linux-kernel - Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to process

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Message-ID: <YjiTn+7vw2rXA6K/@dhcp22.suse.cz>
Date:   Mon, 21 Mar 2022 16:02:55 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Charan Teja Kalla <quic_charante@...cinc.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, surenb@...gle.com,
        vbabka@...e.cz, rientjes@...gle.com, sfr@...b.auug.org.au,
        edgararriaga@...gle.com, nadav.amit@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, "# 5 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to
 process_madvise

On Wed 16-03-22 19:49:38, Charan Teja Kalla wrote:
[...]
> It can return EINTR when:
> -------------------------
> 1) PTRACE_MODE_READ is being checked in mm_access() where it is waiting
> on task->signal->exec_update_lock. EINTR returned from here guarantees
> that process_madvise() didn't event start processing.
> https://elixir.bootlin.com/linux/v5.16.14/source/mm/madvise.c#L1264 -->
> https://elixir.bootlin.com/linux/v5.16.14/source/kernel/fork.c#L1318
> 
> 2) The process_madvise() started processing VMA's but the required
> behavior on a VMA needs mmap_write_lock_killable(), from where EINTR is
> returned.

Please note this will happen if the task has been killed. The return
value doesn't really matter because the process won't run in userspace.

> The current behaviours supported by process_madvise(),
> MADV_COLD, PAGEOUT, WILLNEED, just need read lock here.
> https://elixir.bootlin.com/linux/v5.16.14/source/mm/madvise.c#L1164
>  **Thus I think no way for EINTR can be returned by process_madvise() in
> the middle of processing.** . No?

Maybe not with the current implementation but I can easily imagine that
there is a requirement to break out early when there is a signal pending
(e.g. to support terminating madvise on a large memory rage). You would
get EINTR then somehow need to communicate that to the userspace.

-- 
Michal Hocko
SUSE Labs