Re: [RFC 0/7] Introduce swiotlb throttling

[Christoph Hellwig <hch@lst.de>]

On Thu, Aug 22, 2024 at 11:37:11AM -0700, mhkelley58@gmail.com wrote:
> Because it's not possible to detect at runtime whether a DMA map call
> is made in a context that can block, the calls in key device drivers
> must be updated with a MAY_BLOCK attribute, if appropriate. When this
> attribute is set and swiotlb memory usage is above a threshold, the
> swiotlb allocation code can serialize swiotlb memory usage to help
> ensure that it is not exhausted.

One thing I've been doing for a while but haven't gotten to due to
my lack of semantic patching skills is that we really want to split
the few flags useful for dma_map* from DMA_ATTR_* which largely
only applies to dma_alloc.

Only DMA_ATTR_WEAK_ORDERING (if we can't just kill it entirely)
and for now DMA_ATTR_NO_WARN is used for both.

DMA_ATTR_SKIP_CPU_SYNC and your new SLEEP/BLOCK attribute is only
useful for mapping, and the rest is for allocation only.

So I'd love to move to a DMA_MAP_* namespace for the mapping flags
before adding more on potentially widely used ones.

With a little grace period we can then also phase out DMA_ATTR_NO_WARN
for allocations, as the gfp_t can control that much better.

> In general, storage device drivers can take advantage of the MAY_BLOCK
> option, while network device drivers cannot. The Linux block layer
> already allows storage requests to block when the BLK_MQ_F_BLOCKING
> flag is present on the request queue.

Note that this also in general involves changes to the block drivers
to set that flag, which is a bit annoying, but I guess there is not
easy way around it without paying the price for the BLK_MQ_F_BLOCKING
overhead everywhere.