linux-kernel - Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup

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Message-ID: <c45e1bf8-f91b-4383-8c1a-7e11be02743b@nvidia.com>
Date: Fri, 14 Mar 2025 18:04:19 -0700
From: John Hubbard <jhubbard@...dia.com>
To: David Hildenbrand <david@...hat.com>,
 Christoph Hellwig <hch@...radead.org>, Matthew Wilcox <willy@...radead.org>,
 Jason Gunthorpe <jgg@...dia.com>
Cc: Sooyong Suk <s.suk@...sung.com>, viro@...iv.linux.org.uk,
 linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
 jaewon31.kim@...il.com, spssyr@...il.com
Subject: Re: [RFC PATCH] block, fs: use FOLL_LONGTERM as gup_flags for direct
 IO

On 3/13/25 3:49 PM, David Hildenbrand wrote:
> On 12.03.25 16:21, Christoph Hellwig wrote:
>> On Fri, Mar 07, 2025 at 08:23:08PM +0000, Matthew Wilcox wrote:
>>> Howver, the problem is real.
>>
>> What is the problem?
> 
> I think the problem is the CMA allocation failure, not the latency.
> 
> "if a large amount of direct IO is requested constantly, this can make 
> pages in CMA pageblocks pinned and unable to migrate outside of the 
> pageblock"
> 
> We'd need a more reliable way to make CMA allocation -> page migration 
> make progress. For example, after we isolated the pageblocks and 
> migration starts doing its thing, we could disallow any further GUP 
> pins. (e.g., make GUP spin or wait for migration to end)
> 
> We could detect in GUP code that a folio is soon expected to be migrated 
> by checking the pageblock (isolated) and/or whether the folio is locked.
> 

Jason Gunthorpe and Matthew both had some ideas about how to fix this [1],
which were very close (maybe the same) to what you're saying here: sleep
and spin in an killable loop.

It turns out to be a little difficult to do this--I had trouble making
the folio's "has waiters" bit work for this, for example. And then...squirrel!

However, I still believe, so far, this is the right approach. I'm just not
sure which thing to wait on, exactly.
 
[1] https://lore.kernel.org/20240502183408.GC3341011@nvidia.com

thanks,
-- 
John Hubbard