linux-kernel - Re: Folio discussion recap

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Date:   Sat, 18 Sep 2021 00:51:50 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>
Subject: Re: Folio discussion recap

On Sat, Sep 18, 2021 at 11:04:40AM +1000, Dave Chinner wrote:
> As for long term, everything in the page cache API needs to
> transition to byte offsets and byte counts instead of units of
> PAGE_SIZE and page->index. That's a more complex transition, but
> AFAIA that's part of the future work Willy is intended to do with
> folios and the folio API. Once we get away from accounting and
> tracking everything as units of struct page, all the public facing
> APIs that use those units can go away.

Probably 95% of the places we use page->index and page->mapping aren't necessary
because we've already got that information from the context we're in and
removing them would be a useful cleanup - if we've already got that from context
(e.g. we're looking up the page in the page cache, via i_pageS) eliminating the
page->index or page->mapping use means we're getting rid of a data dependency so
it's good for performance - but more importantly, those (much fewer) places in
the code where we actually _do_ need page->index and page->mapping are really
important places to be able to find because they're interesting boundaries
between different components in the VM.