Re: page->offset

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Thu, Apr 13, 2000 at 10:23:03AM +0530, pnilesh@in.ibm.com wrote:
> 
> Here the call fails .
> I tried to map at / from offset 512 that also failed.
> however with the offset of 1024 it succeded.

Odd, it shouldn't.  Which kernel is this?

>         char *p = mmap (NULL,10,PROT_READ,MAP_SHARED,fd,1024);
>         char *s = mmap (NULL,10,PROT_READ,MAP_SHARED,fd,1024);

strace shows:

  old_mmap(NULL, 10, PROT_READ, MAP_SHARED, 3, 0x400) = -1 EINVAL (Invalid argument)
  old_mmap(NULL, 10, PROT_READ, MAP_SHARED, 3, 0x400) = -1 EINVAL (Invalid argument)

on a 1k blocksize filesystem.

> Does these virtual addresses point to only one physical page ?
> This page is in the page cache if I am not wrong with page->count = 3 ?
> (2.2.x)

Correct.

> If I do read () from 1024 offset the data I will get will be from the above
> phyiscal page or from .... ?

read() _always_ invokes the page cache with pagesize-aligned
page offsets.  If a correctly aligned page is not present, a new one
will be created.

--Stephen
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