From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
cluster-devel <cluster-devel@redhat.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] gfs2: Fix mmap + page fault deadlocks
Date: Thu, 1 Jul 2021 14:41:15 -0700 [thread overview]
Message-ID: <CAHk-=wjk6KP3vSLFNPMjoaZ4xY4u=DjTM+C+hCS3QKt+XAE6OA@mail.gmail.com> (raw)
In-Reply-To: <20210701204246.2037142-1-agruenba@redhat.com>
On Thu, Jul 1, 2021 at 1:43 PM Andreas Gruenbacher <agruenba@redhat.com> wrote:
>
> here's another attempt at fixing the mmap + page fault deadlocks we're
> seeing on gfs2. Still not ideal because get_user_pages_fast ignores the
> current->pagefault_disabled flag
Of course get_user_pages_fast() ignores the pagefault_disabled flag,
because it doesn't do any page faults.
If you don't want to fall back to the "maybe do IO" case, you should
use the FOLL_FAST_ONLY flag - or get_user_pages_fast_only(), which
does that itself.
> For getting get_user_pages_fast changed to fix this properly, I'd need
> help from the memory management folks.
I really don't think you need anything at all from the mm people,
because we already support that whole "fast only" case.
Also, I have to say that I think the direct-IO code is fundamentally
mis-designed. Why it is doing the page lookup _during_ the IO is a
complete mystery to me. Why wasn't that done ahead of time before the
filesystem took the locks it needed?
So what the direct-IO code _should_ do is to turn an ITER_IOVEC into a
ITER_KVEC by doing the page lookup ahead of time, and none of these
issues should even exist, and then the whole pagefault_disabled and/or
FOLL_FAST_ONLY would be a complete non-issue.
Is there any reason why that isn't what it does (other than historical baggage)?
Linus
next prev parent reply other threads:[~2021-07-01 21:41 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-01 20:42 [PATCH] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-07-01 21:41 ` Linus Torvalds [this message]
2021-07-01 21:52 ` Linus Torvalds
2021-07-02 0:20 ` Andreas Gruenbacher
2021-07-02 0:30 ` Linus Torvalds
2021-07-02 0:44 ` Linus Torvalds
2021-07-02 1:14 ` Andreas Grünbacher
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