linux-kernel - Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)

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Message-ID: <3ec34f77-9299-153c-59e2-5c1f129e9415@nvidia.com>
Date:   Mon, 23 Jan 2023 14:53:44 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>
CC:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        "Jeff Layton" <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        <linux-fsdevel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or
 just list)

On 1/23/23 09:33, Matthew Wilcox wrote:
...
> Bleh, I'd forgotten about that problem.  We really do need to keep
> track of which pages are under I/O for this case, because we need to
> tell the filesystem that they are now available for writeback.
> 
> That said, I don't know that we need to keep track of it in the
> pages themselves.  Can't we have something similar to rmap which
> keeps track of a range of pinned pages, and have it taken care of
> at a higher level (ie unpin the pages in the dio_iodone_t rather
> than in the BIO completion handler)?
> 
> I'm not even sure why pinned pagecache pages remain on the LRU.
> They should probably go onto the unevictable list with the mlocked

This is an intriguing idea, but...

> pages, then on unpin get marked dirty and placed on the active list.
> There's no point in writing back a pinned page since it can be
> written to at any instant without any part of the kernel knowing.
> 

There have been filesystems discussions about this: if a page goes
unwritten for "too long", it's not good. To address that, bounce buffers
were proposed for periodic writeback of pinned pages. The idea with
bounce buffers is: even though the page is never guaranteed up to date
(because DMA can instantly make it effectively dirty), it is at least
much less out of date after a periodic writeback, then it was before.

And that is important for some file system use cases.


thanks,
-- 
John Hubbard
NVIDIA