linux-kernel - Re: [PATCH v19 3/7] xbitmap: add more operations

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Date:   Sun, 17 Dec 2017 19:21:35 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     wei.w.wang@...el.com, willy@...radead.org
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
        mhocko@...nel.org, akpm@...ux-foundation.org,
        mawilcox@...rosoft.com, david@...hat.com, cornelia.huck@...ibm.com,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v19 3/7] xbitmap: add more operations

Wei Wang wrote:
> > But passing GFP_NOWAIT means that we can handle allocation failure. There is
> > no need to use preload approach when we can handle allocation failure.
> 
> I think the reason we need xb_preload is because radix tree insertion 
> needs the memory being preallocated already (it couldn't suffer from 
> memory failure during the process of inserting, probably because 
> handling the failure there isn't easy, Matthew may know the backstory of 
> this)

According to https://lwn.net/Articles/175432/ , I think that preloading is needed
only when failure to insert an item into a radix tree is a significant problem.
That is, when failure to insert an item into a radix tree is not a problem,
I think that we don't need to use preloading.

> 
> So, I think we can handle the memory failure with xb_preload, which 
> stops going into the radix tree APIs, but shouldn't call radix tree APIs 
> without the related memory preallocated.

It seems to me that virtio-ballon case has no problem without using preloading.