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

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Date:   Sun, 17 Dec 2017 13:47:21 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
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        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
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        "riel@...hat.com" <riel@...hat.com>
Subject: RE: [PATCH v19 3/7] xbitmap: add more operations

On Saturday, December 16, 2017 3:22 AM, Matthew Wilcox wrote:
> On Fri, Dec 15, 2017 at 10:49:15AM -0800, Matthew Wilcox wrote:
> > Here's the API I'm looking at right now.  The user need take no lock;
> > the locking (spinlock) is handled internally to the implementation.

Another place I saw your comment " The xb_ API requires you to handle your own locking" which seems conflict with the above "the user need take no lock".
Doesn't the caller need a lock to avoid concurrent accesses to the ida bitmap?


> I looked at the API some more and found some flaws:
>  - how does xbit_alloc communicate back which bit it allocated?
>  - What if xbit_find_set() is called on a completely empty array with
>    a range of 0, ULONG_MAX -- there's no invalid number to return.

We'll change it to "bool xb_find_set(.., unsigned long *result)", returning false indicates no "1" bit is found.


>  - xbit_clear() can't return an error.  Neither can xbit_zero().

I found the current xbit_clear implementation only returns 0, and there isn't an error to be returned from this function. In this case, is it better to make the function "void"?


>  - Need to add __must_check to various return values to discourage sloppy
>    programming
> 
> So I modify the proposed API we compete with thusly:
> 
> bool xbit_test(struct xbitmap *, unsigned long bit); int __must_check
> xbit_set(struct xbitmap *, unsigned long bit, gfp_t); void xbit_clear(struct
> xbitmap *, unsigned long bit); int __must_check xbit_alloc(struct xbitmap *,
> unsigned long *bit, gfp_t);
> 
> int __must_check xbit_fill(struct xbitmap *, unsigned long start,
>                         unsigned long nbits, gfp_t); void xbit_zero(struct xbitmap *,
> unsigned long start, unsigned long nbits); int __must_check
> xbit_alloc_range(struct xbitmap *, unsigned long *bit,
>                         unsigned long nbits, gfp_t);
> 
> bool xbit_find_clear(struct xbitmap *, unsigned long *start, unsigned long
> max); bool xbit_find_set(struct xbitmap *, unsigned long *start, unsigned
> long max);
> 
> (I'm a little sceptical about the API accepting 'max' for the find functions and
> 'nbits' in the fill/zero/alloc_range functions, but I think that matches how
> people want to use it, and it matches how bitmap.h works)

Are you suggesting to rename the current xb_ APIs to the above xbit_ names (with parameter changes)? 

Why would we need xbit_alloc, which looks like ida_get_new, I think set/clear should be adequate to the current usages.

Best,
Wei