linux-kernel - Re: [PATCH v7 01/24] mm: Move readahead prototypes from mm.h

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Date:   Fri, 21 Feb 2020 13:48:53 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
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 01/24] mm: Move readahead prototypes from mm.h

On Thu, Feb 20, 2020 at 06:43:31PM -0800, John Hubbard wrote:
> Yes. But I think these files also need a similar change:
> 
>     fs/btrfs/disk-io.c

That gets pagemap.h through ctree.h, so I think it's fine.  It's
already using mapping_set_gfp_mask(), so it already depends on pagemap.h.

>     fs/nfs/super.c

That gets it through linux/nfs_fs.h.

I was reluctant to not add it to blk-core.c because it doesn't seem
necessarily intuitive that the block device core would include pagemap.h.

That said, blkdev.h does include pagemap.h, so maybe I don't need to
include it here.

> ...because they also use VM_READAHEAD_PAGES, and do not directly include
> pagemap.h yet.

> > +#define VM_READAHEAD_PAGES	(SZ_128K / PAGE_SIZE)
> > +
> > +void page_cache_sync_readahead(struct address_space *, struct file_ra_state *,
> > +		struct file *, pgoff_t index, unsigned long req_count);
> 
> Yes, "struct address_space *mapping" is weird, but I don't know if it's
> "misleading", given that it's actually one of the things you have to learn
> right from the beginning, with linux-mm, right? Or is that about to change?
> 
> I'm not asking to restore this to "struct address_space *mapping", but I thought
> it's worth mentioning out loud, especially if you or others are planning on
> changing those names or something. Just curious.

No plans (on my part) to change the name, although I have heard people
grumbling that there's very little need for it to be a separate struct
from inode, except for the benefit of coda, which is not exactly a
filesystem with a lot of users ...

Anyway, no plans to change it.  If there were something _special_ about
it like a theoretical:

void mapping_dedup(struct address_space *canonical,
		struct address_space *victim);

then that's useful information and shouldn't be deleted.  But I don't
think the word 'mapping' there conveys anything useful (other than the
convention is to call a 'struct address_space' a mapping, which you'll
see soon enough once you look at any of the .c files).