Mark Lord wrote:
We had a discussion here today about IOMMUs,
and they *never* split sg list entries -- they only ever *merge*.
And this happens only after the block layer has
already done merging while respecting q->seg_boundary_mask.
So worst case, the IOMMU may merge everything, and then in
libata we unmerge them again. But the end result can never
exceed the max_sg_entries limit enforced by the block layer.
<shrug> Early experience said otherwise. The split in foo_fill_sg()
and resulting sg_tablesize reduction were both needed to successfully
transfer data, when Ben H originally did the work.
If Ben H and everyone on the arch side agrees with the above analysis, I
would be quite happy to remove all those "/ 2".
This can cost a lot of memory, as using NCQ effectively multiplies
everything by 32..
I recommend dialing down the hyperbole a bit :)
"a lot" in this case is... maybe another page or two per table, if
that. Compared with everything else in the system going on, with
16-byte S/G entries, S/G table size is really the least of our worries.
If you were truly concerned about memory usage in sata_mv, a more
effective route is simply reducing MV_MAX_SG_CT to a number closer to
the average s/g table size -- which is far, far lower than 256
(currently MV_MAX_SG_CT), or even 128 (MV_MAX_SG_CT/2).
Or moving to a scheme where you allocate (for example) S/G tables with
32 entries... then allocate on the fly for the rare case where the S/G
table must be larger.
Memory usage is simply not an effective argument in this case. Safety
and correctness are far more paramount.
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