linux-kernel - Re: [RFC 2/3] mm/vmalloc: add support for __GFP

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Date:   Wed, 20 Oct 2021 15:54:23 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Linux Memory Management List <linux-mm@...ck.org>,
        Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>
Subject: Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

> > >
> > I think adding kind of schedule() will not make things worse and in corner
> > cases could prevent a power drain by CPU. It is important for mobile devices.
>
> I suspect you mean schedule_timeout here? Or cond_resched? I went with a
> later for now, I do not have a good idea for how to long to sleep here.
> I am more than happy to change to to a sleep though.
>
cond_resched() reschedules only if TIF_NEED_RESCHED is raised what is not good
here. Because in our case we know that we definitely would like to
take a breath. Therefore
invoking the schedule() is more suitable here. It will give a CPU time
to another waiting
process(if exists) in any case putting the "current" one to the tail.

As for adding a delay. I am not sure about for how long to delay or i
would say i do not
see a good explanation why for example we delay for 10 milliseconds or so.

> > As for vmap space, it can be that a user specifies a short range that does
> > not contain any free area. In that case we might never return back to a caller.
>
> This is to be expected. The caller cannot fail and if it would be
> looping around vmalloc it wouldn't return anyway.
>
> > Maybe add a good comment something like: think what you do when deal with the
> > __vmalloc_node_range() and __GFP_NOFAIL?
>
> We have a generic documentation for gfp flags and __GFP_NOFAIL is
> docuemented to "The allocation could block indefinitely but will never
> return with failure." We are discussing improvements for the generic
> documentation in another thread [1] and we will likely extend it so I
> suspect we do not have to repeat drawbacks here again.
>
> [1] http://lkml.kernel.org/r/163184741778.29351.16920832234899124642.stgit@noble.brown
>
> Anyway the gfp mask description and constrains for vmalloc are not
> documented. I will add a new patch to fill that gap and send it as a
> reply to this one
>
This is really good. People should be prepared for a case when it
never returns back
to a caller :)

-- 
Uladzislau Rezki