Re: Access content of file via inodes

[Anton Altaparmakov <aia21@cam.ac.uk>]

Hi Martin,

On Fri, 27 May 2005, Martin Jambor wrote:
> On 4/8/05, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> > On Fri, 2005-04-08 at 14:01 +0800, Kathy KN (HK) wrote:
> > > Hi Jeff,
> > >
> > > Is it possible to modify the cached page, and invalidate it back
> > > to update the page cache of the new page? I did a recursive grep
> > > and could only find functions that let you read or grab pages in the
> > > cache.
> > 
> > Once you have the page (via read_cache_page() or whatever) you can
> > simply write to it, then do a flush_dcache_page(page), then
> > set_page_dirty(page) and finally do the page_cache_release().  Oh, and
> > don't forget to unmap the page.  Usually done straight after the
> > flush_dcache_page().  And example from ntfs where we get a page, memset
> > it to a value (val), and then mark it dirty for later write out:
> > 
> >         page = read_cache_page(mapping, idx,
> >                         (filler_t*)mapping->a_ops->readpage, NULL);
> >         if (IS_ERR(page)) {
> >                 ntfs_error(vol->sb, "Failed to read first partial "
> >                                 "page (sync error, index 0x%lx).", idx);
> >                 return PTR_ERR(page);
> >         }
> >         wait_on_page_locked(page);
> >         if (unlikely(!PageUptodate(page))) {
> >                 ntfs_error(vol->sb, "Failed to read first partial page "
> >                                 "(async error, index 0x%lx).", idx);
> >                 page_cache_release(page);
> >                 return PTR_ERR(page);
> >         }
> >         size = PAGE_CACHE_SIZE;
> >         if (idx == end)
> >                 size = end_ofs;
> >         kaddr = kmap_atomic(page, KM_USER0);
> >         memset(kaddr + start_ofs, val, size - start_ofs);
> >         flush_dcache_page(page);
> >         kunmap_atomic(kaddr, KM_USER0);
> >         set_page_dirty(page);
> >         page_cache_release(page);
> > 
> > Of course you need to serialise access in some way so multiple writers
> > do not step on each other's toes, etc...
> 
> What do I have to do to when I need to append something to a file?
> BTW, if that matters, I'll be implementing the adress_space_operations
> of that file as well...

That depends on where you want to do that from and what that file is.

If it is a file accessible from user space the size modification should be 
done under i_sem protection which is tricky from address space operations 
but fine from other parts of the kernel.

If it is a private file to you you need to serialize access with your own 
lock then it does not matter from where you do this (well, unless you try 
to do it from an atomic region/with spinlocks held or something).

Basically there are all sorts of considerations.

Could you explain where in the kernel and in what context you need to do 
the file append?  And whether the file is private to you or accessible 
from user space?

It would be easier to explain what you need/can do if I knew exactly what 
you are trying to do.

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/