linux-kernel - Re: [fs/pipe] 5a519c8fe4: WARNING:at_mm/page_alloc.c:#__alloc

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Message-ID: <CAHk-=wj8OCHqYkB2hVQ2FG6n5g4R0H3eetzbo9NrHooY1-4pwg@mail.gmail.com>
Date:   Fri, 22 Apr 2022 10:23:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrei Vagin <avagin@...il.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Pavel Emelyanov <ovzxemul@...il.com>
Subject: Re: [fs/pipe] 5a519c8fe4: WARNING:at_mm/page_alloc.c:#__alloc_pages

On Thu, Apr 21, 2022 at 10:23 PM Andrei Vagin <avagin@...il.com> wrote:
>
> The big advantage of vmsplice is that it can attach real user pages into
> a pipe and then any following changes of these pages by the process
> don't trigger any allocations and extra copies of data. vmsplice in this
> case is fast. After splicing pages to pipes, we resume a process and
> splice pages from pipes to a socket or a file.  The whole process of
> dumping process pages is zero-copy.

Hmm. What happens if you just use /proc/<pid>/mem?

That just takes a reference to the tsk->mm. No page copies at all.
After that you can do anything you want to that mm.

Well, anything a /proc/<pid>/mm fd allows, which is mainly read and
write. But it stays around for as long as you keep it open, and
fundamentally stays coherent with that mm, because it *is* that mm.

And it doesn't affect anything else, because all it literally has is
that mm_struct pointer.

            Linus