> > > > IIRC, it points to the list of buffer heads describing the disk > > location of the data resident in the page. Buffer heads are the > > structs used to schedule traditional (read()/write()) disk > > I/O, whereas page->mapping is used for mmap() I/O. So a > > "buffer" is just a logical block of data within a page > > that correponds to a physical disk block somewhere; and > > the buffer head is a description of that relationship. In the > > older 2.4 kernels, they were complimentary: when a mmap()ed page > > had to be written out, the VM created buffers describing the page > > and then handed those buffers off to the disk I/O subsystem. I'm > > not sure if things work the same now. (Probably so.) > > Each page in page cache belongs to an inode or is anonymous. But > page_launder() writes the pages out independently of what they belong > to. So it one cache. Mappings are there so page_launder knows where to > write the page (it calls down to writepage method in mapping). > > With buffers, IIRC it's still the same way. When a page has to be > written to disk, it's turned into buffers and these are queued for disk > write. > Are you guys saying that pages for traditional I/O(read/write) do not have page->mapping while clean mmap()ed pages do not have page->buffer? As far as I know, a page(not anonymous) with its mapping points to mapping of inode of the file in page_cache_read() function like this: inode = file->f_dentry->d_inode; struct address_space *mapping = inode->i_mapping; page = page_cache_alloc(mapping); page->mapping = mapping; when page got swapped, it's mapping is set to page->mapping = &swapper_space; so when is the condition (!page->mapping) true and what it represents for? If page->buffers is true, Does that ony mean either page is brought in by traditional IO or a disk block is allocated for this mmap()ed page for written(because page is dirty)? Thanks, :) -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
