RE: doubt abt a function in ext2

["Rajat Jain" <Rajat.Jain@xxxxxxxxxxxx>]

Hi Nidhi,

 

You've got the function semantics wrong. It does not intend to give you the "last byte of nth page".

 

Rather, the "last byte OF FILE X in nth page".

 

inode->i_size = file size.

 

Thanks,

 

Rajat

 

---

From: nidhi mittal hada [mailto:nidhimittal19@xxxxxxxxx]
Sent: Wednesday, November 04, 2009 3:16 PM
To: Rajat Jain; Manish Katiyar
Cc: Kernelnewbies
Subject: Re: doubt abt a function in ext2

in simple words--- simple maths

--------------|----------------------------------|----------------------------------|---------------------------------|
0  page1    (1*2^12)     page 2             2*2^12         page3             3*2^12          page4               4*2^12


now page_nr=3
we want last byte of page 3
inode->i_size=4*2^12

then
       last_byte -= page_nr << PAGE_CACHE_SHIFT;
means
last_byte = 4*2^12 - 3*2^12
last_byte = 1*2^12

NOT True that is isnt it --- please CMIIW

but
actually
last_byte of page_nr=3 is 3*2^12


Nidhi

On Wed, Nov 4, 2009 at 2:47 PM, Rajat Jain <Rajat.Jain@xxxxxxxxxxxx> wrote:

Hi,

The function calculates the last byte in a page OF A PARTICULAR FILE.

static unsigned
ext2_last_byte(struct inode *inode, unsigned long page_nr)
{
       unsigned last_byte = inode->i_size;

/* Currently, last_byte = file size */

       last_byte -= page_nr << PAGE_CACHE_SHIFT;

/* Currently, last_byte = file size - no of bytes in file BEFORE this
page */
/* implies last_byte = no of bytes in file in this page and any
subsequent pages */

       if (last_byte > PAGE_CACHE_SIZE)
               last_byte = PAGE_CACHE_SIZE;

/* Simply check if this is NOT the last page, then obviously last byte
of file in THIS page = page size */

       return last_byte;
/* Otherwise since this is the last page, leave last_byte as is */
}

Thanks,

Rajat


________________________________

       From: kernelnewbies-bounce@xxxxxxxxxxxx
[mailto:kernelnewbies-bounce@xxxxxxxxxxxx] On Behalf Of nidhi mittal
hada
       Sent: Wednesday, November 04, 2009 2:19 PM
       To: Kernelnewbies
       Subject: doubt abt a function in ext2

       /*
        * Return the offset into page `page_nr' of the last valid
        * byte in that page, plus one.
        */

       static unsigned
       ext2_last_byte(struct inode *inode, unsigned long page_nr)
       {
               unsigned last_byte = inode->i_size;

               last_byte -= page_nr << PAGE_CACHE_SHIFT;
               if (last_byte > PAGE_CACHE_SIZE)
                       last_byte = PAGE_CACHE_SIZE;
               return last_byte;
       }


       tht's a small function to get last byte of page number page_nr
       but it seems to be wrong
       as
       last byte of page_nr  if byte number start with 0 for  page_nr 1


       last_byte  should be = page_nr * (2^12 )

       why its been subtracted from total size i_size?????

       pl tell me what i am missing ?
       Nidhi





       --
       Thanks & Regards
       Nidhi Mittal Hada

--
Thanks & Regards
Nidhi Mittal Hada