Re: [PATCH v6 09/19] mm: Add page_cache_readahead_limit

From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
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 09/19] mm: Add page_cache_readahead_limit
Date: Tue, 18 Feb 2020 18:46:59 -0800	[thread overview]
Message-ID: <f2d2346d-611c-ebdd-8430-ea4478e044d4@nvidia.com> (raw)
In-Reply-To: <20200219022300.GJ24185@bombadil.infradead.org>

On 2/18/20 6:23 PM, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 05:32:31PM -0800, John Hubbard wrote:
>>> +			page_cache_readahead_limit(inode->i_mapping, NULL,
>>> +					index, LONG_MAX, num_ra_pages, 0);
>>
>>
>> LONG_MAX seems bold at first, but then again I can't think of anything smaller 
>> that makes any sense, and the previous code didn't have a limit either...OK.
> 
> Probably worth looking at Dave's review of this and what we've just
> negotiated on the other subthread ... LONG_MAX is gone.

Great. OK, I see where it's going there.

> 
>> I also wondered about the NULL file parameter, and wonder if we're stripping out
>> information that is needed for authentication, given that that's what the newly
>> written kerneldoc says the "file" arg is for. But it seems that if we're this 
>> deep in the fs code's read routines, file system authentication has long since 
>> been addressed.
> 
> The authentication is for network filesystems.  Local filesystems
> generally don't use the 'file' parameter, and since we're going to be
> calling back into the filesystem's own readahead routine, we know it's
> not needed.
> 
>> Any actually I don't yet (still working through the patches) see any authentication,
>> so maybe that parameter will turn out to be unnecessary.
>>
>> Anyway, It's nice to see this factored out into a single routine.
> 
> I'm kind of thinking about pushing the rac in the other direction too,
> so page_cache_readahead_unlimited(rac, nr_to_read, lookahead_size).
> 
>>> +/**
>>> + * page_cache_readahead_limit - Start readahead beyond a file's i_size.
>>
>>
>> Maybe: 
>>
>>     "Start readahead to a caller-specified end point" ?
>>
>> (It's only *potentially* beyond files's i_size.)
> 
> My current tree has:
>  * page_cache_readahead_exceed - Start unchecked readahead.

Sounds good.

> 
> 
>>> + * @mapping: File address space.
>>> + * @file: This instance of the open file; used for authentication.
>>> + * @offset: First page index to read.
>>> + * @end_index: The maximum page index to read.
>>> + * @nr_to_read: The number of pages to read.
>>
>>
>> How about:
>>
>>     "The number of pages to read, as long as end_index is not exceeded."
> 
> API change makes this irrelevant ;-)
> 
>>> + * @lookahead_size: Where to start the next readahead.
>>
>> Pre-existing, but...it's hard to understand how a size is "where to start".
>> Should we rename this arg?
> 
> It should probably be lookahead_count.
> 
>>> + *
>>> + * This function is for filesystems to call when they want to start
>>> + * readahead potentially beyond a file's stated i_size.  If you want
>>> + * to start readahead on a normal file, you probably want to call
>>> + * page_cache_async_readahead() or page_cache_sync_readahead() instead.
>>> + *
>>> + * Context: File is referenced by caller.  Mutexes may be held by caller.
>>> + * May sleep, but will not reenter filesystem to reclaim memory.
>>
>> In fact, can we say "must not reenter filesystem"? 
> 
> I think it depends which side of the API you're looking at which wording
> you prefer ;-)
> 
> 

Yes. We should try to write these so that it's clear which way we're looking:
in or out. 

thanks,
-- 
John Hubbard
NVIDIA