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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: Sun, 22 Mar 2020 09:28:18 -0700
Message-ID: <20200322162818.GG4971@bombadil.infradead.org> (raw)
In-Reply-To: <20200320182452.GF851@sol.localdomain>

On Fri, Mar 20, 2020 at 11:24:52AM -0700, Eric Biggers wrote:
> On Fri, Mar 20, 2020 at 11:11:32AM -0700, Matthew Wilcox wrote:
> > On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote:
> > > 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)
> 
> The max file size is ((loff_t)ULONG_MAX << PAGE_SHIFT) which means the maximum
> page *index* is ULONG_MAX - 1, not ULONG_MAX.

I see where we set that for _files_ ... I can't find anywhere that we prevent
i_size getting that big for block devices.  Maybe I'm missing something.

> Anyway, I think we may be making this much too complicated.  How about just:
> 
> 	pgoff_t i_nrpages = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
> 
> 	if (index >= i_nrpages)
> 		return;
> 	/* Don't read past the end of the file */
> 	nr_to_read = min(nr_to_read, i_nrpages - index);
> 
> That's 2 branches instead of 4.  (Note that assigning to i_nrpages can't
> overflow, since the max number of pages is ULONG_MAX not ULONG_MAX + 1.)

I like where you're going with this.  Just to be on the safe side, I'd
prefer to do this:

@@ -266,11 +266,8 @@ void __do_page_cache_readahead(struct address_space *mapping,
        end_index = (isize - 1) >> PAGE_SHIFT;
        if (index > end_index)
                return;
-       /* 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)
+       if (nr_to_read > end_index - index)
                nr_to_read = end_index - index + 1;
 
        page_cache_readahead_unbounded(mapping, file, index, nr_to_read,

end_index - index + 1 could only overflow if end_index is ULONG_MAX
and index is 0.  But if end_index is ULONG_MAX and index is 0, then
nr_to_read is necessarily <= ULONG_MAX, so the condition is false.
And if nr_to_read is 0, then the condition is also false, so it won't
increase nr_to_read from 0 to 1.  It might assign x to nr_to_read when
nr_to_read is already x, but that's harmless.

Thanks!

  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
2020-03-20 18:24           ` Eric Biggers
2020-03-22 16:28             ` Matthew Wilcox [this message]
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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