Tyler wrote:
Ketan Mukadam wrote:
Hi all,
I am trying to understand a little bit about linux VMM [Writing a
device driver for linux].
What is "unsigned long vm_pgoff;" ?
The comment in the mm.h says that
/* Offset (within vm_file) in PAGE_SIZE units, *not* PAGE_CACHE_SIZE */
But that did not made it clear....tried to 'google' it but to no avail...
can anyone explain me the use of this variable?
I think it's just the offset within the vma region (the relative adress
to the beginning of the region if you want)
I guess it's only the offset in the file where the vma starts. Only a
copy of the offset value passed in the mmap syscall. And it is stored in
PAGE_SIZE units instead of bytes, maybe to allow mapping huge files,
whose offsets in bytes would not fit in 'unsigned long' variables...
Luciano Stertz
It is generally used in the driver's mmap implementation as following
unsigned long off = vma->vm_pgoff << PAGE_SHIFT;
This is equal to divide the offest by the number of possible pages. So
we get here the offset in pages.
Thanks in advance
Regards
Ketan
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Luciano A. Stertz
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