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

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Message-ID: <cd013218-7735-4bc1-b6b6-80d1129e2b76@redhat.com>
Date: Thu, 13 Mar 2025 23:49:00 +0100
From: David Hildenbrand <david@...hat.com>
To: Christoph Hellwig <hch@...radead.org>,
 Matthew Wilcox <willy@...radead.org>
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 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.

-- 
Cheers,

David / dhildenb