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From:   Uladzislau Rezki <urezki@gmail.com>
Date:   Fri, 29 Oct 2021 19:23:07 +0200
To:     Michal Hocko <mhocko@suse.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Dave Chinner <david@fromorbit.com>, Neil Brown <neilb@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Jeff Layton <jlayton@kernel.org>
Subject: Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL
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> On Fri 29-10-21 16:05:32, Uladzislau Rezki wrote:
> [...]
> > > OK, this looks easier from the code reading but isn't it quite wasteful
> > > to throw all the pages backing the area (all of them allocated as
> > > __GFP_NOFAIL) just to then fail to allocate few page tables pages and
> > > drop all of that on the floor (this will happen in __vunmap AFAICS).
> > >
> > > I mean I do not care all that strongly but it seems to me that more
> > > changes would need to be done here and optimizations can be done on top.
> > >
> > > Is this something you feel strongly about?
> > >
> > Will try to provide some motivations :)
> > 
> > It depends on how to look at it. My view is as follows a more simple code
> > is preferred. It is not considered as a hot path and it is rather a corner
> > case to me.
> 
> Yes, we are definitely talking about corner cases here. Even GFP_KERNEL
> allocations usually do not fail.
> 
> > I think "unwinding" has some advantage. At least one motivation
> > is to release a memory(on failure) before a delay that will prevent holding
> > of extra memory in case of __GFP_NOFAIL infinitelly does not succeed, i.e.
> > if a process stuck due to __GFP_NOFAIL it does not "hold" an extra memory
> > forever.
> 
> Well, I suspect this is something that we can disagree on and both of us
> would be kinda right. I would see it as throwing baby out with the
> bathwater. The vast majority of the memory will be in the area pages and
> sacrificing that just to allocate few page tables or whatever that might
> fail in that code path is just a lot of cycles wasted.
>
We are not talking about performance, no sense to measure cycles here :)

> 
> So unless you really feel strongly about this then I would stick with
> this approach.
>
I have raised one concern. The memory resource is shared between all
process in case of __GFP_NOFAIL it might be that we never return back
to user in that scenario i prefer to release hold memory for other
needs instead of keeping it for nothing.

If you think it is not a problem, then i do not have much to say.

--
Vlad Rezki