linux-kernel - Re: [PATCH v2 0/4] Avoid live-lock in fault-in+uaccess loops with sub-page faults

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Date:   Fri, 3 Dec 2021 19:11:58 +0100
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] Avoid live-lock in fault-in+uaccess loops with
 sub-page faults

On Fri, Dec 3, 2021 at 6:58 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Dec 3, 2021 at 7:29 AM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> > We're trying pretty hard to handle large I/O requests efficiently at
> > the filesystem level. A small, static upper limit in the fault-in
> > functions has the potential to ruin those efforts. So I'm not a fan of
> > that.
>
> I don't think fault-in should happen under any sane normal circumstances.
>
> Except for low-memory situations, and then you don't want to fault in
> large areas.
>
> Do you really expect to write big areas that the user has never even
> touched? That would be literally insane.
>
> And if the user _has_ touched them, then they'll in in-core. Except
> for the "swapped out" case.
>
> End result: this is purely a correctness issue, not a performance issue.

It happens when you mmap a file and write the mmapped region to
another file, for example. I don't think we want to make filesystems
go bonkers in such scenarios. Scaling down in response to memory
pressure sounds perfectly fine though.

Thanks,
Andreas