On Tue, Nov 23, 2021 at 10:18:27AM +0000, David Laight wrote:

From: Vlastimil Babka

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Sent: 22 November 2021 10:46

On 11/22/21 11:36, Christoph Lameter wrote:

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On Mon, 22 Nov 2021, Vlastimil Babka wrote:

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But it seems there's no reason we couldn't do better? I.e. use the value of
SLOB_HDR_SIZE only to align the beginning of actual object (and name the
define different than SLOB_HDR_SIZE). But the size of the header, where we
store the object lenght could be just a native word - 4 bytes on 32bit, 8 on
64bit. The address of the header shouldn't have a reason to be also aligned
to ARCH_KMALLOC_MINALIGN / ARCH_SLAB_MINALIGN as only SLOB itself processes
it and not the slab consumers which rely on those alignments?

Well the best way would be to put it at the end of the object in order to
avoid the alignment problem. This is a particular issue with SLOB because
it allows multiple types of objects in a single page frame.

If only one type of object would be allowed then the object size etc can
be stored in the page struct.

Or just a single byte that is the index of the associated free list structure.
For 32bit and for the smaller kmalloc() area it may be reasonable to have
a separate array indexed by the page of the address.

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So I guess placement at the beginning cannot be avoided. That in turn runs
into trouble with the DMA requirements on some platforms where the
beginning of the object has to be cache line aligned.

It's no problem to have the real beginning of the object aligned, and the
prepended header not.

I'm not sure that helps.
The header can't share a cache line with the previous item (because it
might be mapped for DMA) so will always take a full cache line.

I thought that DMA API allocates buffers that are larger than page size.
DMA pool seems to be able to give out smaller buffers, but underneath it
seems to be calling page allocator.
The SLOB objects that have this header are all less than page size, and
they cannot end up in DMA code paths, or can they?

There might me some strange scheme where you put the size at the end
and the offset of the 'last end' into the page struct.
The DMA API should let you safely read the size from an allocated
buffer - but you can't modify it.

There is also all the code that allocates 'power of 2' sized buffers
under the assumption they are efficient - as soon as you add a size
field that assumption just causes the sizes of item to (often) double.

	David

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The code already does that before this patch for the
kmalloc power-of-two alignments, where e.g. the object can be aligned to 256
bytes, but the prepended header to a smaller ARCH_KMALLOC_MINALIGN /
ARCH_SLAB_MINALIGN.

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I dont know but it seems that making slob that sophisticated is counter
productive. Remove SLOB?

I wouldn't mind, but somebody might :)

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