linux-kernel - Re: [PATCH 1/2] nvme-hwmon: Cache-line-align the NVME SMART log-buffer

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Message-ID: <20220910123542.tzxg2blegw55z5fj@mobilestation>
Date:   Sat, 10 Sep 2022 15:35:42 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        Jonathan Derrick <jonathan.derrick@...el.com>,
        Revanth Rajashekar <revanth.rajashekar@...el.com>,
        Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
        Jens Axboe <axboe@...com>, Sagi Grimberg <sagi@...mberg.me>,
        Guenter Roeck <linux@...ck-us.net>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] nvme-hwmon: Cache-line-align the NVME SMART
 log-buffer

On Sat, Sep 10, 2022 at 07:30:45AM +0200, Christoph Hellwig wrote:
> I think this will work, but unless we have to I'd generally prefer
> to just split dta that is DMAed into into a separate allocation.
> That is, do a separate kmalloc for the nvme_smart_log structure.

Well, both approaches will solve the denoted problem. I am just
wondering why do you think that the kmalloc-ed buffer is more
preferable? IMO it is a bit less suitable since increases the memory
granularity - two kmalloc's instead of one. Moreover it makes the code
a bit more complex for the same reason of having two mallocs and two
frees. Meanwhile using the ____cacheline_aligned qualifier to prevent
the noncoherent DMA problem is a standard approach.

What would be the best solution if we had a qualifier like this:
#ifdef CONFIG_DMA_NONCOHERENT
#define ____dma_buffer ____cacheline_aligned
#else
#define ____dma_buffer
#endif
and used it instead of the direct ____cacheline_aligned utilization.

-Sergey

> 
> Guenter, is this ok with you?