linux-kernel - Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on entire memory range

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Message-ID: <CAJuCfpGQQN-FEoRUymeGSs86Du5DmziNbbr7uXqSTd2xsqMn3g@mail.gmail.com>
Date:   Wed, 23 Dec 2020 09:32:16 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jann Horn <jannh@...gle.com>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Christian Brauner <christian@...uner.io>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on
 entire memory range

On Tue, Dec 22, 2020 at 11:57 PM Christoph Hellwig <hch@....de> wrote:
>
> On Tue, Dec 22, 2020 at 09:48:43AM -0800, Suren Baghdasaryan wrote:
> > Thanks for the feedback! The use case is userspace memory reaping
> > similar to oom-reaper. Detailed justification is here:
> > https://lore.kernel.org/linux-mm/20201124053943.1684874-1-surenb@google.com
>
> Given that this new variant of process_madvise
>
>   a) does not work on an address range

True, however I can see other madvise flavors that could be used on
the entire process. For example process_madvise(MADV_PAGEOUT) could be
used to "shrink" an entire inactive background process.

>   b) is destructive

I agree that memory reaping might be the only case when a destructive
process_madvise() makes sense. Unless the target process is dying, a
destructive process_madvise() would need coordination with the target
process, and if it's coordinated then the target might as well call
normal madvise() itself.

>   c) doesn't share much code at all with the rest of process_madvise

It actually does reuse a considerable part of the code, but the same
code can be refactored and reused either way.

>
> Why not add a proper separate syscall?

I think my answer to (a) is one justification for allowing
process_madvise() to operate on the entire process. Also MADV_DONTNEED
seems quite suitable for this operation.
Considering the above answers, are you still leaning towards a separate syscall?

>
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