linux-ext4 - Re: Kernel Benchmarking

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Date:   Sun, 13 Sep 2020 04:18:33 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Michael Larabel <Michael@...haellarabel.com>,
        Ted Ts'o <tytso@...gle.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernel Benchmarking

On Sun, Sep 13, 2020 at 10:40:57AM +1000, Dave Chinner wrote:
> > The reason the apache benchmark regresses is that it basically does a
> > web server test with a single file ("test.html") that gets served by
> > just mmap'ing it, and sending it out that way. Using lots of threads,
> > and using lots of different mappings. So they *all* fault on the read
> > of that page, and they *all* do that "lock page, check that the
> > mapping is valid, insert page" dance.
> 
> Hmmmm. So this is a typically a truncate race check, but this isn't
> sufficient to protect the fault against all page invalidation races
> as the page can be re-inserted into the same mapping at a different
> page->index now within EOF.

No it can't.  find_get_page() returns the page with an elevated refcount.
The page can't be reused until we call put_page().  It can be removed
from the page cache, but can't go back to the page allocator until the
refcount hits zero.

> 5) filesystems will still need to be able to exclude page faults
> over a file range while they directly manipulate file metadata to
> change the user data in the file

Yes, but they can do that with a lock inside ->readpage (and, for that
matter in ->readahead()), so there's no need to take a lock for pages
which are stable in cache.