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scsi_mq_setup_tags() preallocates a big buffer for the IO SGL. The size is based on scsi_mq_sgl_size() which is determined based on shost->sg_tablesize and SG_CHUNK_SIZE. Modern DMA engines are often capable of dealing with very big segments so the resulting scsi_mq_sgl_size() is often too big. SG_CHUNK_SIZE results in a static 4KB SGL allocation per command. If an HBA has lots of deep queues, preallocation for the sg list can consume substantial amounts of memory. For lpfc, nr_hw_queues can be 70 and each queue's depth 3781. This means the resulting preallocation for the data SGL is 70*3781*2K = 517MB. Switch to runtime allocation for SGL for lists longer than 2 entries. This is the approach used by NVMe PCI so it should be reasonable for SCSI as well. Runtime SGL allocation has always been the case for the legacy I/O path so this is nothing new. [mkp: attempted to clarify commit desc] Cc: Christoph Hellwig <hch@lst.de> Cc: Bart Van Assche <bvanassche@acm.org> Cc: Ewan D. Milne <emilne@redhat.com> Cc: Hannes Reinecke <hare@suse.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.