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Date: Thu, 22 Dec 2022 15:22:01 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Lorenzo Stoakes <lstoakes@...il.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
LKML <linux-kernel@...r.kernel.org>, Baoquan He <bhe@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
Nicholas Piggin <npiggin@...il.com>,
Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH v2 1/3] mm: vmalloc: Avoid of calling __find_vmap_area()
twise in __vunmap()
> Some pedantic grammar/spelling stuff:-
>
> (I know it can be a little annoying to get grammatical suggestions so I do hope
> that it isn't too irritating!)
>
It is absolutely OK :)
>
> For the Subject line:-
> 'mm: vmalloc: Avoid of calling __find_vmap_area() twise in __vunmap()' ->
> 'mm: vmalloc: Avoid calling __find_vmap_area() twice in __vunmap()'
>
Will fix in the v3.
> On Wed, Dec 21, 2022 at 06:44:52PM +0100, Uladzislau Rezki (Sony) wrote:
> > Currently __vunmap() path calls __find_vmap_area() two times. One on
> > entry to check that area exists, second time inside remove_vm_area()
> > function that also performs a new search of VA.
>
> Perhaps slightly tweak to:-
>
> "Currently the __vunmap() path calls __find_vmap_area() twice. Once on entry
> to check that the area exists, then inside the remove_vm_area() function
> which also performs a new search for the VA."
>
Will fix in the v3.
> >
> > In order to improvie it from a performance point of view we split
> > remove_vm_area() into two new parts:
> > - find_unlink_vmap_area() that does a search and unlink from tree;
> > - __remove_vm_area() that does a removing but without searching.
>
> 'that does a removing but without searching' reads better I think as
> 'that removes without searching'.
>
Will fix in the v3.
> >
> > In this case there is no any functional change for remove_vm_area()
> > whereas vm_remove_mappings(), where a second search happens, switches
> > to the __remove_vm_area() variant where already detached VA is passed
> > as a parameter, so there is no need to find it again.
> >
>
> 'where already detached VA' -> 'where the already detached VA' as a minor nit
> here!
>
Will fix it.
> > Performance wise, i use test_vmalloc.sh with 32 threads doing alloc
> > free on a 64-CPUs-x86_64-box:
> >
> > perf without this patch:
> > - 31.41% 0.50% vmalloc_test/10 [kernel.vmlinux] [k] __vunmap
> > - 30.92% __vunmap
> > - 17.67% _raw_spin_lock
> > native_queued_spin_lock_slowpath
> > - 12.33% remove_vm_area
> > - 11.79% free_vmap_area_noflush
> > - 11.18% _raw_spin_lock
> > native_queued_spin_lock_slowpath
> > 0.76% free_unref_page
> >
> > perf with this patch:
> > - 11.35% 0.13% vmalloc_test/14 [kernel.vmlinux] [k] __vunmap
> > - 11.23% __vunmap
> > - 8.28% find_unlink_vmap_area
> > - 7.95% _raw_spin_lock
> > 7.44% native_queued_spin_lock_slowpath
> > - 1.93% free_vmap_area_noflush
> > - 0.56% _raw_spin_lock
> > 0.53% native_queued_spin_lock_slowpath
> > 0.60% __vunmap_range_noflush
> >
> > __vunmap() consumes around ~20% less CPU cycles on this test.
>
> Very nice, amazing work!
>
Thanks!
> >
> > Reported-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> > mm/vmalloc.c | 66 +++++++++++++++++++++++++++++++++-------------------
> > 1 file changed, 42 insertions(+), 24 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 9e30f0b39203..28030d2441f1 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1825,9 +1825,11 @@ static void free_vmap_area_noflush(struct vmap_area *va)
> > unsigned long va_start = va->va_start;
> > unsigned long nr_lazy;
> >
> > - spin_lock(&vmap_area_lock);
> > - unlink_va(va, &vmap_area_root);
> > - spin_unlock(&vmap_area_lock);
> > + if (!list_empty(&va->list)) {
> > + spin_lock(&vmap_area_lock);
> > + unlink_va(va, &vmap_area_root);
> > + spin_unlock(&vmap_area_lock);
> > + }
>
> Do we want to do the same in free_vmap_area()?
>
The free_vmap_area() is a bit special. It only pairs with alloc_vmap_area().
There are two users and both invoke free_vmap_area() in a error path. So probably
it would be good to remove it fully. But it requires some refactoring.
> >
> > nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
> > PAGE_SHIFT, &vmap_lazy_nr);
> > @@ -1871,6 +1873,19 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> > return va;
> > }
> >
> > +static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > +{
> > + struct vmap_area *va;
> > +
> > + spin_lock(&vmap_area_lock);
> > + va = __find_vmap_area(addr, &vmap_area_root);
> > + if (va)
> > + unlink_va(va, &vmap_area_root);
> > + spin_unlock(&vmap_area_lock);
> > +
> > + return va;
> > +}
> > +
> > /*** Per cpu kva allocator ***/
> >
> > /*
> > @@ -2591,6 +2606,20 @@ struct vm_struct *find_vm_area(const void *addr)
> > return va->vm;
> > }
> >
> > +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> > +{
> > + struct vm_struct *vm;
> > +
> > + if (!va || !va->vm)
> > + return NULL;
> > +
> > + vm = va->vm;
> > + kasan_free_module_shadow(vm);
> > + free_unmap_vmap_area(va);
> > +
> > + return vm;
> > +}
> > +
> > /**
> > * remove_vm_area - find and remove a continuous kernel virtual area
> > * @addr: base address
> > @@ -2607,22 +2636,8 @@ struct vm_struct *remove_vm_area(const void *addr)
> >
> > might_sleep();
> >
> > - spin_lock(&vmap_area_lock);
> > - va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> > - if (va && va->vm) {
> > - struct vm_struct *vm = va->vm;
> > -
> > - va->vm = NULL;
> > - spin_unlock(&vmap_area_lock);
> > -
> > - kasan_free_module_shadow(vm);
> > - free_unmap_vmap_area(va);
> > -
> > - return vm;
> > - }
> > -
> > - spin_unlock(&vmap_area_lock);
> > - return NULL;
> > + va = find_unlink_vmap_area((unsigned long) addr);
> > + return __remove_vm_area(va);
> > }
>
> Really nice separation of concerns and cleanup.
>
Thanks!
> >
> > static inline void set_area_direct_map(const struct vm_struct *area,
> > @@ -2637,15 +2652,16 @@ static inline void set_area_direct_map(const struct vm_struct *area,
> > }
> >
> > /* Handle removing and resetting vm mappings related to the vm_struct. */
> > -static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
> > +static void vm_remove_mappings(struct vmap_area *va, int deallocate_pages)
>
> Perhaps rename this to va_remove_mappings() or vmap_area_remove_mappings() since
> it is now explicitly accepting a vmap_area rather than vm_struct?
>
I agree. There is a discrepancy. I can rename it to the va_remove_mappings()
if there are no any complains from others.
> > {
> > + struct vm_struct *area = va->vm;
> > unsigned long start = ULONG_MAX, end = 0;
> > unsigned int page_order = vm_area_page_order(area);
> > int flush_reset = area->flags & VM_FLUSH_RESET_PERMS;
> > int flush_dmap = 0;
> > int i;
> >
> > - remove_vm_area(area->addr);
> > + __remove_vm_area(va);
> >
> > /* If this is not VM_FLUSH_RESET_PERMS memory, no need for the below. */
> > if (!flush_reset)
> > @@ -2690,6 +2706,7 @@ static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
> > static void __vunmap(const void *addr, int deallocate_pages)
> > {
> > struct vm_struct *area;
>
> Feels like it's getting a bit confusing with 'va' representing vmap_area and
> 'area' which represents... vm_struct (this file has a bunch of naming
> inconsistencies like this actually), perhaps rename this to 'vm'?
>
We can. I think it should be a separate patch-set for refactoring of
things like: va, vm, area, vmap, etc :)
> > + struct vmap_area *va;
> >
> > if (!addr)
> > return;
> > @@ -2698,19 +2715,20 @@ static void __vunmap(const void *addr, int deallocate_pages)
> > addr))
> > return;
> >
> > - area = find_vm_area(addr);
> > - if (unlikely(!area)) {
> > + va = find_unlink_vmap_area((unsigned long)addr);
> > + if (unlikely(!va)) {
> > WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
> > addr);
> > return;
> > }
> >
> > + area = va->vm;
> > debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
> > debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
> >
> > kasan_poison_vmalloc(area->addr, get_vm_area_size(area));
> >
> > - vm_remove_mappings(area, deallocate_pages);
> > + vm_remove_mappings(va, deallocate_pages);
> >
> > if (deallocate_pages) {
> > int i;
> > --
> > 2.30.2
> >
>
> Other than some pendatic points about grammar/naming this looks really good!
>
Thank you for the review!
--
Uladzislau Rezki
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