[lkml]   [2019]   [Mar]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC 0/2] guarantee natural alignment for kmalloc()
On 3/20/19 7:20 PM, Christopher Lameter wrote:
>>> Currently all kmalloc objects are aligned to KMALLOC_MIN_ALIGN. That will
>>> no longer be the case and alignments will become inconsistent.
>> KMALLOC_MIN_ALIGN is still the minimum, but in practice it's larger
>> which is not a problem.
> "In practice" refers to the current way that slab allocators arrange
> objects within the page. They are free to do otherwise if new ideas come
> up for object arrangements etc.
> The slab allocators already may have to store data in addition to the user
> accessible part (f.e. for RCU or ctor). The "natural alighnment" of a
> power of 2 cache is no longer as you expect for these cases. Debugging is
> not the only case where we extend the object.

For plain kmalloc() caches, RCU and ctors don't apply, right.

>> Also let me stress again that nothing really changes except for SLOB,
>> and SLUB with debug options. The natural alignment for power-of-two
>> sizes already happens as SLAB and SLUB both allocate objects starting on
>> the page boundary. So people make assumptions based on that, and then
>> break with SLOB, or SLUB with debug. This patch just prevents that
>> breakage by guaranteeing those natural assumptions at all times.
> As explained before there is nothing "natural" here. Doing so restricts
> future features

Well, future features will have to deal with the existing named caches
created with specific alignment.

> and creates a mess within the allocator of exceptions for
> debuggin etc etc (see what happened to SLAB).

SLAB could be fixed, just nobody cares enough I guess. If I want to
debug wrong SL*B usage I'll use SLUB.

> "Natural" is just a
> simplistic thought of a user how he would arrange power of 2 objects.
> These assumption should not be made but specified explicitly.

Patch 1 does this explicitly for plain kmalloc(). It's unrealistic to
add 'align' parameter to plain kmalloc() as that would have to create
caches on-demand for 'new' values of align parameter.

>>> I think its valuable that alignment requirements need to be explicitly
>>> requested.
>> That's still possible for named caches created by kmem_cache_create().
> So lets leave it as it is now then.

That however doesn't work well for the xfs/IO case where block sizes are
not known in advance:

 \ /
  Last update: 2019-03-21 08:42    [W:0.125 / U:0.052 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site