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From: Matthew Wilcox <willy@infradead.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
	ocfs2-devel@oss.oracle.com, linux-xfs@vger.kernel.org
Subject: Re: [PATCH v6 01/19] mm: Return void from various readahead functions
Date: Tue, 18 Feb 2020 13:21:15 -0800	[thread overview]
Message-ID: <20200218212115.GG24185@bombadil.infradead.org> (raw)
In-Reply-To: <29d2d7ca-7f2b-7eb4-78bc-f2af36c4c426@nvidia.com>

On Tue, Feb 18, 2020 at 01:05:29PM -0800, John Hubbard wrote:
> This is an easy review and obviously correct, so:
> 
>     Reviewed-by: John Hubbard <jhubbard@nvidia.com>

Thanks

> Thoughts for the future of the API:
> 
> I will add that I could envision another patchset that went in the
> opposite direction, and attempted to preserve the information about
> how many pages were successfully read ahead. And that would be nice
> to have (at least IMHO), even all the way out to the syscall level,
> especially for the readahead syscall.

Right, and that was where I went initially.  It turns out to be a
non-trivial aount of work to do the book-keeping to find out how many
pages were _attempted_, and since we don't wait for the I/O to complete,
we don't know how many _succeeded_, and we also don't know how many
weren't attempted because they were already there, and how many weren't
attempted because somebody else has raced with us and is going to attempt
them themselves, and how many weren't attempted because we just ran out
of memory, and decided to give up.

Also, we don't know how many pages were successfully read, and then the
system decided to evict before the program found out how many were read,
let alone before it did any action based on that.

So, given all that complexity, and the fact that nobody actually does
anything with the limited and incorrect information we tried to provide
today, I think it's fair to say that anybody who wants to start to do
anything with that information can delve into all the complexity around
"what number should we return, and what does it really mean".  In the
meantime, let's just ditch the complexity and pretense that this number
means anything.

  reply	other threads:[~2020-02-18 21:21 UTC|newest]

Thread overview: 111+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-17 18:45 [PATCH v6 00/19] Change readahead API Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 01/19] mm: Return void from various readahead functions Matthew Wilcox
2020-02-18  4:47   ` Dave Chinner
2020-02-18 21:05   ` John Hubbard
2020-02-18 21:21     ` Matthew Wilcox [this message]
2020-02-18 21:52       ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 02/19] mm: Ignore return value of ->readpages Matthew Wilcox
2020-02-18  4:48   ` Dave Chinner
2020-02-18 21:33   ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 03/19] mm: Use readahead_control to pass arguments Matthew Wilcox
2020-02-18  5:03   ` Dave Chinner
2020-02-18 13:56     ` Matthew Wilcox
2020-02-18 22:46       ` Dave Chinner
2020-02-18 22:52         ` Matthew Wilcox
2020-02-18 22:22   ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 04/19] mm: Rearrange readahead loop Matthew Wilcox
2020-02-18  5:08   ` Dave Chinner
2020-02-18 13:57     ` Matthew Wilcox
2020-02-18 22:48       ` Dave Chinner
2020-02-18 22:33   ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 04/16] mm: Tweak readahead loop slightly Matthew Wilcox
2020-02-18 22:57   ` John Hubbard
2020-02-18 23:00     ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 05/16] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 05/19] mm: Remove 'page_offset' from readahead loop Matthew Wilcox
2020-02-18  5:14   ` Dave Chinner
2020-02-18 23:08   ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 06/16] mm: Add readahead address space operation Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 06/19] mm: rename readahead loop variable to 'i' Matthew Wilcox
2020-02-18  5:33   ` Dave Chinner
2020-02-18 23:11   ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 07/16] mm: Add page_cache_readahead_limit Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 07/19] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-02-18  6:14   ` Dave Chinner
2020-02-18 15:42     ` Matthew Wilcox
2020-02-19  0:59       ` Dave Chinner
2020-02-19  0:01   ` John Hubbard
2020-02-19  1:02     ` Matthew Wilcox
2020-02-19  1:13       ` John Hubbard
2020-02-19  3:24       ` John Hubbard
2020-02-19 14:41     ` Matthew Wilcox
2020-02-19 14:52       ` Christoph Hellwig
2020-02-19 15:01         ` Matthew Wilcox
2020-02-19 20:24           ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 08/16] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 08/19] mm: Add readahead address space operation Matthew Wilcox
2020-02-18  6:21   ` Dave Chinner
2020-02-18 16:10     ` Matthew Wilcox
2020-02-19  1:04       ` Dave Chinner
2020-02-19  0:12   ` John Hubbard
2020-02-19  3:10   ` Eric Biggers
2020-02-19  3:35     ` Eric Biggers
2020-02-19 16:52     ` Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 09/16] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 09/19] mm: Add page_cache_readahead_limit Matthew Wilcox
2020-02-18  6:31   ` Dave Chinner
2020-02-18 19:54     ` Matthew Wilcox
2020-02-19  1:08       ` Dave Chinner
2020-02-19  1:32   ` John Hubbard
2020-02-19  2:23     ` Matthew Wilcox
2020-02-19  2:46       ` John Hubbard
2020-02-17 18:45 ` [PATCH v6 10/16] erofs: Convert uncompressed files from readpages to readahead Matthew Wilcox
2020-02-17 18:45 ` [PATCH v6 10/19] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-02-18  1:51   ` [Ocfs2-devel] " Joseph Qi
2020-02-18  6:37   ` Dave Chinner
2020-02-19  2:48   ` John Hubbard
2020-02-19  3:28   ` Eric Biggers
2020-02-19  3:47     ` Matthew Wilcox
2020-02-19  3:55       ` Eric Biggers
2020-02-17 18:45 ` [PATCH v6 11/19] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-02-18  6:57   ` Dave Chinner
2020-02-18 21:12     ` Matthew Wilcox
2020-02-19  1:23       ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 11/16] erofs: Convert compressed files " Matthew Wilcox
2020-02-19  2:34   ` Gao Xiang
2020-02-17 18:46 ` [PATCH v6 12/19] erofs: Convert uncompressed " Matthew Wilcox
2020-02-19  2:39   ` Gao Xiang
2020-02-19  3:04   ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 12/16] ext4: Convert " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 13/19] erofs: Convert compressed files " Matthew Wilcox
2020-02-19  3:08   ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 13/16] f2fs: Convert " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 14/19] ext4: " Matthew Wilcox
2020-02-19  3:16   ` Dave Chinner
2020-02-19  3:29   ` Eric Biggers
2020-02-17 18:46 ` [PATCH v6 14/16] fuse: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 15/19] f2fs: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 15/16] iomap: " Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 16/19] fuse: " Matthew Wilcox
2020-02-19  3:22   ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 16/16] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor Matthew Wilcox
2020-02-19  3:17   ` John Hubbard
2020-02-19  5:35     ` Matthew Wilcox
2020-02-19  3:29   ` Dave Chinner
2020-02-19  6:04     ` Matthew Wilcox
2020-02-19  6:40       ` Dave Chinner
2020-02-19 17:06         ` Matthew Wilcox
2020-02-17 18:46 ` [PATCH v6 18/19] iomap: Convert from readpages to readahead Matthew Wilcox
2020-02-19  3:40   ` Dave Chinner
2020-02-17 18:46 ` [PATCH v6 19/19] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-02-19  3:43   ` Dave Chinner
2020-02-19  5:22     ` Matthew Wilcox
2020-02-17 18:48 ` [PATCH v6 00/19] Change readahead API Matthew Wilcox
2020-02-18  4:56 ` Dave Chinner
2020-02-18 13:42   ` Matthew Wilcox
2020-02-18 21:26     ` Dave Chinner
2020-02-19  3:45       ` Dave Chinner
2020-02-19  3:48         ` Matthew Wilcox
2020-02-19  3:57           ` Dave Chinner
2020-02-18 20:49 ` John Hubbard

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