Question on mmap / expecting more calls to vfs_read

Sebastian Pipping sebastian at
Thu Jan 6 18:49:25 EST 2011

On 01/06/11 07:53, Rajat Sharma wrote:
> Hi Sebastian,
> you guess for ELF header seems to be valid to me. When executables or
> binaries are loaded memory, it is done through mmap call to the file,
> and to understand what file is and what binary handler in kernel can
> handle its section, kernel needs to know its header first, which is
> within the first page of the header with fixed location for a magic
> number (identifier for binary handler e.g. ELF handler which further
> loads its other sections by reading section table). Note that there
> are multiple binary format handles within the kernel e.g. ELF, A.OUT
> which are tried sequentially to identify the file format.
> From the file system perspective, mmap does not use vfs_read or
> vfs_write calls at all, thats why you don't see them. It directly
> works on address space operations of an inode (file) to populate data
> in page-cache. For a mmapped region, if you don't see a page in
> memory, kenel page faults and tries to read-in the page using readpage
> method of address_space_operations. Similarly when you modify a page,
> writepage method is called, but since executables are accessed
> read-only, you won't see writepage method getting called either.
> Hope this makes it clearer.

Excellent, thank you!  I find calls to readpages on file
/lib/ now.  That may be the missing reads.

Any ideas how get offset and length (like with vfs_read) for a certain
page passed to readpage(file, page) ?



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