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

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Message-ID: <72b4183b-2ec7-dc73-0c21-b12f342860d4@quicinc.com>
Date:   Tue, 22 Mar 2022 10:49:56 +0530
From:   Charan Teja Kalla <quic_charante@...cinc.com>
To:     Michal Hocko <mhocko@...e.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 3/21/2022 8:32 PM, Michal Hocko 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.

Okay, thanks here.

> 
>> 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.

Agree. Will implement this.