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Date: Fri, 23 Dec 2022 17:42:30 +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 v3 1/3] mm: vmalloc: Avoid calling __find_vmap_area()
twice in __vunmap()
On Thu, Dec 22, 2022 at 08:06:11PM +0000, Lorenzo Stoakes wrote:
> n Thu, Dec 22, 2022 at 08:00:20PM +0100, Uladzislau Rezki (Sony) wrote:
> > 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.
> >
> > 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 removes without searching.
> >
> > 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 the already detached VA is
> > passed as a parameter, so there is no need to find it again.
> >
> > 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.
> >
> > v2 -> v3:
> > - update commit message;
> > - rename the vm_remove_mappings() to the va_remove_mappings();
> > - move va-unlinking to the callers so the free_vmap_area_noflush()
> > now expects a VA that has been disconnected;
> > - eliminate a local variable in the remove_vm_area().
> >
> > Reported-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> > ---
> > mm/vmalloc.c | 77 ++++++++++++++++++++++++++++++++--------------------
> > 1 file changed, 47 insertions(+), 30 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 9e30f0b39203..eb91ecaa7277 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1815,9 +1815,9 @@ static void drain_vmap_area_work(struct work_struct *work)
> > }
> >
> > /*
> > - * Free a vmap area, caller ensuring that the area has been unmapped
> > - * and flush_cache_vunmap had been called for the correct range
> > - * previously.
> > + * Free a vmap area, caller ensuring that the area has been unmapped,
> > + * unlinked and flush_cache_vunmap had been called for the correct
> > + * range previously.
> > */
> > static void free_vmap_area_noflush(struct vmap_area *va)
> > {
> > @@ -1825,9 +1825,8 @@ 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 (WARN_ON_ONCE(!list_empty(&va->list)))
> > + return;
> >
> > nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
> > PAGE_SHIFT, &vmap_lazy_nr);
> > @@ -1871,6 +1870,19 @@ 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 ***/
> >
> > /*
> > @@ -2015,6 +2027,10 @@ static void free_vmap_block(struct vmap_block *vb)
> > tmp = xa_erase(&vmap_blocks, addr_to_vb_idx(vb->va->va_start));
> > BUG_ON(tmp != vb);
> >
> > + spin_lock(&vmap_area_lock);
> > + unlink_va(vb->va, &vmap_area_root);
> > + spin_unlock(&vmap_area_lock);
> > +
> > free_vmap_area_noflush(vb->va);
> > kfree_rcu(vb, rcu_head);
> > }
> > @@ -2591,6 +2607,20 @@ 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
> > @@ -2603,26 +2633,10 @@ struct vm_struct *find_vm_area(const void *addr)
> > */
> > struct vm_struct *remove_vm_area(const void *addr)
> > {
> > - struct vmap_area *va;
> > -
> > 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;
> > + return __remove_vm_area(
> > + find_unlink_vmap_area((unsigned long) addr));
> > }
> >
> > static inline void set_area_direct_map(const struct vm_struct *area,
> > @@ -2636,16 +2650,17 @@ static inline void set_area_direct_map(const struct vm_struct *area,
> > set_direct_map(area->pages[i]);
> > }
> >
> > -/* Handle removing and resetting vm mappings related to the vm_struct. */
> > -static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
> > +/* Handle removing and resetting vm mappings related to the VA's vm_struct. */
> > +static void va_remove_mappings(struct vmap_area *va, int deallocate_pages)
> > {
> > + 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 +2705,7 @@ 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;
> > + struct vmap_area *va;
> >
> > if (!addr)
> > return;
> > @@ -2698,19 +2714,20 @@ 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);
> > + va_remove_mappings(va, deallocate_pages);
> >
> > if (deallocate_pages) {
> > int i;
> > --
> > 2.30.2
> >
>
> All looks good to me! Great work! Feel free to add the below to all patches in series:-
>
> Reviewed-by: Lorenzo Stoakes <lstoakes@...il.com>
>
Added :)
Thank you for review!
--
Uladzislau Rezki
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