linux-kernel - Re: [PATCH] ib_srpt: use kvmalloc to allocate ring pointers

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Message-ID: <5196ea6e33825dd5fde58d23b38630afafcb77f1.camel@wdc.com>
Date:   Tue, 10 Jul 2018 15:08:38 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "leon@...nel.org" <leon@...nel.org>,
        "jan.dakinevich@...tuozzo.com" <jan.dakinevich@...tuozzo.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "den@...tuozzo.com" <den@...tuozzo.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "khorenko@...tuozzo.com" <khorenko@...tuozzo.com>
Subject: Re: [PATCH] ib_srpt: use kvmalloc to allocate ring pointers

On Tue, 2018-07-10 at 08:39 +0300, Leon Romanovsky wrote:
> On Mon, Jul 09, 2018 at 04:51:08PM +0300, Jan Dakinevich wrote:
> > An array of pointers to SRPT contexts in ib_device is over 30KiB even
> > in default case, in which an amount of contexts is 4095. The patch
> > is intended to weed out large contigous allocation for non-DMA memory.
> 
> kvmalloc* doesn't "weed out" large contiguous allocations, but tries to
> allocate them and gracefully fallback to vmalloc if kmalloc fails.
> 
> More on that, for allocations less than page (64KB for PowerPC), it will
> not call to vmalloc at all, which is fine too, because pages granularity.
> 
> If you want to get rid of "contiguous allocations", use vmalloc instead.

vmalloc() has a very significant disadvantage compared to kvmalloc(): with
vmalloc() it is guaranteed that memory allocations will be slow. That's not
the case for kvmalloc() if the number of bytes that is allocated is small
enough.

Bart.