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

From: Suren Baghdasaryan <surenb@google.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Christoph Hellwig <hch@infradead.org>,
	Jann Horn <jannh@google.com>, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@kernel.org>, Michal Hocko <mhocko@suse.com>,
	David Rientjes <rientjes@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
	Christian Brauner <christian@brauner.io>,
	Oleg Nesterov <oleg@redhat.com>,
	Tim Murray <timmurray@google.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@android.com>
Subject: Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on entire memory range
Date: Wed, 23 Dec 2020 09:32:16 -0800	[thread overview]
Message-ID: <CAJuCfpGQQN-FEoRUymeGSs86Du5DmziNbbr7uXqSTd2xsqMn3g@mail.gmail.com> (raw)
In-Reply-To: <20201223075712.GA4719@lst.de>

On Tue, Dec 22, 2020 at 11:57 PM Christoph Hellwig <hch@lst.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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