Re: Access content of file via inodes

[Martin Jambor <jamborm@gmail.com>]

Hi Anton,

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...

Thanks very much in advance,

Martin