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

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Date:   Mon, 18 Dec 2017 16:05:28 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        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

On 12/17/2017 11:16 PM, Tetsuo Handa wrote:
> Wang, Wei W wrote:
>>> 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".
> If you read what radix_tree_node_alloc() is doing, you will find that
> radix_tree_node_alloc() returns NULL when memory allocation failed.
>
> I think that "this assumes that the caller has performed appropriate preallocation"
> means "The caller has to perform appropriate preallocation if the caller does not
> want radix_tree_node_alloc() to return NULL".

For the radix tree, I agree that we may not need preload. But 
ida_bitmap, which the xbitmap is based on, is allocated via preload, so 
I think we cannot bypass preload, otherwise, we get no ida_bitmap to use.

Best,
Wei