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From: Matthew Wilcox <willy@infradead.org>
To: Jaewon Kim <jaewon31.kim@gmail.com>
Cc: gregkh@linuxfoundation.org, m.szyprowski@samsung.com,
	linux-mm@kvack.org, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Jaewon Kim <jaewon31.kim@samsung.com>,
	ytk.lee@samsung.com
Subject: Re: [RFC PATCH] usb: host: xhci: allow __GFP_FS in dma allocation
Date: Fri, 17 May 2019 09:34:20 -0700	[thread overview]
Message-ID: <20190517163420.GG31704@bombadil.infradead.org> (raw)
In-Reply-To: <CAJrd-UuMRdWHky4gkmiR0QYozfXW0O35Ohv6mJPFx2TLa8hRKg@mail.gmail.com>

On Sat, May 18, 2019 at 01:02:28AM +0900, Jaewon Kim wrote:
> Hello I don't have enough knowledge on USB core but I've wondered
> why GFP_NOIO has been used in xhci_alloc_dev for
> xhci_alloc_virt_device. I found commit ("a6d940dd759b xhci: Use
> GFP_NOIO during device reset"). But can we just change GFP_NOIO
> to __GFP_RECLAIM | __GFP_FS ?

No.  __GFP_FS implies __GFP_IO; you can't set __GFP_FS and clear __GFP_IO.

It seems like the problem you have is using the CMA to do DMA allocation.
Why would you do such a thing?

> Please refer to below case.
> 
> I got a report from Lee YongTaek <ytk.lee@samsung.com> that the
> xhci_alloc_virt_device was too slow over 2 seconds only for one page
> allocation.
> 
> 1) It was because kernel version was v4.14 and DMA allocation was
> done from CMA(Contiguous Memory Allocator) where CMA region was
> almost filled with file page and  CMA passes GFP down to page
> isolation. And the page isolation only allows file page isolation only to
> requests having __GFP_FS.
> 
> 2) Historically CMA was changed at v4.19 to use GFP_KERNEL
> regardless of GFP passed to  DMA allocation through the
> commit 6518202970c1 "(mm/cma: remove unsupported gfp_mask
> parameter from cma_alloc()".
> 
> I think pre v4.19 the xhci_alloc_virt_device could be very slow
> depending on CMA situation but free to USB deadlock issue. But as of
> v4.19, I think, it will be fast but can face the deadlock issue.
> Consequently I think to meet the both cases, I think USB can pass
> __GFP_FS without __GFP_IO.
> 
> If __GFP_FS is passed from USB core, of course, the CMA patch also
> need to be changed to pass GFP.



  reply	other threads:[~2019-05-17 16:34 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-17 16:02 Jaewon Kim
2019-05-17 16:34 ` Matthew Wilcox [this message]
2019-05-18  1:53   ` Jaewon Kim
2019-05-20  5:56 ` Christoph Hellwig
2019-05-20  9:09   ` Oliver Neukum
2019-05-20 10:12     ` Christoph Hellwig
2019-05-20 14:16       ` Alan Stern
2019-05-20 14:23         ` Christoph Hellwig
2019-05-21  8:54           ` Oliver Neukum
2019-05-21 13:27             ` Christoph Hellwig
2019-05-21 13:11         ` Oliver Neukum
2019-05-21 14:00           ` Alan Stern
2019-05-22  6:31             ` Oliver Neukum
2019-05-22 14:56               ` Alan Stern
2019-05-22 20:47                 ` Oliver Neukum
2019-05-23 14:01                   ` Alan Stern
2019-05-28 12:34                     ` Oliver Neukum
2019-05-28 14:25                       ` Alan Stern
2019-05-23 12:32   ` Oliver Neukum
2019-05-23 16:35     ` Christoph Hellwig

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