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

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Message-ID: <286AC319A985734F985F78AFA26841F739387B68@shsmsx102.ccr.corp.intel.com>
Date:   Sun, 17 Dec 2017 11:50:03 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        "willy@...radead.org" <willy@...radead.org>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "david@...hat.com" <david@...hat.com>,
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        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
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        "nilal@...hat.com" <nilal@...hat.com>,
        "riel@...hat.com" <riel@...hat.com>
Subject: RE: [PATCH v19 3/7] xbitmap: add more operations



> -----Original Message-----
> From: Tetsuo Handa [mailto:penguin-kernel@...ove.SAKURA.ne.jp]
> Sent: Sunday, December 17, 2017 6:22 PM
> To: Wang, Wei W <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.

It also mentions that the preload attempts to allocate sufficient memory to *guarantee* that the next radix tree insertion cannot fail.

If we check radix_tree_node_alloc(), the comments there says "this assumes that the caller has performed appropriate preallocation".

So, I think we would get a risk of triggering some issue without preload().

> >
> > 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.

Why is that?

Best,
Wei