linux-ext4 - Re: [PATCH v7 11/24] mm: Move end_index check out of readahead loop

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Date:   Fri, 21 Feb 2020 11:41:28 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <linux-btrfs@...r.kernel.org>,
        <linux-erofs@...ts.ozlabs.org>, <linux-ext4@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>,
        <cluster-devel@...hat.com>, <ocfs2-devel@....oracle.com>,
        <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v7 11/24] mm: Move end_index check out of readahead loop

On 2/21/20 7:35 AM, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 07:50:39PM -0800, John Hubbard wrote:
>> This tiny patch made me pause, because I wasn't sure at first of the exact
>> intent of the lines above. Once I worked it out, it seemed like it might
>> be helpful (or overkill??) to add a few hints for the reader, especially since
>> there are no hints in the function's (minimal) documentation header. What
>> do you think of this?
>>
>> 	/*
>> 	 * If we can't read *any* pages without going past the inodes's isize
>> 	 * limit, give up entirely:
>> 	 */
>> 	if (index > end_index)
>> 		return;
>>
>> 	/* Cap nr_to_read, in order to avoid overflowing the ULONG type: */
>> 	if (index + nr_to_read < index)
>> 		nr_to_read = ULONG_MAX - index + 1;
>>
>> 	/* Cap nr_to_read, to avoid reading past the inode's isize limit: */
>> 	if (index + nr_to_read >= end_index)
>> 		nr_to_read = end_index - index + 1;
> 
> A little verbose for my taste ... How about this?


Mine too, actually. :)  I think your version below looks good.


thanks,
-- 
John Hubbard
NVIDIA

> 
>          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)
>                  nr_to_read = end_index - index + 1;
>