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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 15 Sep 2020 11:47:45 -0700
Message-ID: <CAHk-=wimjnAsoDUjkanC2BQTntwK4qtzmPdBbtmgM=MMhR6B2w@mail.gmail.com>
Subject: Re: Kernel Benchmarking
To:     Matthieu Baerts <matthieu.baerts@tessares.net>
Cc:     Michael Larabel <Michael@michaellarabel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Amir Goldstein <amir73il@gmail.com>,
        "Ted Ts'o" <tytso@google.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Tue, Sep 15, 2020 at 11:27 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Every one of them is in the "io_schedule()" in the filemap_fault()
> path, although two of them seem to be in file_fdatawait_range() rather
> than in the lock_page() code itself (so they are also waiting on a
> page bit, but they are waiting for the writeback bit to clear).

No, that seems to be just stale entries on the stack from a previous
system call, rather than a real trace. There's no way to reach
file_fdatawait_range() from mlockall() that I can see.

So I'm not entirely sure why the stack trace for two of the processes
looks a bit different, but they all look like they should be in
__lock_page_killable().

It's possible those two were woken up (by another CPU) and their stack
is in flux. They also have "wake_up_page_bit()" as a stale entry on
their stack, so that's not entirely unlikely.

So that sysrq-W state shows that yes, people are stuck waiting for a
page, but that wasn't exactly unexpected.

                 Linus