linux-kernel - Re: [PATCH 1/3] block: loop: use kmap(page) instead of page

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Message-ID: <CACVXFVMcC4G5s0-JT1Z8APup18MKwGAkN7qyubxDW-ByTVxtRA@mail.gmail.com>
Date:	Tue, 24 Mar 2015 18:49:00 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Maxim Patlasov <mpatlasov@...allels.com>
Subject: Re: [PATCH 1/3] block: loop: use kmap(page) instead of page_address(page)

On Tue, Mar 24, 2015 at 6:29 PM, Christoph Hellwig <hch@....de> wrote:
> On Sun, Mar 22, 2015 at 04:14:52PM +0800, Ming Lei wrote:
>> The raw page allocated in lo_send() can be a highmem page,
>> so kmap(page) should be used for read/write on the page.
>>
>> Also the patch removes kmap()/kunmap() in lo_send() because
>> it isn't needed at all.
>
> The description seems odd, what it seems to do is to shift the
> kmap from lo_send into do_lo_send_write.  This does not seem

Looks I ignore the fact which kmap() can setup page_address(),
so this change isn't needed, please ignore the patch.

> very useful as it now means a kmap per segment instead of just
> a single one per I/O.  What would make more sense is to just
> not allocate a highmem page.

On 32bit system, this still may cause pressure on page allocation, since
requests are handled concurrently now after using workqueue.

But it is a good idea to keep kmap() per segment, and will do that in v2.

Thanks,
Ming Lei
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