linux-kernel - Re: [PATCH] mm: Remove redundant test from find_get_pages

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Message-ID: <20190108213621.GH6310@bombadil.infradead.org>
Date:   Tue, 8 Jan 2019 13:36:21 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Remove redundant test from find_get_pages_contig

On Tue, Jan 08, 2019 at 01:26:49PM -0800, Andrew Morton wrote:
> On Tue, 8 Jan 2019 12:26:35 -0800 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > > Would it be excessively cautious to put a WARN_ON_ONCE() in there for a
> > > while?
> > 
> > I think it would ... it'd get in the way of a subsequent patch to store
> > only head pages in the page cache.
> 
> OK, shall grab.  Perhaps the changelog could gain a few words
> explaining the history, etc.

Yeah, I suck at changelogs.  Particularly when I've encountered something
that's distracting me from the thing I was trying to do.  How about this:

mm: Remove redundant test from find_get_pages_contig

After we establish a reference on the page, we check the pointer
continues to be in the correct position in i_pages.  Checking page->index
afterwards is unnecessary; if it were to change, then the pointer to it
from the page cache would also move.  The check used to be done before
grabbing a reference on the page which was racy (see 9cbb4cb21b19f
("mm: find_get_pages_contig fixlet")), but nobody noticed that moving
the check after grabbing the reference was redundant.

Signed-off-by: Matthew Wilcox <willy@...radead.org>