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From: Matthew Wilcox <willy@infradead.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-xfs@vger.kernel.org,
	William Kucharski <william.kucharski@oracle.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-kernel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com,
	linux-mm@kvack.org, ocfs2-devel@oss.oracle.com,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-erofs@lists.ozlabs.org, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v9 12/25] mm: Move end_index check out of readahead loop
Date: Fri, 20 Mar 2020 11:11:32 -0700
Message-ID: <20200320181132.GD4971@bombadil.infradead.org> (raw)
In-Reply-To: <20200320180017.GE851@sol.localdomain>

On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote:
> On Fri, Mar 20, 2020 at 10:30:40AM -0700, Matthew Wilcox wrote:
> > On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers wrote:
> > > On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote:
> > > > +	/* Avoid wrapping to the beginning of the file */
> > > > +	if (index + nr_to_read < index)
> > > > +		nr_to_read = ULONG_MAX - index + 1;
> > > > +	/* Don't read past the page containing the last byte of the file */
> > > > +	if (index + nr_to_read >= end_index)
> > > > +		nr_to_read = end_index - index + 1;
> > > 
> > > There seem to be a couple off-by-one errors here.  Shouldn't it be:
> > > 
> > > 	/* Avoid wrapping to the beginning of the file */
> > > 	if (index + nr_to_read < index)
> > > 		nr_to_read = ULONG_MAX - index;
> > 
> > I think it's right.  Imagine that index is ULONG_MAX.  We should read one
> > page (the one at ULONG_MAX).  That would be ULONG_MAX - ULONG_MAX + 1.
> > 
> > > 	/* Don't read past the page containing the last byte of the file */
> > > 	if (index + nr_to_read > end_index)
> > > 		nr_to_read = end_index - index + 1;
> > > 
> > > I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that
> > > 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0.
> > > 
> > > Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index',
> > > since otherwise nr_to_read can be increased by 1 rather than decreased or stay
> > > the same as expected.
> > 
> > Ooh, I missed the overflow case here.  It should be:
> > 
> > +	if (index + nr_to_read - 1 > end_index)
> > +		nr_to_read = end_index - index + 1;
> > 
> 
> But then if someone passes index=0 and nr_to_read=0, this underflows and the
> entire file gets read.

nr_to_read == 0 doesn't make sense ... I thought we filtered that out
earlier, but I can't find anywhere that does that right now.  I'd
rather return early from __do_page_cache_readahead() to fix that.

> The page cache isn't actually supposed to contain a page at index ULONG_MAX,
> since MAX_LFS_FILESIZE is at most ((loff_t)ULONG_MAX << PAGE_SHIFT), right?  So
> I don't think we need to worry about reading the page with index ULONG_MAX.
> I.e. I think it's fine to limit nr_to_read to 'ULONG_MAX - index', if that makes
> it easier to avoid an overflow or underflow in the next check.

I think we can get a page at ULONG_MAX on 32-bit systems?  I mean, we can buy
hard drives which are larger than 16TiB these days:
https://www.pcmag.com/news/seagate-will-ship-18tb-and-20tb-hard-drives-in-2020
(even ignoring RAID devices)

  reply index

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-20 14:22 [PATCH v9 00/25] Change readahead API Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 01/25] mm: Move readahead prototypes from mm.h Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 02/25] mm: Return void from various readahead functions Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 03/25] mm: Ignore return value of ->readpages Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 04/25] mm: Move readahead nr_pages check into read_pages Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 05/25] mm: Add new readahead_control API Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 06/25] mm: Use readahead_control to pass arguments Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 07/25] mm: Rename various 'offset' parameters to 'index' Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 08/25] mm: rename readahead loop variable to 'i' Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 09/25] mm: Remove 'page_offset' from readahead loop Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 10/25] mm: Put readahead pages in cache earlier Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 11/25] mm: Add readahead address space operation Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 12/25] mm: Move end_index check out of readahead loop Matthew Wilcox
2020-03-20 16:58   ` Eric Biggers
2020-03-20 17:30     ` Matthew Wilcox
2020-03-20 18:00       ` Eric Biggers
2020-03-20 18:11         ` Matthew Wilcox [this message]
2020-03-20 18:24           ` Eric Biggers
2020-03-22 16:28             ` Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 13/25] mm: Add page_cache_readahead_unbounded Matthew Wilcox
2020-03-20 17:27   ` Eric Biggers
2020-03-20 14:22 ` [PATCH v9 14/25] mm: Document why we don't set PageReadahead Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 15/25] mm: Use memalloc_nofs_save in readahead path Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 16/25] fs: Convert mpage_readpages to mpage_readahead Matthew Wilcox
2020-03-20 23:24   ` Namjae Jeon
2020-03-20 14:22 ` [PATCH v9 17/25] btrfs: Convert from readpages to readahead Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 18/25] erofs: Convert uncompressed files " Matthew Wilcox
2020-03-21 12:38   ` [f2fs-dev] " Chao Yu
2020-03-20 14:22 ` [PATCH v9 19/25] erofs: Convert compressed " Matthew Wilcox
2020-03-21 12:41   ` [f2fs-dev] " Chao Yu
2020-03-20 14:22 ` [PATCH v9 20/25] ext4: Convert " Matthew Wilcox
2020-03-20 17:37   ` Eric Biggers
2020-03-20 17:48     ` Matthew Wilcox
2020-03-20 18:40       ` Eric Biggers
2020-03-20 14:22 ` [PATCH v9 21/25] ext4: Pass the inode to ext4_mpage_readpages Matthew Wilcox
2020-03-20 18:44   ` Eric Biggers
2020-03-20 14:22 ` [PATCH v9 22/25] f2fs: Convert from readpages to readahead Matthew Wilcox
2020-03-20 18:51   ` Eric Biggers
2020-03-21 12:34   ` [f2fs-dev] " Chao Yu
2020-03-23  3:55   ` Jaegeuk Kim
2020-03-20 14:22 ` [PATCH v9 23/25] f2fs: Pass the inode to f2fs_mpage_readpages Matthew Wilcox
2020-03-20 18:52   ` Eric Biggers
2020-03-21 12:35   ` [f2fs-dev] " Chao Yu
2020-03-23  3:53   ` Jaegeuk Kim
2020-03-20 14:22 ` [PATCH v9 24/25] fuse: Convert from readpages to readahead Matthew Wilcox
2020-03-20 14:22 ` [PATCH v9 25/25] iomap: " Matthew Wilcox

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