting and surrounding vmas
* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [240821 05:59]:
> On Tue, Aug 20, 2024 at 07:57:15PM GMT, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> >
> > Clean up the code by changing the munmap operation to use a structure
> > for the accounting and munmap variables.
> >
> > Since remove_mt() is only called in one location and the contents will
> > be reduced to almost nothing. The remains of the function can be added
> > to vms_complete_munmap_vmas().
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> > ---
> > mm/vma.c | 79 ++++++++++++++++++++++++++++----------------------------
> > mm/vma.h | 6 +++++
> > 2 files changed, 46 insertions(+), 39 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index e1aee43a3dc4..7b8b8b983399 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
>
> [snip]
>
> > mm = vms->mm;
> > @@ -731,21 +708,26 @@ static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> > if (vms->unlock)
> > mmap_write_downgrade(mm);
> >
> > - prev = vma_iter_prev_range(vms->vmi);
> > - next = vma_next(vms->vmi);
> > - if (next)
> > - vma_iter_prev_range(vms->vmi);
> > -
> > /*
> > * We can free page tables without write-locking mmap_lock because VMAs
> > * were isolated before we downgraded mmap_lock.
> > */
> > mas_set(mas_detach, 1);
> > - unmap_region(mm, mas_detach, vms->vma, prev, next, vms->start, vms->end,
> > - vms->vma_count, !vms->unlock);
> > - /* Statistics and freeing VMAs */
> > + unmap_region(mm, mas_detach, vms->vma, vms->prev, vms->next,
> > + vms->start, vms->end, vms->vma_count, !vms->unlock);
> > + /* Update high watermark before we lower total_vm */
> > + update_hiwater_vm(mm);
> > + /* Stat accounting */
> > + WRITE_ONCE(mm->total_vm, READ_ONCE(mm->total_vm) - vms->nr_pages);
> > + mm->exec_vm -= vms->exec_vm;
> > + mm->stack_vm -= vms->stack_vm;
> > + mm->data_vm -= vms->data_vm;
>
> See below, but I bisected a bug to this patch that manifested because of
> miscalculated accounting. So I wonder whether it'd be a good idea to take
> this opportunity, when updating mm->... stats to add some:
>
> VM_WARN_ON(vms->exec_vm > mm->exec_vm);
>
> etc. for each of the fields updated. This would help catch any accounting
> issues like this with CONFIG_DEBUG_VM switched on.
Sounds good.
>
> [snip]
>
> > @@ -824,6 +807,22 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> > goto munmap_gather_failed;
> >
> > vma_mark_detached(next, true);
> > + nrpages = vma_pages(next);
> > +
> > + vms->nr_pages += nrpages;
> > + if (next->vm_flags & VM_LOCKED)
> > + vms->locked_vm += nrpages;
>
> So I bisected a self-test failure, memfd_secret specifically, to this
> commit. This is because you are double-counting VM_LOCKED...
>
> > +
> > + if (next->vm_flags & VM_ACCOUNT)
> > + vms->nr_accounted += nrpages;
> > +
> > + if (is_exec_mapping(next->vm_flags))
> > + vms->exec_vm += nrpages;
> > + else if (is_stack_mapping(next->vm_flags))
> > + vms->stack_vm += nrpages;
> > + else if (is_data_mapping(next->vm_flags))
> > + vms->data_vm += nrpages;
> > +
> > if (next->vm_flags & VM_LOCKED)
> > vms->locked_vm += vma_pages(next);
>
> ...the double counting being right here :) so I think we should drop the
> above couple lines.
Yeah, sure.
Thanks for reviewing!
>
> >
> > @@ -847,7 +846,9 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> > BUG_ON(next->vm_start < vms->start);
> > BUG_ON(next->vm_start > vms->end);
> > #endif
> > - } for_each_vma_range(*(vms->vmi), next, vms->end);
> > + }
> > +
> > + vms->next = vma_next(vms->vmi);
> >
> > #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
> > /* Make sure no VMAs are about to be lost. */
> > diff --git a/mm/vma.h b/mm/vma.h
> > index cb67acf59012..cbf55e0e0c4f 100644
> > --- a/mm/vma.h
> > +++ b/mm/vma.h
> > @@ -33,12 +33,18 @@ struct vma_munmap_struct {
> > struct vma_iterator *vmi;
> > struct mm_struct *mm;
> > struct vm_area_struct *vma; /* The first vma to munmap */
> > + struct vm_area_struct *prev; /* vma before the munmap area */
> > + struct vm_area_struct *next; /* vma after the munmap area */
> > struct list_head *uf; /* Userfaultfd list_head */
> > unsigned long start; /* Aligned start addr (inclusive) */
> > unsigned long end; /* Aligned end addr (exclusive) */
> > int vma_count; /* Number of vmas that will be removed */
> > unsigned long nr_pages; /* Number of pages being removed */
> > unsigned long locked_vm; /* Number of locked pages */
> > + unsigned long nr_accounted; /* Number of VM_ACCOUNT pages */
> > + unsigned long exec_vm;
> > + unsigned long stack_vm;
> > + unsigned long data_vm;
> > bool unlock; /* Unlock after the munmap */
> > };
> >
> > --
> > 2.43.0
> >
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