Thanks Matthew!!
On 1/23/2024 6:56 PM, Matthew Wilcox wrote:
>> I am just curious about the purpose of maple node slab objects to be its
>> size aligned, but I can understand why they need to be cache aligned.
> Because we encode various information in the bottom few bits of the
> maple node pointer.
>
> /*
> * The Maple Tree squeezes various bits in at various points which aren't
> * necessarily obvious. Usually, this is done by observing that pointers are
> * N-byte aligned and thus the bottom log_2(N) bits are available for use. We
> * don't use the high bits of pointers to store additional information because
> * we don't know what bits are unused on any given architecture.
> *
> * Nodes are 256 bytes in size and are also aligned to 256 bytes, giving us 8
> * low bits for our own purposes. Nodes are currently of 4 types:
> * 1. Single pointer (Range is 0-0)
> * 2. Non-leaf Allocation Range nodes
> * 3. Non-leaf Range nodes
> * 4. Leaf Range nodes All nodes consist of a number of node slots,
> * pivots, and a parent pointer.
> */
>
I got it. Looks like I need to revisit the maple tree documentation
before asking such questions.
> That seems like a very badly implemented patch. Rather than make all
> objects left & right redzone, we should simply insert a redzone at
> the beginning of the slab. ie
>
> 0 redzone
> 256 node
> 512 redzone
> 768 node
> 1024 redzone
> 1280 node
> [...]
> 3072 redzone
> 3382 node
> 3584 redzone
> 3840 wasted space
>
This seems to work when only redzone is enabled?
I think it will again 768b aligned if any other debug option enabled,
say U. It is:
(size aligned red zone + maple node + right red zone (size of (void*))
+ alloc/free track).
My understanding to have both left and right red zone is:
/*
* Add some empty padding so that __we can catch
* overwrites from earlier objects rather than let
* tracking information or the free pointer be
* corrupted if a user writes before the start
* of the object__.
*/
When all the debug options enabled, the slab object will roughly look
like below:
Left red zone | object | right red zone | free pointer | alloc/free
track | padding
> Instead of getting only five nodes per 4kB page, we'd get seven; about
> a 30% reduction in memory usage.
>
> Slab redzoning is not a feature people turn on often, so I'm not
> surprised nobody's noticed before now.
+Vlastimil. The patch in discussion is d86bd1bece6f ("mm/slub: support
left redzone").
Thanks,
Charan
