linux-kernel - Re: [PATCH 2/2] fs: Do not update nr_thps for mappings which support THPs

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Date:   Wed, 16 Sep 2020 12:46:49 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Song Liu <songliubraving@...com>,
        Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH 2/2] fs: Do not update nr_thps for mappings which support
 THPs

On Wed, Sep 16, 2020 at 06:21:26AM +0100, Christoph Hellwig wrote:
> On Wed, Sep 16, 2020 at 04:27:17AM +0100, Matthew Wilcox (Oracle) wrote:
> > The nr_thps counter is to support THPs in the page cache when the
> > filesystem doesn't understand THPs.  Eventually it will be removed, but
> > we should still support filesystems which do not understand THPs yet.
> > Move the nr_thp manipulation functions to filemap.h since they're
> > page-cache specific.
> 
> Honestly I don't think we should support the read-only THP crap.  We
> should in fact never have merged that bandaid to start with given that
> you did good progress on the real thing.

I'd like to see the feature ripped out again, yes.  Once we have a few more
filesystems converted, I think that'll be a reasonable thing to do.

It was a good step along the way; Song fixed a number of problems,
and worked on other things that I never had to learn anything about
(like uprobes and khugepaged).  I wouldn't go so far as to say we should
never have merged it, but I think we can remove it in about six months.