On Wed, Jul 29, 2026 at 03:47:23PM +0530, Anuj Gupta/Anuj Gupta wrote:

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But I also don't understand what the use case for this function is to start with. struct sg_table tells us how many segments exist on the DMA side in the nents member, which should be just fine for the SGL threshold calculation.

sg_table->nents covers the entire exported buffer (<=1GiB), while a request only covers a subrange[bi_offset, bi_offset+payload). Using nents would overcount the request's segments.

Urgg, yes.

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• if (!entries)

• return BLK_STS_IOERR;
  

• if (entries > NVME_MAX_SEGS)

• return BLK_STS_AGAIN;
  

Given that the block layer enforced data in rw/command and the max_segments limit, why do we need the extra check here?

A dmabuf bio reports nsegs=1 (bio_split_io_at) to the block layer, so max_segments isn't enforced against the SG entries actually spanned by the request. Hence the explicit check.

We'll need to expose the actual nsegs to the block layer and split based on that. Otherwise I/O might work or fail based on the device capabilities.