linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Message-ID: <20200814121544.GA32598@pc636>
Date:   Fri, 14 Aug 2020 14:15:44 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

> On Thu 13-08-20 19:09:29, Thomas Gleixner wrote:
> > Michal Hocko <mhocko@...e.com> writes:
> [...]
> > > Why should we limit the functionality of the allocator for something
> > > that is not a real problem?
> > 
> > We'd limit the allocator for exactly ONE new user which was aware of
> > this problem _before_ the code hit mainline. And that ONE user is
> > prepared to handle the fail.
> 
> If we are to limit the functionality to this one particular user then
> I would consider a dedicated gfp flag a huge overkill. It would be much
> more easier to have a preallocated pool of pages and use those and
> completely avoid the core allocator. That would certainly only shift the
> complexity to the caller but if it is expected there would be only that
> single user then it would be probably better than opening a can of worms
> like allocator usable from raw spin locks.
> 
Vlastimil raised same question earlier, i answered, but let me answer again:

It is hard to achieve because the logic does not stick to certain static test
case, i.e. it depends on how heavily kfree_rcu(single/double) are used. Based
on that, "how heavily" - number of pages are formed, until the drain/reclaimer
thread frees them.

Preloading pages and keeping them for internal use, IMHO, seems not optimal
from the point of resources wasting. It is better to have a fast mechanism to
request a page and release it back for needs of others. As described about
we do not know how much we will need.

--
Vlad Rezki