linux-kernel - Re: strace of io

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Message-ID: <20200721195602.qtncgzddl7y55b6l@alap3.anarazel.de>
Date:   Tue, 21 Jul 2020 12:56:02 -0700
From:   Andres Freund <andres@...razel.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Kees Cook <keescook@...omium.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        strace-devel@...ts.strace.io, io-uring@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Stefan Hajnoczi <stefanha@...hat.com>
Subject: Re: strace of io_uring events?

Hi,

On 2020-07-21 12:44:09 -0700, Andy Lutomirski wrote:
> Can you enlighten me?  I don't see any iov_iter_get_pages() calls or
> equivalents.  If an IO is punted, how does the data end up in the
> io_uring_enter() caller's mm?

For operations needing that io_op_def.needs_mm is true. Which is checked
by io_prep_async_work(), adding the current mm to req. On the wq side
io_wq_switch_mm() uses that mm when executing the queue entry.

Greetings,

Andres Freund