Hi, Chris.
Is it made on purpose not to drop VM_LAZY_FREE flag in
__purge_vmap_area_lazy()? With your patch va->flags
will have two bits set: VM_LAZY_FREE | VM_LAZY_FREEING.
Seems it is not that bad, because all other code paths
do not care, but still the change is not clear.
Also, did you consider to avoid taking static purge_lock
in __purge_vmap_area_lazy() ? Because, with your change
it seems that you can avoid taking this lock at all.
Just be careful when you observe llist as empty, i.e.
nr == 0.
And one comment is below:
On Tue, Apr 12, 2016 at 8:57 AM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
> When mixing lots of vmallocs and set_memory_*() (which calls
> vm_unmap_aliases()) I encountered situations where the performance
> degraded severely due to the walking of the entire vmap_area list each
> invocation. One simple improvement is to add the lazily freed vmap_area
> to a separate lockless free list, such that we then avoid having to walk
> the full list on each purge.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Roman Pen <r.peniaev@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Toshi Kani <toshi.kani@xxxxxx>
> Cc: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> include/linux/vmalloc.h | 3 ++-
> mm/vmalloc.c | 29 ++++++++++++++---------------
> 2 files changed, 16 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 8b51df3ab334..3d9d786a943c 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -4,6 +4,7 @@
> #include <linux/spinlock.h>
> #include <linux/init.h>
> #include <linux/list.h>
> +#include <linux/llist.h>
> #include <asm/page.h> /* pgprot_t */
> #include <linux/rbtree.h>
>
> @@ -45,7 +46,7 @@ struct vmap_area {
> unsigned long flags;
> struct rb_node rb_node; /* address sorted rbtree */
> struct list_head list; /* address sorted list */
> - struct list_head purge_list; /* "lazy purge" list */
> + struct llist_node purge_list; /* "lazy purge" list */
> struct vm_struct *vm;
> struct rcu_head rcu_head;
> };
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 293889d7f482..5388bf64dc32 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -21,6 +21,7 @@
> #include <linux/debugobjects.h>
> #include <linux/kallsyms.h>
> #include <linux/list.h>
> +#include <linux/llist.h>
> #include <linux/notifier.h>
> #include <linux/rbtree.h>
> #include <linux/radix-tree.h>
> @@ -282,6 +283,7 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
> static DEFINE_SPINLOCK(vmap_area_lock);
> /* Export for kexec only */
> LIST_HEAD(vmap_area_list);
> +static LLIST_HEAD(vmap_purge_list);
> static struct rb_root vmap_area_root = RB_ROOT;
>
> /* The vmap cache globals are protected by vmap_area_lock */
> @@ -628,7 +630,7 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> int sync, int force_flush)
> {
> static DEFINE_SPINLOCK(purge_lock);
> - LIST_HEAD(valist);
> + struct llist_node *valist;
> struct vmap_area *va;
> struct vmap_area *n_va;
> int nr = 0;
> @@ -647,20 +649,15 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
> if (sync)
> purge_fragmented_blocks_allcpus();
>
> - rcu_read_lock();
> - list_for_each_entry_rcu(va, &vmap_area_list, list) {
> - if (va->flags & VM_LAZY_FREE) {
> - if (va->va_start < *start)
> - *start = va->va_start;
> - if (va->va_end > *end)
> - *end = va->va_end;
> - nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
> - list_add_tail(&va->purge_list, &valist);
> - va->flags |= VM_LAZY_FREEING;
> - va->flags &= ~VM_LAZY_FREE;
> - }
> + valist = llist_del_all(&vmap_purge_list);
> + llist_for_each_entry(va, valist, purge_list) {
> + if (va->va_start < *start)
> + *start = va->va_start;
> + if (va->va_end > *end)
> + *end = va->va_end;
> + nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
> + va->flags |= VM_LAZY_FREEING;
> }
> - rcu_read_unlock();
>
> if (nr)
> atomic_sub(nr, &vmap_lazy_nr);
> @@ -670,7 +667,7 @@ static void __purge_vmap_area_lazy(unsigned long *start, unsigned long *end,
>
> if (nr) {
> spin_lock(&vmap_area_lock);
> - list_for_each_entry_safe(va, n_va, &valist, purge_list)
> + llist_for_each_entry_safe(va, n_va, valist, purge_list)
> __free_vmap_area(va);
> spin_unlock(&vmap_area_lock);
> }
> @@ -706,6 +703,8 @@ static void purge_vmap_area_lazy(void)
> static void free_vmap_area_noflush(struct vmap_area *va)
> {
> va->flags |= VM_LAZY_FREE;
> + llist_add(&va->purge_list, &vmap_purge_list);
> +
> atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
it seems to me that this a very long-standing problem: when you mark
va->flags as VM_LAZY_FREE, va can be immediately freed from another CPU.
If so, the line:
atomic_add((va->va_end - va->va_start)....
does use-after-free access.
So I would also fix it with careful line reordering with barrier:
(probably barrier is excess here, because llist_add implies cmpxchg,
but I simply want to be explicit here, showing that marking va as
VM_LAZY_FREE and adding it to the list should be at the end)
- va->flags |= VM_LAZY_FREE;
atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
+ smp_mb__after_atomic();
+ va->flags |= VM_LAZY_FREE;
+ llist_add(&va->purge_list, &vmap_purge_list);
What do you think?
--
Roman
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