linux-kernel - Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library

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Date:   Tue, 09 Feb 2021 21:10:42 +0000
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Trond Myklebust <trondmy@...merspace.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        ceph-devel@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-cachefs@...hat.com, CIFS <linux-cifs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] fscache: I/O API modernisation and netfs helper library

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> The PG_fscache bit waiting functions are completely crazy. The comment
> about "this will wake up others" is actively wrong,

You mean this?

/**
 * unlock_page_fscache - Unlock a page pinned with PG_fscache
 * @page: The page
 *
 * Unlocks the page and wakes up sleepers in wait_on_page_fscache().  Also
 * wakes those waiting for the lock and writeback bits because the wakeup
 * mechanism is shared.  But that's OK - those sleepers will just go back to
 * sleep.
 */

Actually, you're right.  The wakeup check func is evaluated by the
waker-upper.  I can fix the comment with a patch.

> and the waiting function looks insane, because you're mixing the two names
> for "fscache" which makes the code look totally incomprehensible. Why would
> we wait for PF_fscache, when PG_private_2 was set? Yes, I know why, but the
> code looks entirely nonsensical.

IIRC someone insisted that I should make it a generic name and put the
accessor functions in the fscache headers (which means they aren't available
to core code), but I don't remember who (maybe Andrew? it was before mid-2007)
- kind of like PG_checked is an alias for PG_owner_priv_1.

I'd be quite happy to move the accessors for PG_fscache to the
linux/page-flags.h as that would simplify things.

David