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SubjectRe: [PATCH] mm: refactor __vunmap() to avoid duplicated call to find_vm_area()
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On Wed, May 15, 2019 at 01:11:46PM +0530, Anshuman Khandual wrote:
>
>
> On 05/15/2019 05:21 AM, Roman Gushchin wrote:
> > __vunmap() calls find_vm_area() twice without an obvious reason:
> > first directly to get the area pointer, second indirectly by calling
> > vm_remove_mappings()->remove_vm_area(), which is again searching
> > for the area.
> >
> > To remove this redundancy, let's split remove_vm_area() into
> > __remove_vm_area(struct vmap_area *), which performs the actual area
> > removal, and remove_vm_area(const void *addr) wrapper, which can
> > be used everywhere, where it has been used before. Let's pass
> > a pointer to the vm_area instead of vm_struct to vm_remove_mappings(),
> > so it can pass it to __remove_vm_area() and avoid the redundant area
> > lookup.
> >
> > On my test setup, I've got 5-10% speed up on vfree()'ing 1000000
> > of 4-pages vmalloc blocks.
> >
> > Perf report before:
> > 29.44% cat [kernel.kallsyms] [k] free_unref_page
> > 11.88% cat [kernel.kallsyms] [k] find_vmap_area
> > 9.28% cat [kernel.kallsyms] [k] __free_pages
> > 7.44% cat [kernel.kallsyms] [k] __slab_free
> > 7.28% cat [kernel.kallsyms] [k] vunmap_page_range
> > 4.56% cat [kernel.kallsyms] [k] __vunmap
> > 3.64% cat [kernel.kallsyms] [k] __purge_vmap_area_lazy
> > 3.04% cat [kernel.kallsyms] [k] __free_vmap_area
> >
> > Perf report after:
> > 32.41% cat [kernel.kallsyms] [k] free_unref_page
> > 7.79% cat [kernel.kallsyms] [k] find_vmap_area
> > 7.40% cat [kernel.kallsyms] [k] __slab_free
> > 7.31% cat [kernel.kallsyms] [k] vunmap_page_range
> > 6.84% cat [kernel.kallsyms] [k] __free_pages
> > 6.01% cat [kernel.kallsyms] [k] __vunmap
> > 3.98% cat [kernel.kallsyms] [k] smp_call_function_single
> > 3.81% cat [kernel.kallsyms] [k] __purge_vmap_area_lazy
> > 2.77% cat [kernel.kallsyms] [k] __free_vmap_area
> >
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > ---
> > mm/vmalloc.c | 52 +++++++++++++++++++++++++++++-----------------------
> > 1 file changed, 29 insertions(+), 23 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index c42872ed82ac..8d4907865614 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2075,6 +2075,22 @@ struct vm_struct *find_vm_area(const void *addr)
> > return NULL;
> > }
> >
> > +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> > +{
> > + struct vm_struct *vm = va->vm;
> > +
> > + spin_lock(&vmap_area_lock);
> > + va->vm = NULL;
> > + va->flags &= ~VM_VM_AREA;
> > + va->flags |= VM_LAZY_FREE;
> > + spin_unlock(&vmap_area_lock);
> > +
> > + kasan_free_shadow(vm);
> > + free_unmap_vmap_area(va);
> > +
> > + return vm;
> > +}
> > +
> > /**
> > * remove_vm_area - find and remove a continuous kernel virtual area
> > * @addr: base address
> > @@ -2087,26 +2103,14 @@ struct vm_struct *find_vm_area(const void *addr)
> > */
> > struct vm_struct *remove_vm_area(const void *addr)
> > {
> > + struct vm_struct *vm = NULL;
> > struct vmap_area *va;
> >
> > - might_sleep();
>
> Is not this necessary any more ?

We've discussed it here: https://lkml.org/lkml/2019/4/17/1098 .
Tl;dr it's not that useful.

>
> > -
> > va = find_vmap_area((unsigned long)addr);
> > - if (va && va->flags & VM_VM_AREA) {
> > - struct vm_struct *vm = va->vm;
> > -
> > - spin_lock(&vmap_area_lock);
> > - va->vm = NULL;
> > - va->flags &= ~VM_VM_AREA;
> > - va->flags |= VM_LAZY_FREE;
> > - spin_unlock(&vmap_area_lock);
> > -
> > - kasan_free_shadow(vm);
> > - free_unmap_vmap_area(va);
> > + if (va && va->flags & VM_VM_AREA)
> > + vm = __remove_vm_area(va);
> >
> > - return vm;
> > - }
> > - return NULL;
> > + return vm;
> > }
>
> Other callers of remove_vm_area() cannot use __remove_vm_area() directly as well
> to save a look up ?
>

I'll take a look. Good idea, thanks!

Roman

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