Date:   Fri, 24 Feb 2023 11:24:21 -0700
From:   Keith Busch <kbusch@kernel.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Keith Busch <kbusch@meta.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Subject: Re: [PATCH] dmapool: push new blocks in ascending order
Message-ID: <Y/kA1Tp5wIZSiY4q@kbusch-mbp.dhcp.thefacebook.com>
References: <20230221165400.1595247-1-kbusch@meta.com>
 <Y/UHOohEMg+oyO8j@infradead.org>
 <Y/UIZC+plt9Y0nr1@kbusch-mbp>
 <20230223124137.e6fe921659e6f6f1c10668b6@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230223124137.e6fe921659e6f6f1c10668b6@linux-foundation.org>
Precedence: bulk

On Thu, Feb 23, 2023 at 12:41:37PM -0800, Andrew Morton wrote:
> On Tue, 21 Feb 2023 11:07:32 -0700 Keith Busch <kbusch@kernel.org> wrote:
> 
> > On Tue, Feb 21, 2023 at 10:02:34AM -0800, Christoph Hellwig wrote:
> > > On Tue, Feb 21, 2023 at 08:54:00AM -0800, Keith Busch wrote:
> > > > From: Keith Busch <kbusch@kernel.org>
> > > > 
> > > > Some users of the dmapool need their allocations to happen in ascending
> > > > order. The recent optimizations pushed the blocks in reverse order, so
> > > > restore the previous behavior by linking the next available block from
> > > > low-to-high.
> > > 
> > > Who are those users?
> > > 
> > > Also should we document this behavior somewhere so that it isn't
> > > accidentally changed again some time in the future?
> > 
> > usb/chipidea/udc.c qh_pool called "ci_hw_qh".
> 
> It would be helpful to know why these users need this side-effect.  Did
> the drivers break?   Or just get slower?

The affected driver was reported to be unusable without this behavior.
 
> Are those drivers misbehaving by assuming this behavior?   Should we

I do think they're using the wrong API. You you shouldn't use the dmapool if
your blocks need to be arranged in a contiguous address order. They should just
directly use dma_alloc_coherent() instead.

> require that they be altered instead of forever constraining the dmapool
> implementation in this fashion?

This change isn't really constraining dmapool where it matters. It's just an
unexpected one-time initialization thing.

As far as altering those drivers, I'll reach out to someone on that side for
comment (I'm currently not familiar with the affected subsystem).