Question on mmap / expecting more calls to vfs_read

[Manish Katiyar]

On Thu, Jan 6, 2011 at 3:49 PM, Sebastian Pipping <sebastian at pipping.org> wrote:
> 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/libc-2.11.2.so 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) ?

Add your stats hooks to do_mmap or mmap_region() ???

-- 
Thanks -
Manish
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