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SubjectRe: [PATCH v10 14/69] kernel/fork: use maple tree for dup_mmap() during forking
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* David Hildenbrand <david@redhat.com> [220623 14:42]:
> On 21.06.22 22:46, Liam Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
> >
> > The maple tree was already tracking VMAs in this function by an earlier
> > commit, but the rbtree iterator was being used to iterate the list.
> > Change the iterator to use a maple tree native iterator and switch to the
> > maple tree advanced API to avoid multiple walks of the tree during insert
> > operations. Unexport the now-unused vma_store() function.
> >
> > For performance reasons we bulk allocate the maple tree nodes. The node
> > calculations are done internally to the tree and use the VMA count and
> > assume the worst-case node requirements. The VM_DONT_COPY flag does not
> > allow for the most efficient copy method of the tree and so a bulk loading
> > algorithm is used.
>
> You could track the #VM_DONT_COPY per mm and optimize? Do you have that
> planned or maybe it already is part of this patch set? :)
>
> >
> > Link: https://lkml.kernel.org/r/20220504010716.661115-16-Liam.Howlett@oracle.com
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: SeongJae Park <sj@kernel.org>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Davidlohr Bueso <dave@stgolabs.net>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > ---
> > include/linux/mm.h | 2 --
> > kernel/fork.c | 12 +++++++++---
> > 2 files changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index f22c6f71a18c..27fa36a7c093 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2612,8 +2612,6 @@ extern bool arch_has_descending_max_zone_pfns(void);
> > /* nommu.c */
> > extern atomic_long_t mmap_pages_allocated;
> > extern int nommu_shrink_inode_mappings(struct inode *, size_t, size_t);
> > -/* mmap.c */
> > -void vma_mas_store(struct vm_area_struct *vma, struct ma_state *mas);
> >
> > /* interval_tree.c */
> > void vma_interval_tree_insert(struct vm_area_struct *node,
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 1840da0732f6..4ecca79556eb 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -583,9 +583,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> > struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
> > struct rb_node **rb_link, *rb_parent;
> > int retval;
> > - unsigned long charge;
> > - LIST_HEAD(uf);
> > + unsigned long charge = 0;
>
> ^ why that change?

mas_for_each() may not be executed - according to the compiler. We know
it will be but this makes the compiler happy.

>
> > + MA_STATE(old_mas, &oldmm->mm_mt, 0, 0);
> > MA_STATE(mas, &mm->mm_mt, 0, 0);
> > + LIST_HEAD(uf);
>
> ^ why that reshuffling?

I'll revert this.

>
> >
> > uprobe_start_dup_mmap();
> > if (mmap_write_lock_killable(oldmm)) {
> > @@ -620,7 +621,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> > goto out;
> >
> > prev = NULL;
> > - for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
> > +
> > + retval = mas_expected_entries(&mas, oldmm->map_count);
> > + if (retval)
> > + goto out;
>
> mas_expected_entries is a fancy function :)
>
>
> > +
> > + mas_for_each(&old_mas, mpnt, ULONG_MAX) {
> > struct file *file;
> >
> > if (mpnt->vm_flags & VM_DONTCOPY) {
>
>
> --
> Thanks,
>
> David / dhildenb
>
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