Re: Where does a process wait after requesting some data file ?

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Jan 15, 2009 at 02:50:27PM +0100, Francis Moreau wrote:
> Let's say that a process wants to read a page of data stored in a file.
> For that it issues a sys_read().
> 
> Now the data page is not in the page cache and it must be fetched from
> the disk.
> 
> So I assume while the data page is recovered from the disk, the process
> is suspended until the data are ready to be read.
> 
> Could anybody show me where in the source code where the process is
> suspended ?

Sure.  Take a look in mm/filemap.c.  Filesystems that use the page cache
will usually end up calling do_generic_file_read() one way or another.
It tries to find the page in the page cache, when it doesn't find the
page, it creates it, then calls ->readpage() to start the read, and
lock_page_killable() to wait for the page to be read.  As the comment
says, when the read finishes, it will unlock the page and the reader
will continue.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."