linux-ext4 - Re: Kernel Benchmarking

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Date:   Thu, 17 Sep 2020 11:30:00 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michael Larabel <Michael@...haellarabel.com>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        Amir Goldstein <amir73il@...il.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 Thu, Sep 17, 2020 at 11:23 AM Matthew Wilcox <willy@...radead.org> wrote:
>
>             Something like taking
> the i_mmap_lock_read(file->f_mapping) in filemap_fault, then adding a
> new VM_FAULT_I_MMAP_LOCKED bit so that do_read_fault() and friends add:
>
>         if (ret & VM_FAULT_I_MMAP_LOCKED)
>                 i_mmap_unlock_read(vmf->vma->vm_file->f_mapping);
>         else
>                 unlock_page(page);
>
> ... want me to turn that into a real patch?

I can't guarantee it's the right model - it does worry me how many
places we might get that i_mmap_rwlock, and how long we migth hold it
for writing, and what deadlocks it might cause when we take it for
reading in the page fault path.

But I think it might be very interesting as a benchmark patch and a
trial balloon. Maybe it "just works".

I would _love_ for the page lock itself to be only (or at least
_mainly_) about the actual IO synchronization on the page.

That was the origin of it, the whole "protect all the complex state of
a page" behavior kind of grew over time, since it was the only
per-page lock we had.

              Linus