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

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 09/19] mm: Add page_cache_readahead_limit
Date: Tue, 18 Feb 2020 18:23:00 -0800	[thread overview]
Message-ID: <20200219022300.GJ24185@bombadil.infradead.org> (raw)
In-Reply-To: <1263603d-f446-c447-2eac-697d105fa76c@nvidia.com>

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.

> 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.

> > + * @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 ;-)