Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1

[Uladzislau Rezki]

Hello.

Let me have a look at it.

Vlad Rezki

On Wed, Sep 8, 2021 at 3:27 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> find_vmap_lowest_match() is imprecise such that it won't always
> find "the first free block ... that will accomplish the request" if
> an alignment > 1 was specified: especially also when the alignment is
> PAGE_SIZE. Unfortuantely, the way the vmalloc data structures were
> designed, propagating the max size without alignment information through
> the tree, it's hard to make it precise again when an alignment > 1 is
> specified.
>
> The issue is that in order to be able to eventually align later,
> find_vmap_lowest_match() has to search for a slightly bigger area and
> might skip some applicable subtrees just by lookign at the result of
> get_subtree_max_size(). While this usually doesn't matter, it matters for
> exact allocations as performed by KASAN when onlining a memory block,
> when the free block exactly matches the request.
> (mm/kasn/shadow.c:kasan_mem_notifier()).
>
> In case we online memory blocks out of order (not lowest to highest
> address), find_vmap_lowest_match() with PAGE_SIZE alignment will reject
> an exact allocation if it corresponds exactly to a free block. (there are
> some corner cases where it would still work, if we're lucky and hit the
> first is_within_this_va() inside the while loop)
>
> [root@vm-0 fedora]# echo online > /sys/devices/system/memory/memory82/state
> [root@vm-0 fedora]# echo online > /sys/devices/system/memory/memory83/state
> [root@vm-0 fedora]# echo online > /sys/devices/system/memory/memory85/state
> [root@vm-0 fedora]# echo online > /sys/devices/system/memory/memory84/state
> [  223.858115] vmap allocation for size 16777216 failed: use vmalloc=<size> to increase size
> [  223.859415] bash: vmalloc: allocation failure: 16777216 bytes, mode:0x6000c0(GFP_KERNEL), nodemask=(null),cpuset=/,mems_allowed=0
> [  223.860992] CPU: 4 PID: 1644 Comm: bash Kdump: loaded Not tainted 4.18.0-339.el8.x86_64+debug #1
> [  223.862149] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> [  223.863580] Call Trace:
> [  223.863946]  dump_stack+0x8e/0xd0
> [  223.864420]  warn_alloc.cold.90+0x8a/0x1b2
> [  223.864990]  ? zone_watermark_ok_safe+0x300/0x300
> [  223.865626]  ? slab_free_freelist_hook+0x85/0x1a0
> [  223.866264]  ? __get_vm_area_node+0x240/0x2c0
> [  223.866858]  ? kfree+0xdd/0x570
> [  223.867309]  ? kmem_cache_alloc_node_trace+0x157/0x230
> [  223.868028]  ? notifier_call_chain+0x90/0x160
> [  223.868625]  __vmalloc_node_range+0x465/0x840
> [  223.869230]  ? mark_held_locks+0xb7/0x120
>
> While we could fix this in kasan_mem_notifier() by passing an alignment
> of "1", this is actually an implementation detail of vmalloc and to be
> handled internally.
>
> So use an alignment of 1 when calling find_vmap_lowest_match() for exact
> allocations that are expected to succeed -- if the given range can
> satisfy the alignment requirements.
>
> Fixes: 68ad4a330433 ("mm/vmalloc.c: keep track of free blocks for vmap allocation")
> Reported-by: Ping Fang <pifang@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/vmalloc.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index d5cd52805149..c6071f5f8de3 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1153,7 +1153,8 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
>  /*
>   * Find the first free block(lowest start address) in the tree,
>   * that will accomplish the request corresponding to passing
> - * parameters.
> + * parameters. Note that with an alignment > 1, this function
> + * can be imprecise and skip applicable free blocks.
>   */
>  static __always_inline struct vmap_area *
>  find_vmap_lowest_match(unsigned long size,
> @@ -1396,7 +1397,15 @@ __alloc_vmap_area(unsigned long size, unsigned long align,
>         enum fit_type type;
>         int ret;
>
> -       va = find_vmap_lowest_match(size, align, vstart);
> +       /*
> +        * For exact allocations, ignore the alignment, such that
> +        * find_vmap_lowest_match() won't search for a bigger free area just
> +        * able to align later and consequently fail the search.
> +        */
> +       if (vend - vstart == size && IS_ALIGNED(vstart, align))
> +               va = find_vmap_lowest_match(size, 1, vstart);
> +       else
> +               va = find_vmap_lowest_match(size, align, vstart);
>         if (unlikely(!va))
>                 return vend;
>
>
> base-commit: 7d2a07b769330c34b4deabeed939325c77a7ec2f
> --
> 2.31.1
>