linux-kernel - Re: [PATCH v3 1/3] lib: bitmap: introduce bitmap_print_to

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Message-ID: <YLjSbSeFBygxl799@smile.fi.intel.com>
Date:   Thu, 3 Jun 2021 16:00:29 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     "tiantao (H)" <tiantao6@...wei.com>,
        Tian Tao <tiantao6@...ilicon.com>, gregkh@...uxfoundation.org,
        rafael@...nel.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, song.bao.hua@...ilicon.com,
        Randy Dunlap <rdunlap@...radead.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        "Ma, Jianpeng" <jianpeng.ma@...el.com>,
        Yury Norov <yury.norov@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 1/3] lib: bitmap: introduce bitmap_print_to_buf

On Thu, Jun 03, 2021 at 01:39:19PM +0100, Jonathan Cameron wrote:
> On Thu, 3 Jun 2021 14:11:16 +0300
> Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> 
> > On Thu, Jun 03, 2021 at 06:33:25PM +0800, tiantao (H) wrote:
> > > 在 2021/6/3 17:50, Andy Shevchenko 写道:  
> > > > On Thu, Jun 03, 2021 at 05:22:40PM +0800, Tian Tao wrote:  
> > > > > New API bitmap_print_to_buf() with bin_attribute to avoid maskp
> > > > > exceeding PAGE_SIZE. bitmap_print_to_pagebuf() is a special case
> > > > > of bitmap_print_to_buf(), so in bitmap_print_to_pagebuf() call
> > > > > bitmap_print_to_buf().  
> > 
> > ...
> > 
> > > > > + * @count: the maximum number of bytes to print  
> > 
> > > > > +	size = memory_read_from_buffer(buf, count, &off, data, strlen(data) + 1);  
> > > > Are you sure you have put parameters in the correct order?  
> > > 
> > > yes, I already test it.
> > > 
> > > ssize_t memory_read_from_buffer(void *to, size_t count, loff_t *ppos,
> > >                                 const void *from, size_t available)  
> > 
> > Have you read the meaning of count and available?
> > Please, double check that they are filled with correct values.
> 
> Ok, I don't get this one either so can you give us more of a hint?

There is no hint, as you noticed the documentation of the function is a bit
confusing.

> /**
>  * memory_read_from_buffer - copy data from the buffer
>  * @to: the kernel space buffer to read to
>  * @count: the maximum number of bytes to read
>  * @ppos: the current position in the buffer
>  * @from: the buffer to read from
>  * @available: the size of the buffer
>  *
>  * The memory_read_from_buffer() function reads up to @count bytes from the
>  * buffer @from at offset @ppos into the kernel space address starting at @to.
>  *
>  * On success, the number of bytes read is returned and the offset @ppos is
>  * advanced by this number, or negative value is returned on error.
>  **/
> 
> These docs do end up rather confusing by using the term buffer for multiple things
> but taking what is passed in.
> 
> Count is the maximum in the sense of how many bytes we are requesting are read
> which indeed should be count here as that reflects what userspace asked for.
> 
> Avail is the size of the buffer we are reading from.  Now that's slightly
> ambiguous in the docs in the sense of 'buffer' could mean the to buffer or
> the from buffer.  However, I'd assume count is definitely <= size of the space
> after address to in the to buffer, so I would assume that means available
> is the size of the from buffer.  Here that is strlen() + 1, so looks fine.
> 
> This interpretation also lines up with the implementation.
> 
> So what are we missing?

Thanks for double checking and explaining.

> > > > I guess you have to provide the test case(s).

Just test cases is what we are missing. Then we can play around with different
input to see if it's all correct.

-- 
With Best Regards,
Andy Shevchenko