From: Christoph Hellwig <hch@lst.de>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: Sagi Grimberg <sagi@grimberg.me>,
Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>,
Christoph Hellwig <hch@lst.de>,
linux-nvme@lists.infradead.org,
Douglas Gilbert <dgilbert@interlog.com>
Subject: Re: [PATCH] nvmet-passthru: Cleanup nvmet_passthru_map_sg()
Date: Fri, 16 Oct 2020 15:57:59 +0200 [thread overview]
Message-ID: <20201016135759.GA4685@lst.de> (raw)
In-Reply-To: <b798e7f6-0afc-551c-f6d9-f7900b400137@deltatee.com>
On Thu, Oct 15, 2020 at 12:40:52PM -0600, Logan Gunthorpe wrote:
> > But those are entirely unrelated to the bio size. BIO_MAX_PAGES is
> > 256, so with 4k pages and assuming none can't be merged that is 1MB,
> > while max_segments/max_hw_sectors could be something much larger.
>
> Isn't it constrained by max_segments which is set to NVME_MAX_SEGS=127
> (for PCI)... less than BIO_MAX_PAGES...
nvmet-passthrough is not limited to work on PCIe controllers. And even
if it did relying on an implicit limit like that is a time bomb. I
have work pending for one of the next merge windows to lift that limit
for example, and I would not have thought that nvmet-passthrough
relies on it in any way.
> Would the NVME driver even work if max_segments was greater than
> BIO_MAX_PAGES? Correct me if I'm wrong, but it looks like
> blk_rq_map_sg() will only map one bio within a request. So there has to
> be one bio per request by the time it hits nvme_map_data()...
blk_rq_map_sg maps the bio chain. Take a look at the for_each_bio
loop in __blk_bios_map_sg.
> If you want me to send a patch to future proof the MDTS limit with
> BIO_MAX_PAGES, I can do that, but it doesn't look like it will have any
> effect right now unless big things change.
Yes, please do.
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next prev parent reply other threads:[~2020-10-16 13:58 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-09 23:18 [PATCH] nvmet-passthru: Cleanup nvmet_passthru_map_sg() Logan Gunthorpe
2020-10-13 22:26 ` Sagi Grimberg
2020-10-13 22:38 ` Douglas Gilbert
2020-10-14 0:16 ` Chaitanya Kulkarni
2020-10-14 0:20 ` Logan Gunthorpe
2020-10-14 0:25 ` Chaitanya Kulkarni
2020-10-14 15:47 ` Logan Gunthorpe
2020-10-15 7:56 ` Christoph Hellwig
2020-10-15 16:01 ` Logan Gunthorpe
2020-10-15 17:24 ` Douglas Gilbert
2020-10-15 18:01 ` Christoph Hellwig
2020-10-15 18:40 ` Logan Gunthorpe
2020-10-16 13:57 ` Christoph Hellwig [this message]
2020-10-16 16:49 ` Logan Gunthorpe
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